Cabletron Systems MMAC-5FNB, MMAC-FNB Networking Manual

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Cabletron Systems

Networking Guide

MMAC-FNB™ Solutions

Notice

Cabletron Systems reserves the right to make changes in speciﬁcations and other information contained in this document without prior notice. The reader should in all cases consult Cabletron Systems to determine whether any such changes have been made.

The hardware, ﬁrmware, or software described in this manual is subject to change without notice.

IN NO EVENT SHALL CABLETRON SYSTEMS BE LIABLE FOR ANY INCIDENTAL, INDIRECT, SPECIAL, OR CONSEQUENTIAL DAMAGES WHATSOEVER (INCLUDING BUT NOT LIMITED TO LOST PROFITS) ARISING OUT OF OR RELATED TO THIS MANUAL OR THE INFORMATION CONTAINED IN IT, EVEN IF CABLETRON SYSTEMS HAS BEEN ADVISED OF, KNOWN, OR SHOULD HAVE KNOWN, THE POSSIBILITY OF SUCH DAMAGES.

Printed in the United States of America.

Order Number: 9031706-01 March 1996

Cabletron Systems, Inc. P.O. Box 5005 Rochester, NH 03866-5005

Cabletron Systems , SPECTRUM , BRIM , DNI , FNB , INA , Integrated Network Architecture , LANVIEW , LANVIEW Secure , Multi Media Access Center , MiniMMAC , MicroMMAC , and TRMM are registered trademarks, and Bridge/Router Interface Modules , CRXMIM , CXRMIM , Desktop Network Interface , Distributed LAN Monitoring , Distributed Network Server , DLM , EMM-E6 , EMME , EPIM , EPIM-A , EPIM-C , EPIM-F1 , EPIM-F2 , EPIM-F3 , EPIM-T , EPIM-X , ESXMIM , ESXMIM-F2 , ETWMIM , FDCMIM-04 , FDCMIM-08 , FDCMIM-24 , FDCMIM-28 , FDCMIM-44 , FDCMIM-48 , FDMMIM , FDMMIM-04 , FDMMIM-24, FDMMIM-44, Flexible Network Bus , FOMIM , FORMIM , HubSTACK , IRBM , IRM , IRM-2 , IRM-3 , Media Interface Module , MIM , MMAC , MMAC-3 , MMAC-3FNB , MMAC-5 , MMAC-5FNB , MMAC-8 , MMAC8FNB , MMAC-M8FNB , MMAC-Plus , MRX , MRXI , MRXI-24 , Multichannel , NB20E , NB25E , NB30 , NB35 , NBR-220/420/620 , RMIM , SecureFast Packet Switching , SFPS , SPECTRUM Element Manager , SPECTRUM for Open Systems , SPIM-A , SPIM-C , SPIM-F1 , SPIM-F2 , SPIM-T , SPIM-T1 , TPMIM , TPMIM-22 , TPMIM-T1 , TPRMIM , TPRMIM-36 , TPT-T , TRBMIM , TRMM-2 , TRMMIM , and



other Cabletron product names are trademarks of Cabletron Systems, Inc.

All other product names mentioned in this document may be trademarks or registered trademarks of their respective companies.

Chapter 1 Introduction

Using This Guide.........................................................................................................................1-1

Organization of Document.........................................................................................................1-1

Conventions of This Document.................................................................................................1-3

Warnings and Notiﬁcations ................................................................................................1-3

Formats and Measures.........................................................................................................1-3

Additional Assistance .................................................................................................................1-4

Associated Documentation ........................................................................................................1-4

Contents

Chapter 2 Overview of Networking

Discussion of Networking..........................................................................................................2-1

Why Network?...................................................................................................................... 2-1

What Is a Network?.....................................................................................................................2-3

The Classiﬁcation of Networks..................................................................................................2-3

Network Topology ...............................................................................................................2-4

Network Technologies.........................................................................................................2-6

Media......................................................................................................................................2-7

Interoperability and Standards Bodies...................................................................................2-13

Interoperability, the Ideal of Networking.......................................................................2-13

Standards and Compliance...............................................................................................2-13

The OSI Model, Basis of Standards.................................................................................. 2-14

Application of the OSI Model........................................................................................... 2-18

Chapter 3 Technology Basics

Ethernet.........................................................................................................................................3-1

Abstract..................................................................................................................................3-1

Theory .................................................................................................................................... 3-2

Operation...............................................................................................................................3-2

Segmentation.........................................................................................................................3-4

Strengths and Weaknesses ..................................................................................................3-6

Special Design Considerations ........................................................................................... 3-8

Token Ring....................................................................................................................................3-9

Abstract..................................................................................................................................3-9

Theory .................................................................................................................................... 3-9

Operation...............................................................................................................................3-9

Segmentation....................................................................................................................... 3-11

Strengths and Weaknesses ................................................................................................3-13

Special Design Considerations ......................................................................................... 3-15

iii

Fiber Distributed Data Interface..............................................................................................3-15

Abstract ................................................................................................................................3-15

Theory ..................................................................................................................................3-15

Operation.............................................................................................................................3-16

Strengths and Weaknesses.................................................................................................3-17

Special Design Considerations .........................................................................................3-18

Chapter 4 Network Design

Workgroup Creation....................................................................................................................4-2

What Is a Workgroup? .........................................................................................................4-2

Workgroup Establishment Criteria....................................................................................4-2

Selecting Workgroup Organization....................................................................................4-7

Selecting Workgroup Technologies..................................................................................4-12

Backbone Planning ....................................................................................................................4-13

What Is a Backbone?...........................................................................................................4-13

Methods of Conﬁguring Backbones ................................................................................4-14

Choosing Backbone Technologies....................................................................................4-17

Creating a Manageable Plan.....................................................................................................4-18

Logical Layout.....................................................................................................................4-19

Fault Aversion .....................................................................................................................4-21

Network Maps and Record Keeping ...............................................................................4-22

Network Expandability.............................................................................................................4-24

Network Migration....................................................................................................................4-24

Designing with the MMAC......................................................................................................4-25

Modular Chassis .................................................................................................................4-25

Reliability and Recovery....................................................................................................4-26

Technology Flexibility........................................................................................................4-27

Power Redundancy ............................................................................................................4-29

Chapter 5 Ethernet

Description....................................................................................................................................5-1

CSMA/CD.............................................................................................................................5-1

Media......................................................................................................................................5-4

Connectivity/Transceivers..................................................................................................5-4

Rules and Regulations .........................................................................................................5-5

Repeating ...............................................................................................................................5-6

Repeaters/Hubs....................................................................................................................5-6

Simple Ethernet Conﬁguration................................................................................................5-10

Design Philosophy..............................................................................................................5-10

Design Example ..................................................................................................................5-10

Segmentation..............................................................................................................................5-15

Bridges..................................................................................................................................5-16

Multichannel Ethernet .......................................................................................................5-21

Segmented Ethernet Conﬁguration.........................................................................................5-23

Design Philosophy..............................................................................................................5-23

Design Example ..................................................................................................................5-24

Segmentation - Special Cases...................................................................................................5-30

Port Assignment .................................................................................................................5-30

Port Assignment Conﬁguration............................................................................................... 5-31

Design Philosophy .............................................................................................................5-31

Design Example..................................................................................................................5-32

Ethernet Switching ....................................................................................................................5-34

Switching Conﬁgurations......................................................................................................... 5-35

Permutations.......................................................................................................................5-37

Chapter 6 Token Ring

Description ...................................................................................................................................6-1

Fault Isolation .......................................................................................................................6-4

Fault Recovery ......................................................................................................................6-6

Media......................................................................................................................................6-9

Connectivity/Transceivers................................................................................................6-10

Token Ring Network Rules...............................................................................................6-12

Single Ring Conﬁguration........................................................................................................6-13

Design Philosophy .............................................................................................................6-13

Design Example..................................................................................................................6-13

Extending the Ring (Ring-In/Ring-Out)......................................................................... 6-20

Brief Review of MAUs.......................................................................................................6-21

Segmentation..............................................................................................................................6-22

Multi-Ring Conﬁguration.........................................................................................................6-24

Design Philosophy .............................................................................................................6-24

Design Example..................................................................................................................6-24

Multichannel Token Ring..................................................................................................6-33

Multichannel Token Ring Conﬁguration ...............................................................................6-34

Design Philosophy .............................................................................................................6-34

Design Example..................................................................................................................6-35

Chapter 7 FDDI

Description ...................................................................................................................................7-1

Media......................................................................................................................................7-2

Rings and Devices ................................................................................................................7-3

Concentrators........................................................................................................................7-5

Bridges ...................................................................................................................................7-7

FDDI Backbone Conﬁguration ..................................................................................................7-7

Design Philosophy ...............................................................................................................7-7

Design Example....................................................................................................................7-7

FDDI Workgroup Conﬁguration...............................................................................................7-8

Design Philosophy ...............................................................................................................7-8

Design Example....................................................................................................................7-8

Chapter 8 Expansion - Ethernet

Simple Ethernet............................................................................................................................8-1

Adding Stations ....................................................................................................................8-1

Adding Segmentation..........................................................................................................8-3

Incorporating Token Ring....................................................................................................8-5

Incorporating FDDI..............................................................................................................8-6

Segmented Ethernet.....................................................................................................................8-8

Adding Users to One Segment ...........................................................................................8-8

Adding Users to Several Segments....................................................................................8-9

Incorporating Port Assignment..........................................................................................8-9

Incorporating Token Ring....................................................................................................8-9

Incorporating FDDI............................................................................................................8-10

Port Assignment and Virtual LANs........................................................................................8-11

Adding Users to Any Segment.........................................................................................8-11

Incorporating Token Ring..................................................................................................8-11

Incorporating FDDI............................................................................................................8-11

Chapter 9 Expansion - Token Ring

Single Ring....................................................................................................................................9-1

Adding Stations ....................................................................................................................9-1

Adding New Rings...............................................................................................................9-2

Incorporating New Technologies .......................................................................................9-2

Multi-Ring.....................................................................................................................................9-3

Adding Stations to Any Ring..............................................................................................9-3

Incorporating New Technologies .......................................................................................9-4

Port Assignment...........................................................................................................................9-4

Adding Stations to Any Ring..............................................................................................9-4

Incorporating New Technologies .......................................................................................9-5

Chapter 10 Expansion - FDDI

FDDI Workgroups .....................................................................................................................10-1

Adding Single Attached Stations .....................................................................................10-1

Adding Dual Attached Concentrators.............................................................................10-2

Connecting Multiple Rings ...............................................................................................10-3

Chapter 11 Product Descriptions

Chassis.........................................................................................................................................11-2

Ethernet ....................................................................................................................................... 11-5

Token Ring ................................................................................................................................11-16

FDDI...........................................................................................................................................11-25

Miscellaneous/Multiprotocol................................................................................................ 11-31

Appendix A Charts & Tables

Network Design Flowcharts .....................................................................................................A-2

Ethernet Network Design Flowchart................................................................................A-2

ESXMIM Network Design Flowchart...............................................................................A-3

Single Token Ring Network Design Flowchart...............................................................A-4

Segmented Token Ring Network Design Flowchart ......................................................A-5

Multichannel Token Ring Network Design Flowchart ..................................................A-6

FDDI Backbone Network Design Flowchart...................................................................A-7

FDDI Workgroup Network Design...................................................................................A-8

MMAC Design Tables ................................................................................................................A-9

Ethernet Design Tables .......................................................................................................A-9

Token Ring Design Tables ................................................................................................A-13

FDDI Design Tables...........................................................................................................A-16

Networking Standards and Limitations................................................................................A-17

Ethernet...............................................................................................................................A-17

Token Ring..........................................................................................................................A-18

FDDI ....................................................................................................................................A-20

vii

viii

Introduction

Using This Guide

The purpose of this Networking Guide is to provide the customers and strategic partners of Cabletron Systems with information which allows them to conﬁgure and expand their own networks. As it is impossible to foresee every possible situation that may arise when a new network must be created or an existing one expanded, this guide deals with several of the most common networking situations.

Chapter 1

If you are unfamiliar with the basics of the Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) networking technologies, read the chapters in order. Pay particular attention to Chapters 3 through 7. These chapters should provide you with the basic information necessary to generate a relatively straightforward network design.

If you have experience with the design or operation of Cabletron networking products, you may wish to refer mainly to the chapters dealing with the expansion of existing networks, Chapters 8 through 10.

Organization of Document

In the interests of making this document accessible to as many customers of differing levels of experience as possible, this Networking Guide begins with a discussion of the various reasons for networking and a short overview of Local Area Networks (LANs). Following this brief overview, a series of short, introductory level training documents is provided. These documents provide general information on Ethernet, Token Ring and Fiber Distributed Data Interface networking, including examples of needs analysis and initial network designs utilizing Cabletron equipment. A short treatment of other networking technologies follows these individual subsections.

1-1

Introduction

Following the discussions of the major networking technologies supported, this guide shows how networks, based on the examples from the training sections, can be expanded.

The remainder of this guide contains brief descriptions of Cabletron Systems modular chassis products, charts and tables which supply much of the information that the network design process requires, and an extensive glossary of the terms used within this guide and in other Cabletron Systems publications.

The following summarizes the organization of this manual:

Chapter 1, Introduction, discusses the use and contents of this guide.

Chapter 2, Overview of Networking, explains the fundamentals of Local Area Networks (LANs), including the reasons to utilize a network, and discussions of factors such as standards-compliance and interoperability.

Chapter 3, Technology Basics, provides some introductory training into the LAN technologies which are treated in this Networking Guide.

Chapter 4, Network Design, discusses the procedures and steps involved in planning out a network, including the selection of a LAN technology and its organization.

Chapter 5, Ethernet, offers training in greater depth for the Ethernet LAN technology. This training includes the creation of example network conﬁgurations based on the concepts introduced.

Chapter 6, Token Ring, provides training and conﬁguration instruction for the Token Ring LAN technology, including example conﬁgurations.

Chapter 7, FDDI, is a training chapter which details the operation of the Fiber Distributed Data Interface (FDDI) technology and the creation of FDDI networks using Cabletron products.

Chapter 8, Expansion - Ethernet, shows how an existing Ethernet network can be expanded to include new users or capabilities.

Chapter 9, Expansion - Token Ring, provides instructions and guidelines for expanding existing Cabletron Token Ring network conﬁgurations.

Chapter 10, Expansion - FDDI, describes the means by which FDDI conﬁgurations which use Cabletron products may be expanded to include new users or stations.

Chapter 11, Product Descriptions, provides short descriptions of many Cabletron products referred to in this Networking Guide.

Following Chapter 11, Appendix A, Charts & Tables, provides quickly-accessible tables and ﬂowcharts for network design.

Following Appendix A, the Cabletron Systems Glossary of Terms may be found.

1-2 Organization of Document

Conventions of This Document

Warnings and Notiﬁcations

Introduction

NOTE

CAUTION

TIP

Note symbol. Used to provide additional information

concerning subsequent steps or actions that enhance the operator’s knowledge of the step or action.

Caution symbol. Used to caution against an action that could result in damage to equipment or poor equipment performance.

Tip symbol. Used to convey helpful hints concerning procedures or actions which would assist the operator in performing the task in a more timely manner in the future.

Warning symbol. Used to warn against an action that could result in personal injury or death and equipment damage.

Formats and Measures

Figures throughout the document are identiﬁed by chapter and illustration number. Many ﬁgures contain small numbers at the lower right-hand corner of the illustration. These are Cabletron Systems document control numbers and are not essential to understanding of the document.

When measurements are given in the text, they will be presented in both metric and U.S. Customary units. The metric unit will precede the U.S. Customary unit, and the latter will be contained in parentheses.

References to chapters or sections within this document will be printed in boldface type.

References to other publications or documents will be printed in italic type.

Conventions of This Document 1-3

Introduction

Additional Assistance

This publication describes many possible network conﬁgurations and designs. Due to the nearly limitless possibilities involved in network design, there are some aspects of the design process which are not addressed in this guide. If you have any doubts about your conﬁguration or expansion plans, Cabletron Systems maintains a staff of network design personnel and a sizable team of highly-trained cabling and hardware installation technicians. The services of the Networking Services group are available to customers at any time. If you are interested in obtaining design assistance or a network installation plan from the Networking Services group, contact your Cabletron Systems sales representative.

In addition to the availability of Networking Services, The Cabletron Systems Technical Support department is available to answer customer questions regarding existing Cabletron Systems networks or planned expansion issues. Contact Cabletron Systems at (603) 335-9400 to reach the Technical Support department with any speciﬁc product-related questions you may have.

Associated Documentation

The following publications may be of assistance to you in the design process. Several of these documents treat concepts raised in this Networking Guide in greater detail than they are presented here.

• The Cabletron Systems Product Catalog

1-4 Additional Assistance

Chapter 2

Overview of Networking

This chapter introduces and discusses several basic concepts and deﬁnitions essential to the understanding of local area networking.

Discussion of Networking

Why Network?

In this day and age, all companies and agencies have two resources in common, information and ability. No matter what a company may produce, market, offer or observe, the ﬁnal result is one that comes from a combination of ability and information. Every company with more than one employee is in the business of sharing information abilities. Information and ability may take many forms: stock reports and market prediction; manufacturing processes and skilled quality assurance; accounting information and efﬁciency examination.

Information is naturally separated from ability. It is the nature of information to accumulate beyond the point that every person has all the information required to perform productively. This separation is a factor that has been part of the business world for centuries. The rise of printing, a postal service, and the telephone can be viewed as ways that businesses and agencies have created and adopted means of providing information to those with ability.

While the separation of these factors has always existed and will always exist, the limiting factor for businesses is not the presence of the separation, but its magnitude. The more difﬁcult it is to bridge the gap between information and expertise, the less efﬁciently both factors are used.

A Local Area Network (LAN) allows for the rapid and direct sharing of critical information. This sharing enables employees to act more rapidly and with more complete access to information and resources than may have been possible with a system of ﬁling cabinets and interofﬁce mail.

2-1

Overview of Networking

The basis of the LAN is sharing. The LAN allows users to transfer information and completed documents without the overhead and delay introduced by hardcopy information. In addition, the LAN increases the utility of expensive resources such as printers, disk arrays, and plotters. For example, a high-speed printer on every desktop is an expensive and wasteful proposition, but allowing 20 users to share access to one high-speed printer reduces the overall cost of each document printed.

As very few users create things for only their own use, the LAN allows employees to discover means to speed the process of work or increase its efﬁciency. For example, a document that was once developed, printed, carried to the Order Entry department, signed, photocopied, sent to the Records department, updated, and ﬁled can now be printed in all three locations at once, freeing up each department to perform the tasks it is intended to.

If a LAN is used on a scale that encompasses the entirety of a facility, company, or corporation, the beneﬁts can be enormous. The nearly instantaneous sharing of information between several worldwide sites greatly improves the consistency of company documents and products, provides for the rapid integration of new policies, and supplies a system for seamless worldwide collaboration on projects. By reducing the delay inherent in the operations of business, a LAN increases productivity.

By providing users, management, sales, and production alike, with rapid and monitored access to the information base on which a corporation is built, a network empowers employees on a company-wide scale, giving them a chance to increase their own abilities and use their talents more fully in the corporation. The LAN is a means of bridging the chasm between information and expertise, enabling the ﬂow of essential information between workers, departments, ofﬁces, and corporate partners.

The LAN is a means of bringing things together: information and ability, customers and producers, employees and equipment. In an age where combination leads to security and strength for corporations, streamlining the combinations of information and ability helps erase the borders that have traditionally slowed the business process. By reducing the overhead related in doing business, a LAN allows your current employees to do more, improving efﬁciency and effectiveness to attain greater levels of productivity.

2-2 Discussion of Networking

What Is a Network?

Simply put, a Local Area Network, or LAN, is a number of related computers and electronic devices which share information over a transmission media. This can be as simple as a series of electronic cash registers which send updates of products sold during the course of the day to an inventory computer or as complex as a network spanning an entire corporate facility or University campus, providing high-powered communication services for hundreds of applications and the swapping of thousands of ﬁles each day.

The Classiﬁcation of Networks

While all networks are different, they all have a number of common deﬁning characteristics which serve to illustrate the type of network being discussed or examined. The most important characteristics for the purposes of this guide are topology, technology, media, and devices.

For the purposes of explanation, this chapter of the Networking Guide uses one cohesive metaphor to describe the portions of a network, the “meeting metaphor.” In the sections which follow, the topologies, technologies, and devices of networks will be viewed as aspects of a business or public meeting. By viewing the various facets of a network as parts of one common process, we can describe the distinct concepts as parts of a larger overall system.

Overview of Networking

What Is a Network? 2-3

Overview of Networking

Network T opology

The topology of a network refers to its physical layout or “shape.” The topology characteristic describes how components and cabling are interconnected. Using the meeting metaphor, the topology of a network can be seen as the organizational structure of the meeting itself; will the meeting consist of several committees making reports to each other when necessary? Will pre-determined representatives speak for the members of their group? Will an intermediary or moderator determine who has the “ﬂoor,” or will the opportunity to speak pass from participant to participant in a particular order?

Although the growing complexity and scale of networks has caused some topologies to bleed over into others, the topologies can be useful starting points for describing the overall layout of a network or network segment. There are a number of widely used topology descriptions for the most common topologies (see Figure 2-1).

Tree

Bus

Ring

Figure 2-1. Network Topologies

Star

Hub

1706n01

2-4 The Classiﬁcation of Networks

Bus

Tree

Overview of Networking

The bus topology uses a single common cable or link (coaxial cable, broadcast radio frequency) to connect the stations of the network to one another. The bus topology is strictly an Ethernet phenomenon, and is frequently encountered in existing Ethernet environments. Stations connect to the common media through a series of taps, located a speciﬁed distance from one another along the common cable, and only one station may successfully transmit onto the common media at any one time. Bus topologies are noted for their simplicity, but are notoriously difﬁcult to troubleshoot. In addition, a failure of the bus media itself, due to disruption or poor conﬁguration, causes the network to cease functioning.

The tree topology arranges links and stations into distinct hierarchies in order to allow greater control and troubleshooting. In order to function well, networks using tree topologies must incorporate some form of “trafﬁc control,” determining when trafﬁc is allowed to travel up and down the branches of the tree. Similar to a well-deﬁned chain of command, the tree topology shields disparate network groups from affecting each other.

Ring

The tree topology also facilitates much more straightforward troubleshooting procedures. The main downfall of the tree topology is its own organization. If there is a failure on one of the branches of the tree, every branch that forks from that point of failure becomes unable to communicate with the rest of the network.

The ring topology connects every station on the network to every other station in the network in a contiguous circle. Most common in the Token Ring and FDDI network technologies (discussed later), ring topologies rely on well-deﬁned rules of transmission and reception in order to function. Stations are connected in a deﬁnite series, with information going from one station to the next in a pre-deﬁned order. Since each station is expecting transmissions from the station before it and sending transmissions only to the station following it, ring topologies can be made to incorporate automatic fault location and recovery procedures.

The Classiﬁcation of Networks 2-5

Overview of Networking

Star

The star topology consists of a number of individual stations which communicate through a common central point. Similar to the bus topology, star topology network stations all share a single common interface. In place of a section of cable, however, the common central point in star topology networks is often a concentrator device, or “hub.” Each station connects to the hub through its own physical interface, and many concentrators incorporate their own troubleshooting and monitoring functions, allowing network managers to determine faulty stations and remove them from the concentrator without disrupting the remaining network. Unfortunately, a failure of the concentrator can lead to the same disabling of the network that can occur in bus topologies.

Hybrid

Any network topology that incorporates elements of two or more of the previously discussed topologies is a hybrid. For example, a tree that led down to a series of buses (the ‘leaves’) would be a hybrid called a “tree of buses.” A ring topology network with a series of concentrators acting as stations on the ring would be called a “ring of stars.” As networks grow to encompass more and more of a facility’s needs, topologies tend to hybridize in order to fulﬁll the individual needs of workgroups and departments.

Network Technologies

While a topology describes the way a network is physically laid out, the network technology deﬁnes how the devices that make up the network receive and transmit information, deal with faults and problems, and control the actual operation of the network. The technology of the network can be seen as the Rules of Order that will be followed at the meeting. These concrete and binding rules will determine who may speak, at what time, for how long, to whom, in what language, and so on. The technology determines how problems are identiﬁed and what methods may be used to resolve them. If arm wrestling is going to be considered a valid means of breaking a deadlock between two sides in a vote, the rules of order must clearly state that, as well as provide the rules for arm wrestling.

There are several commonly-encountered technologies in networking. Among the most common of the ‘client-server’ (non mainframe-terminals) technologies are Ethernet, Token Ring, and the Fiber Distributed Data Interface (FDDI). These network technologies and their operation are individually described in their own chapters later in this document.

2-6 The Classiﬁcation of Networks

Media

Overview of Networking

The term media has come to mean several different things in today’s English language. For the purposes of networking, media always refers to the physical entity that is used for the purposes of transmitting and receiving the impulses that make up data exchange. While in some networks, radio frequencies and nationwide telephone service providers are considered to be media, the term most commonly refers to the physical chunks of cable that connect one network device to another. In the meeting metaphor, media is simply the communications system that is to be used, be it speech, writing, or tin-can telephone.

While media can conceivably be any system of physically transmitting the impulses indicating the zeroes and ones of electronic data, there are a number of media common to most networks. Unshielded Twisted Pair cabling, or UTP, is a commonly-used media in networks. UTP is small-gauge, inexpensive cabling made up of a series of copper strands which are twisted together inside the insulating jacket, in the same manner as many telephone cables are. Several Ethernet networks use telephone-grade UTP as their primary media. Other common media include coaxial cable, ﬁber optics (both Light Emitting Diode and Laser driven), Shielded Twisted Pair (STP), and speciﬁc technology-related cable types such as the Attachment Unit Interface (AUI) cable deﬁned in the Ethernet technology speciﬁcation.

It is important to note that some media are not supported by some technologies. For example; neither the Token Ring nor FDDI standards support the use of coaxial cables. Each technology will deﬁne what media it supports, and to what extent. Some technologies are very demanding of certain types of media, and all technologies place limitations on the extent to which a particular media may be used. These limitations, for the purposes of designing networks, are discussed in greater detail within the chapters providing individual descriptions of the major networking technologies, as well as in the Charts and Tables section at the end of this Networking Guide.

This guide does not intend to describe in detail the procedures for cable installation and testing. Characteristic properties of the individual media and any special installation-related information that should be kept in mind during planning will be presented. This information should be used to assist the selection of a media based on the needs of a speciﬁc location to be networked.

To provide a rough footing, a brief description of each of the cabling media to be referred to within this Networking Guide follows.

Do not run electrically conductive cabling (cable media with metal strands or shielding) between buildings. Exposed conductive cabling is susceptible to lightning strikes.

The Classiﬁcation of Networks 2-7

Overview of Networking

Thick Coaxial Cable

Thick coaxial cable (also known as thick Ethernet cable, “thicknet”, or 10BASE5 cable) is a cable constructed with a single solid core, which carries the network signals, and a series of layers of shielding and insulator material. The shielding of thick coaxial cable consists of four stages. The outermost shield is a braided metal screen. Working inward, the second stage shield is usually a metal foil, but in some brands of coaxial cable may be made up of a second screen. The third stage consists of a second braided shield followed by the fourth stage, a second foil shield. The various shields are separated by non-conductive insulator materials.

Foil Shield

Solid Core

Outer Jacket

Insulator

Braided Shield

1706n02

Figure 2-2. Thick Coaxial Cable Cross-Section

Thick coaxial cable is a media used exclusively in Ethernet installations, commonly as a backbone media. Transceivers (devices designed to TRANSmit and reCIEVE network signals) are connected to the cable at speciﬁed distances from one another, and standard transceiver (AUI) cables connect these transceivers to the network devices.

Due to the extensive shielding, thick coaxial cable is highly resistant to electrical interference by outside sources such as lighting, machinery, etc. Because of the bulkiness and limited ﬂexibility of the cable (typically 0.405 inch in diameter or thicker), thick coaxial cable is primarily used as a backbone media and is placed in cable runways or laid above ceiling tiles to keep it out of the way. The 10BASE5 standard, which standardizes minimum and maximum characteristics for Ethernet networks using thick coaxial cable, speciﬁes a maximum cable length of 500meters (1,640 ft).

Thick coaxial cable is designed to be accessed as a shared media. Multiple transceivers can be attached to the thick coaxial cable at multiple points along the cable itself.

2-8 The Classiﬁcation of Networks

Thin Coaxial Cable

Overview of Networking

Thin coaxial cable (also known as thin Ethernet cable, “thinnet,” “cheapernet,” RG58 A/U, BNC or 10BASE2 cable) is a less shielded, and thus less expensive, type of coaxial cabling. Also used exclusively for Ethernet networks, thin coaxial cable is smaller, lighter, and more ﬂexible than thick coaxial cable. The cable itself resembles (but is not identical to) television coaxial cable.

Thin coaxial cable is made up of a single outer copper shield, which may be braided or foil, a layer beneath that of non-conductive dielectric material, and a stranded center conductor. This shielding makes thin coaxial cable resistant to electromagnetic interference as the shielding of thick coaxial cable does, but does not provide the same extent of protection. Thin coaxial cable can be run to a maximum length of 185 meters (606.7 ft.).

As with thick coaxial cable, thin coaxial cable allows multiple devices to connect to a single cable. Up to 30 transceivers may be connected to a single length of thin coaxial cable, spaced a minimum of 0.5 meter from one another. Connections to the cable are typically made using T-connectors, which provide taps for additional runs of coaxial cable to workstations or network devices. T-connectors, as shown in Figure 2-3, below, provide three RG58 connections, two of which attach to RG58 female connectors on the cable itself and one of which is used for connection to the male RG58 connection of a transceiver or Desktop Network Interface Card on a workstation.

Front

Side

Bottom

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Figure 2-3. Thin Coaxial Cable T-Connector

The Classiﬁcation of Networks 2-9

Overview of Networking

Attachment Unit Interface (AUI)

Attachment Unit Interface cable (referred to hereafter as AUI cable, but which may also be called ofﬁce transceiver cable or standard transceiver cable in other publications) is a shielded, multistranded cable that is used to connect Ethernet network devices to Ethernet transceivers. AUI cable should be used for no other purpose.

AUI cable is made up of four individually shielded pairs of wire surrounded by an overall cable shielding sheath. The gauge of the internal cables determines the thickness and relative ﬂexibility of the AUI cable. Heavy-gauge AUI cable (containing pairs of wire of 20 or 22 AWG wire) is capable of reaching a maximum distance of 50 meters (164 ft.) between transceivers and the network device, but is thick, and somewhat inﬂexible. The lighter-gauge AUI cable (consisting of 28 AWG wire) is thinner and much more ﬂexible, but can only be run to a maximum distance of 16.5 meters (54.1 ft.).

Unshielded Twisted Pair (UTP)

Unshielded Twisted Pair cabling (referred to here as UTP, but also may be termed copper wire, 10BASE-T wire, Category 3, 4, or 5 Ethernet wire, telephone cable, or twisted pair without shielded or unshielded qualiﬁer) is commonly made up of two or, ideally, four pairs of 22, 24 or 26 AWG unshielded copper solid or stranded wires. These pairs of wires are twisted together throughout the length of the cable. The twisting of associated pairs helps to reduce the interference of the other strands of wire throughout the cable. UTP cable used in network installations is the same type of cable used in the installation of telephone lines within buildings.

Tx+ Tx-

RxRx+

1706n04

Figure 2-4. UTP Pair Association

As UTP cabling already exists in many facilities, and as it is inexpensive, available in bulk and easy to install, the use of twisted pair cabling is often a signiﬁcant savings over the use of other media in a networking installation. As UTP cabling is an accepted media in all common network technologies, it is considered a somewhat ‘future-proof’ media. In opposition to a length of thin coaxial cable, which can only be used for Ethernet communications between devices, UTP cabling could initially be used for Ethernet, then be used to support Token Ring and FDDI network equipment as the network grew.

2-10 The Classiﬁcation of Networks

UTP cabling is differentiated by the quality of the cable. UTP is divided into Categories, which indicate the relative quality of the materials used and the processes used to manufacture the cables. The categories used in LANs range from Category 3 to Category 5, with Category 5 being the highest quality.

Shielded Twisted Pair (STP)

Shielded Twisted Pair cabling (referred to in this document as STP, but also seen as “IBM-type” cable or “shielded copper”) is a cable type which is constructed in much the same fashion as UTP cabling (see Figure 2-4) but incorporates more elaborate shielding methods. These cables are most commonly used in Token Ring networks.

As with UTP cable, STP cable consists of two or more pairs of wire. Each wire is shielded with a layer of insulation, twisted together with a related wire, then bundled with the other pairs and wrapped in a metal foil. The metal foil provides additional resistance to the effects of external electrical ﬁelds produced by electrical equipment or other cabling.

STP cabling, like UTP cabling, is divided into groups based on ﬁtness for a particular purpose. Where UTP referred to Categories of cable, STP cabling is divided into “types.” These types are based on the IBM Cabling System, and are often labeled “IBM Type 1 STP.” IBM Type 1 cable is usually the highest quality, incorporating several layers of shielding and made with heavy-gauge wire, while Types 6 and 9 STP are usually best suited for use as short jumper cables in low-interference areas.

Overview of Networking

Fiber Optics

Fiber optic cable is a high performance media constructed of glass or plastic which uses pulses of light as a transmission method. Because ﬁber optics do not use electrical charges to pass data, they are free from the possibility of interference due to proximity to electrical ﬁelds. This, combined with the extremely low rate of signal degradation and dB loss makes ﬁber optics able to traverse extremely long distances. The actual maximums are dependant upon the technology being used, but distances upwards of 2 kilometers (1.2 miles) are not uncommon.

The Classiﬁcation of Networks 2-11

Overview of Networking

Fiber optic cabling is made up of a glass strand, the core, which allows for the easy transmission of light; the cladding, a glass layer around the core which helps keep the light within the core; and a plastic buffer which protects the cable.

Cladding

There are two basic types of ﬁber optics: multimode and single mode. The names come from the types of light used in the transmission process.

Multimode Fiber Optics

Multimode ﬁber optic cabling uses inexpensive Light Emitting Diodes (LEDs) to produce the signals that travel through the core of the cable. Due to the nature of the LED, the signal produced is made up of a number of differing wavelengths of light, ﬁred outward from the center of the LED. Not all the rays of light enter the ﬁber, and those that do often do so at an angle, which reduces the amount of distance the signal can effectively cover.

Single Mode Fiber Optics

Single mode ﬁber optics, driven by the concentrated beams of light which can only be produced by lasers, are constructed in the same fashion as multimode ﬁber optics, but may use a narrower core strand. The use of lasers to drive the signal greatly increases the expense involved in producing single mode ﬁber optic devices as opposed to multimode ﬁber optic devices. The expense involved in single mode ﬁber optic equipment often causes it to be reserved for applications requiring its ability to traverse much greater distances than multimode ﬁber optics.

Transmissive Core

PVC Buffer (Jacketing)

Figure 2-5. Fiber Optic Cable Cross-Section

1706n05

Single mode ﬁber optics and their hardware can transmit and receive signals at distances of 3 kilometers (1.8 miles) or more. As such, it is often best reserved for extremely long-distance LAN communications.

NOTE

2-12 The Classiﬁcation of Networks

While the media is capable of supporting these distances, the technology being used may not be able to function with a link of that distance. Be sure to remain within the maximum distances and limitations imposed on any network technology by the appropriate IEEE or ANSI standard.

Interoperability and Standards Bodies

Interoperability, the Ideal of Networking

Ideally, all devices placed on any network should be able to transfer information in a usable fashion and understandable format to any other station. For some time, however, this was not always the case. Different companies, even within the same industry, have different ways of designing, developing, and constructing their products. Different views of how a network should operate led to radically different products and methods of networking. These early networking implementations were speciﬁc to one particular vendor, and would often only work in homogenous environments, where all components used in the network were produced by that single vendor. This method of networking locked customers into relying on a single vendor for all of their networking needs, current and future, which could lead to problems if the network implementation was unsatisfactory. Ripping out all of your present networking equipment in order to use the proprietary solution of another vendor can become an extremely costly proposition.

Overview of Networking

To combat this, the idea of interoperability grew in popularity. Ideally, interoperability means that the networking devices of Vendor X can communicate, problem-free, with the networking devices of Vendor Y.

Standards and Compliance

Interoperability requires the following of standards, distinct rules and ﬁnite margins within which network operation and performance must be kept. If a network does not meet the minimums, or exceeds the maximums of the networking standard that the industry uses, it is said to be “out of speciﬁcations,” and may not operate at an acceptable level. For example, the Token Ring network standard speciﬁes the maximum number of stations that may be placed on one network, or “ring.” If this number of stations is exceeded, the network will suffer erratic performance and may cease to function correctly. By providing a single deﬁnition for the maximum number of stations per ring, the Token Ring standard allows devices from multiple vendors to operate in the same fashion.

Standards are deﬁned by committee, through the operation of standards institutes. Standards institutes are made up of personnel from several ﬁrms in the industry who volunteer their time and effort. These volunteers work to compose and ratify an acceptable standard, which, when accepted and ratiﬁed, will need to at least be met by any product that refers to itself as “standards-compliant.” Products that are not standards-compliant may cause or experience interoperability problems when operating in a standards-based network. Of course, even in a fully standards-based network, there may still be problems. Most vendors in the industry, realizing the importance of providing a ﬂexible and open network to all customers, seek to eliminate any interoperability problems they notice.

Interoperability and Standards Bodies 2-13

Overview of Networking

The most common Local Area Networking technologies (Ethernet, Token Ring, and FDDI) have standards ratiﬁed and in place for their operation and conﬁguration. ATM, still in the draft stages in some aspects, is operating under a working interim standard, which is intended to allow ATM equipment to be produced which will be compatible with future ATM standards. The standards bodies that this document is concerned with and the standards they oversee are listed below:

Standards Committee Technology Standard Code

Table 2-1. Networking Standards Bodies

Institute of Electrical and Electronic Engineers (IEEE)

American National Standards Institute

ATM Forum Asynchronous Transfer Mode UNI V3.0

Ethernet IEEE 802.3

Token Ring IEEE 802.5

Fiber Distributed Data Interface (FDDI) ANSI X3T9.5

The OSI Model, Basis of Standards

The International Organization for Standardization (ISO) Open Systems Interconnect (OSI) Model provides a framework for the development of system connection standards by deﬁning a consistent hierarchy of rules. The OSI model deﬁnes where the needed tasks of system interconnection are performed but not how they are performed. How tasks are performed on a given layer is determined by the protocols, or rules, written for that particular network based on the OSI model. The layers may be implemented in hardware, software, or both. Each layer in a network based on the OSI model performs speciﬁc types of functions required for proper system interconnection.

2-14 Interoperability and Standards Bodies

Overview of Networking

There are seven layers in the OSI Model (see Figure 2-6). They begin with the Physical Layer and end with the Application Layer. Each layer provides services to the layer above it. As the seventh layer is the ‘topmost’ layer, it servers the user directly, and is considered the top of the OSI model.

Application

7. Presentation

6. Session

5. Transport

4. Network

3. Data Link

2. Physical

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Figure 2-6. OSI Model

Layer Seven: Application

The Application Layer is the user’s interface with the network. This layer directly interacts with user application programs to provide access to the network. All other layers exist to support the requirements of the Application layer. The Application layer is usually involved with network-oriented end-user tasks such as electronic mail, network ﬁle transfers, and collaborative document preparation.

Layer Six: Presentation

The Presentation Layer deals with data translation and code conversion between devices with different data formats (i.e. ASCII to EBCDIC). This layer also handles translation between differing device types and ﬁle formats, as well as data encryption and decryption services. In the transmit mode, the presentation layer passes information from the application layer to the Session layer after it has appropriately modiﬁed or converted the data. In the receive mode, the Presentation layer works in reverse passing information from the Session layer to the Application layer.

Layer Five: Session

The Session layer manages the communications dialogue between two communicating devices. The Session layer establishes rules for initiating and terminating communications between devices and can provide error recovery.

Interoperability and Standards Bodies 2-15

Overview of Networking

Layer Four: Transport

The Transport layer deals with the optimization of data transfer from source to destination by managing network data ﬂow and implementing the quality of service requested by the Session layer. The Transport layer determines the packet size requirements for transmission based on the amount of data to be sent and the maximum packet size allowed by the network architecture. If the data to be sent is larger than the maximum packet size allowed on the network, the Transport layer is responsible for dividing the data into acceptable sizes and sequencing each packet for transmission.

When receiving data from the Network layer, the Transport layer ensures that the data is received in order and checks for duplicate and lost packets. If data is received out of order, the Transport layer correctly orders the data and passes the data up to the Session layer for additional processing.

Layer Three: Network

The Network layer accepts data from the Transport layer and adds the appropriate information to the packet to provide proper network routing and some level of error control. Data is formatted by this layer for the appropriate communications method, such as IP, IPX, or X.25.

Layer Two: Data Link

The Data Link layer is involved with transmission, error detection, and ﬂow control of the data. The major function of the Data Link layer is to act as a shield for the higher layers of the OSI model, controlling the actual processes of transmission and reception. Error detection and control of the Physical layer are the primary functions of this layer, ensuring that data received by the upper layers is error-free. For purposes of understanding networking, it is useful to divide the Data Link layer into two sub-layers; the Logical Link Control layer and the Media Access Control layer (see Figure 2-7).

Application

7. Presentation

6. Session

5. Transport

4. Network

3. Data Link

2. Physical

Figure 2-7. Data Link Layer

Logical Link Control

Media Access Control

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2-16 Interoperability and Standards Bodies

Layer One: Physical

Overview of Networking

Logical Link Control: The Logical Link Control sub-layer is responsible for shielding the upper layers from any particular access method or media. The upper layers need not worry about wether they are connected to a Token Ring or Ethernet network because the Logical Link Control sub-layer handles the interface. The Logical Link Control provides for a common interface of the layers above to any physical network implementation.

Media Access Control: The Media Access Control, or MAC, sub-layer is responsible for severla areas of operation. On the transmit side the MAC layer is responsible for receiving data from the Logical Link Control sub-layer and encapsulating it into a packet ready for transmission. The MAC sub-layer is also responsible for determining if the communications channel is available, and for handling retransmission in the event of a collision on some networks.

At this layer, the transmission of data between devices is deﬁned. That deﬁnition includes cables and connectors, connector pinouts, voltage levels that represent digital logic levels, bit timing, and the actual network device interface.

Interoperability and Standards Bodies 2-17

Overview of Networking

Application of the OSI Model

A user’s perception of network operation appears as direct peer to peer communications. The user message appears to go from the sending application directly to the receiving application. In actuality, the user message is routed from the sending application down through the other OSI Model layers of the system (see Figure 2-8). Each layer adds to or modiﬁes the message according to the network operating system’s protocol for each layer. The message passes through all the layers of the system before appearing on the data channel at the Physical layer, where transmission and reception of signals takes place.

Application

7. Presentation

6. Session

5. Transport

4. Network

3. Data Link

2. Physical

Perceived Path

Actual Path

Application

7. Presentation

6. Session

5. Transport

4. Network

3. Data Link

2. Physical

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Figure 2-8. Transmission through OSI Model

From the data channel the message passes upward through the same layers at the destination device. As the message proceeds from layer to layer, each layer strips off information that was added by its counterpart in the transmitting station. The result is the message as it was originally sent, arriving at the destination station’s Application layer.

2-18 Interoperability and Standards Bodies

Chapter 3

Technology Basics

This chapter presents the three main networking technologies that will be discussed throughout this book. The chapter does not cover these technologies in detail.

This chapter introduces the fundamentals of the technologies to be discussed in this document. The information is intended to provide a level of basic understanding of the general operation, capabilities, strengths, and weaknesses of the three technologies. This is necessary to understand Chapter 4, Network Design, which deals with the selection of technologies, topologies, and organization of the network.

Ethernet

Abstract

As this chapter intends to cover only the information necessary to get a start on the network design process, it speciﬁcally avoids detailed discussions and enumerations of the limits and capacities of Ethernet, Token Ring, and FDDI. For more detailed information, refer to the more extensive discussions of each technology contained within chapters 5, 6, and 7.

Ethernet is a local area networking technology that was initially developed in the 1970s by the Xerox Corporation. It is based on the principles of workstations being responsible for their own transmissions and operation. It is sometimes referred to as 802.3 networking, in reference to the number of the IEEE standards body which subsumes all Ethernet operations.

Ethernet networks provide an operating bandwidth of 10 megabits per second (Mbps). Bandwidth is a networking term which describes the operating speed of a technology. In the case of Ethernet, a perfectly operating, theoretical Ethernet network, can move 10,000,000 bits of data between two stations on the network.

3-1

Technology Basics

Theory

Ethernet, in its basic form, operates like a series of ofﬁces arranged along a central hallway. Each workstation in an Ethernet network can be viewed as an ofﬁce along this giant hallway. When one of these hypothetical ofﬁces needs to send information to another, the worker in the ofﬁce leans out into the corridor to see if anyone else is sending a message, takes a deep breath, and yells the message, which all the other ofﬁces receive. Thus, like sound in a corridor, the Ethernet transmission travels to all the stations on the network. Every person in every room hears the message and determines if it is for them.

1001011001110110011101000111011001110101

1706n09

Operation

Packets

Figure 3-1. The Ethernet Hallway

Because all the Ethernet stations on the network share this central corridor, increases in trafﬁc makes the availability of this corridor less and less likely. This translates to a decrease in the available bandwidth as more stations are added to the network. While a network may be able to move 9 Mbps with 10 stations on it, the same network with 300 stations on it may only be able to move 2 or 3 Mbps.

The message that was shouted out into the corridor represents what is called a packet. A packet is a collection of network data that a station transmits or receives. It is a series of electrical impulses that represents binary information (strings of “1”s and “0”s). The Ethernet packet, in the most elementary terms, contains the following:

• A destination address - Where the packet is headed to

• A source address - Where the packet originated from

• A data ﬁeld - The content of the message

3-2 Ethernet

Procedures

Technology Basics

All the information necessary for a station to receive and comprehend the network transmission is contained in the packet. The Ethernet packet contains other ﬁelds related to Ethernet operation which are not essential to a basic understanding of the technology.

Ethernet stations follow four basic procedures when dealing with transmission and reception. The treatment of these procedures given below is a highly simpliﬁed view of network packet operations. The four procedures are preparation, transmission, reception, and examination. Preparation and transmission are both performed by the sending station, while reception and examination are performed by all receiving stations.

Preparation: When the workstation or device has information that it needs to send out to the network, it prepares that information for transmission by generating a packet. The packet, as described above, consists of some network control information, the station or stations to which the packet must go, the address of the sending station, and the actual data of the transmission.

Transmission: Once the packet is ready to be transmitted, the Ethernet device checks its connection to the network. This monitoring determines if the network is currently being used. If the network is in use, transmitting would cause both the signals to become garbled. This is called a collision. The monitoring of the media helps to avoid collisions.

If the media is not in use, the station will add some futrther control information to the packet, effectively encapsulating it in what is called an Ethernet frame. Once the frame has been generated, it will be transmitted. It then returns to monitoring the network to see if the frame it just sent is involved in a collision. Even though the media was monitored before the frame was sent, collisions can, and do, still occur in Ethernet networks.

Reception: The frame that has just been sent spreads along the Ethernet network. As the signal travels throughout the cables and devices, all the stations on the network read in the entire frame.

Examination: Once a station has received the frame in its entirety, the portion of the frame that contains the destination address is examined by the receiving station. If the destination address of the frame matches the address of the receiving station, the frame is processed. The station removes the frame information from the transmission, stripping it back down to a packet. The packet is passed up to the higher layers of the OSI model and the information in the data portion of the packet is acted upon. If the destination address of the received frame does not match the address of the receiving station, the frame is discarded.

More complete treatments of the organization, content, and processing of frames may be found in documents dedicated exclusively to Ethernet operations. A more complete depiction of the operation of the Ethernet network technology may be found in Chapter 5, Ethernet.

Ethernet 3-3

Technology Basics

Segmentation

The operation of Ethernet networks is based on the fact that every workstation or device on the network is connected to every other device on the network. Due to this treatment, the larger the Ethernet network gets, the more congested it becomes with added trafﬁc. The added trafﬁc also increases the probability of collisions occurring. In time, the Ethernet network will grow to encompass so many stations that it is excessively difﬁcult for a station to receive an opportunity to transmit frames. The only way to reduce this congestion is by breaking the one large network up into smaller networks through segmentation.

It is the function of segmentation to separate networks while allowing information to ﬂow between them when necessary. An Ethernet bridge acts in much the same way as a bridge in the physical world. It provides a single point of access between both networks, allowing trafﬁc which must cross to the other network to do so, while keeping unrelated network activity on one side of the bridge from impacting the operation of the network on the other side of the bridge.

Segmentation devices available for Ethernet include bridges, switches, and routers. While all of these devices perform the same basic function they carry out the task in different ways and with different results.

Bridges

Bridges operate by reading in frames from one interface to determine if they are intended for the network they are read from. If the bridge determines that a frame being examined is, in fact, intended for a station on the network it was received from, it will discard the frame. After all, the destination station is on the same side as the transmitting station, and has already received the frame. If, however, the bridge determines that the destination of the frame is a station which is not on that network, the bridge will repeat that frame to one of its other interfaces.

In addition, Ethernet bridges can connect an Ethernet network to another networking technology, provided that the networks are similar. While the deﬁnition of “similar” in respect to bridges is a complex one, we can simplify it for design purposes: Ethernet, Token Ring, and FDDI are all similar network technologies from a bridge’s point of view.

This bridging functionality is the basis of creating backbones that use a different technology than workgroup networks. The most common backbone strategy is to interconnect several Ethernet workgroups, which operate at a maximum of 10 Mbps, to each other with an FDDI backbone, which operates at 100 Mbps. An FDDI Management Media Interface Module, or FDMMIM, conﬁgured in an Ethernet chassis would provide the bridging functions necessary to allow the chassis to connect to such a backbone.

3-4 Ethernet

Switches

Routers

Technology Basics

Switches, in an Ethernet environment, act like bridges. A switch connects one network to another. A switch, however, provides a dedicated connection at full Ethernet speeds between devices. The important thing to realize about Ethernet switches is that they may be used in the place of a bridge for the interconnection of two or more Ethernet networks.

Since most Ethernet switches provide more than two ports for connecting Ethernet networks, they are often used as outlined in Chapter 5 to provide connections between high-bandwidth demand workstations.

A router is a network device that performs the same overall functions as a bridge or a switch, the connection of one network to another. The means by which a router connects networks together are more complex, however. While bridges and switches can connect similar networks to each other, routers can connect dissimilar networks.

Routers are often used in situations where the functions of a bridge would not be robust enough for the required operations of the network. Routers, by examining packets of information on the network, can determine not only if those packets are intended for another network, but what is the shortest path for them to take to get to their destination. This is accomplished by the router monitoring the operation of the network and examining each received packet of data in more detail than a bridge.

The practical upshot of this is that while a bridge is able to operate faster for not having to perform the same in-depth analysis of every packet that a router must do, the bridge may not be able to provide the level of efﬁciency in the utilization of the network that a router is able to.

Ethernet 3-5

Technology Basics

Strengths and Weaknesses

Ethernet

Performance: At 10 Mbps, Ethernet networks are not the fastest category in the list of LAN technologies, but they are perfectly capable of handling most types of ofﬁce and technical trafﬁc.

Reliability: While Ethernet networks are reasonably reliable, they are not always entirely predictable. Due to the manner in which Ethernet networks handle congestion and collisions, the network tends to become less available as more users attempt to transmit frames at the same time. The greater the number of users, the greater the chance of collisions occurring, and the greater the amount of waiting for the network to be free for transmission. This translates directly into delays in network access.

Additionally, the operation and design strategy of the single Ethernet network can be a reliability problem. Ethernet communications go automatically to all points on the network. This means that, unless some hardware controls are part of the network, that an error produced by one station will affect every other station on the network.

Ease of Conﬁguration: Simple Ethernet networks are very easy to design and set up. There are a number of well-deﬁned rules set up regarding the maximum lengths of certain cables and the number of stations that may be serviced by a single repeater, but these are hard and fast rules which are relatively easy to check before implementing a network installation.

Troubleshooting: The design of a simple Ethernet network is based on the “many cables - one LAN” philosophy. Every Ethernet station is connected to every other Ethernet station through the shared medium of the many cables and repeaters. As every station is connected to every other station, locating and eliminating network problems can be difﬁcult, as the search for problem segments, cables, or devices must begin with the network as a whole. One of the common means of troubleshooting early Ethernet networks was the “binary debug,” where half the network would be unplugged or otherwise disabled. If the problem continued, it was on the half still operating. The remaining portion would then be ‘cut’ in half. By following this down to its logical conclusion, the segment or segments with the error condition could be located.

Cost: Ethernet networks are simple, straightforward, and ubiquitous. The presence of many vendors of Ethernet equipment keeps the prices of components low in comparison to those required for other networking technologies. Also, simple Ethernet networks do not require a large investment in networking hardware to set up. This combination of minimal required hardware and the availability of that hardware at a low cost makes the simple Ethernet the least expensive to implement in terms of initial startup expenses. Whether the low cost of a simple Ethernet is cost-effective is a matter to be determined by the expected demands that will be placed on the network by its users.

3-6 Ethernet

Segmented Ethernet

Technology Basics

Performance: Segmented Ethernet provides the same 10 Mbps that simple Ethernet networks do, but allows that bandwidth to be more effectively and efﬁciently utilized. A bridge connects two separate Ethernet networks. By separating the Ethernet stations from one another based on a workgroup criteria (as outlined in Chapter 4, under Workgroup Creation), the segmented Ethernet design attempts to provide more specialized networks. By making the networks more specialized, the segmented Ethernet network design allows local network trafﬁc to circulate without impacting the operation of trafﬁc on other network segments.

Reliability: By separating the Ethernet network into a number of related segments, it becomes easier to locate and correct network faults. Since each network segment is a network in and of itself, a network suffering from an error condition will not impact the operation of other Ethernet networks to which it is connected.

Ease of Conﬁguration: Segmented Ethernet networks are relatively easy to design and conﬁgure. The segmentation of the workgroups for the initial conﬁguration must be carefully thought out and examined in order to achieve an optimized efﬁciency. When planning any segmented network, keep in mind that the more thought that is put into the segmentation of the network, the less difﬁcult accommodating current and future needs will be.

Troubleshooting: A segmented Ethernet makes troubleshooting, on the whole, less difﬁcult and network-crippling than a simple Ethernet. Although the troubleshooting process must include the bridges being used to segment the network, which are more complicated than repeaters, the division of the network into segments allows one segment to be examined without being affected by trafﬁc on other segments, and without the troubleshooting process affecting error-free segments.

Cost: The addition of bridging to an Ethernet network introduces a number of hardware and cabling costs. Since the network still utilizes the relatively common Ethernet hardware and cabling, the costs of the devices are quite low in comparison to most other technologies. The technology requires a number of Ethernet networks be set up, which requires the design and conﬁguration of a number of simple Ethernets. The additional expense comes from purchasing the bridge or bridges which will perform the interconnection of these network segments and from the duplication of some cabling in order to set up inter-bridge connections.

Ethernet 3-7

Technology Basics

Switched Ethernet

Performance: Switched Ethernet provides the same 10 Mbps that simple Ethernet networks do, but allows that bandwidth to be divided in much the same way as segmented Ethernet (see above). Switches are used to provide dedicated connections between Ethernet stations and other Ethernet devices. As the switched connection does not utilize a shared media, the chance of the network being available for use by the station or device connected to it increases appreciably.

Reliability: While the inclusion of an Ethernet switch into the network adds much the same complexity to a network as the addition of a bridge, the overall reliability of a switched network is greater than that of a simple Ethernet. The reason for that is the isolation of faulty segments from the rest of the network. A switch naturally ﬁlters Ethernet signals which are not intended for other Ethernet segments. By including error-checking capabilities into the switching hardware (which Cabletron Systems switching devices do), network error conditions and faults can be blocked at the switch before they go on to affect the other Ethernet segments in the network. In this way, a segment experiencing trouble will not bring the other segments of the network down. Since each link is a dedicated connection between two devices, a loss of link should not jeopardize the network.

Ease of Conﬁguration: The design process for a switched Ethernet network is exactly the same as that for a segmented Ethernet network (see above).

Troubleshooting: Troubleshooting a switched Ethernet network is similar to the troubleshooting of a segmented Ethernet (see above). One advantage to the switched Ethernet network design from a troubleshooting point of view is that Ethernet switches typically provide for the interconnection of more ports from one device than an Ethernet bridge. This means that the network, as a whole, may be further segmented or that a port or two on the Ethernet switch can remain unused as a back-up in the event of a failure of another switch port.

Cost: The expenses inherent in a switched Ethernet network are similar to those of a segmented Ethernet. The Ethernet switch is relatively expensive, but works with standard, low-cost Ethernet hardware. The more extensive the use of switching in the network, the greater the costs will be, due to the requirement of multiple switches. For example, a network which uses an Ethernet switch to make connections between a number of single Ethernet hubs will be signiﬁcantly less expensive than the same network which uses switches for all connections between every network device.

Special Design Considerations

• The more stations you have on a single network, the greater the probability of collisions. Collisions introduce delay and reduce available bandwidth.

• As an Ethernet network grows it remains one segment until that segment is broken up by a bridge or other segmentation device.

• Any time frames cross a bridge, switch, or router, delay is introduced.

3-8 Ethernet

Token Ring

Abstract

Technology Basics

Token Ring is a networking technology developed in the early 1970s by researchers in Sweden and the United States. The technology was embraced by IBM, and was standardized in 1985 by the IEEE 802.5 group. The Token Ring standard is often referred to as the IEEE 802.5 standard.

Token Ring LANs provide an operating bandwidth of either 4 Mbps or 16 Mbps. The throughput which the network will provide is determined when the network is designed.

Token Ring is very popular with companies or facilities requiring fault tolerance and guaranteed throughput under heavy network loads. Customers such as Insurance Agencies, Banks, or Sales Organizations, which have regular high-usage periods where delay is detrimental to proﬁt, or mission-critical networks for facilities such as hospitals, often ﬁnd Token Ring to be more beneﬁcial than Ethernet, while being less costly to implement than the much faster Fiber Distributed Data Interface (FDDI) technology.

Theory

Operation

Rings

Token Ring network operation is based on the principle that the operation of the entire network determines when a station may transmit and when it will receive. Stations monitor one another, and one station acts as a ring monitor, keeping track of important statistics. Token Ring stations are connected to one another in a predetermined order, and network frames pass from one station to the next, following that order. A speciﬁc, specialized network frame (a collection of data), called a token, is passed around the ring at regular intervals. The transmission of the token helps establish some of the operational statistics for the network, and receiving it allows a station to transmit.

Every station that connects to a Token Ring network is a member of a ring. The ring is made up of a series of stations which are connected to one another through a Token Ring device, and the cabling, or media which connects them together. The ring in Token Ring networks is a single network segment.

Token Ring 3-9

Technology Basics

No station may transmit normal data unless it has received the token from the station before it and ‘claimed’ it by not transmitting the token to the next station. It then transmits a data frame which is passed from station to station, with each station receiving the frame, passing it on, then examining the frame to determine if it is intended for them. If it is, the frame is passed up into the station to be processed. If the frame is for another station, the Token Ring device discards the frame and returns to a waiting state.

Direction of data travel (rotation)

1706n10

Figure 3-2. Token Ring Data Transfer

Once the data frame has been passed around the entire ring, it will return to the station that sent it. This station, which still holds the token, will examine the frame and see that it was transmitted from its own location. Seeing this, the station will strip the frame from the network by not transmitting it to the next station in the ring. It then releases the token, allowing another station to claim it.

The orderly fashion in which Token Ring networks operate allows them to incorporate built-in fault location and recovery features. These features help the network, through its normal operation, to detect problems with stations or cabling on the ring, and to either correct or bypass them. The fault recovery process occurs without human intervention, and faulty networks can eliminate problems without the users ever experiencing a signiﬁcant loss of network access.

Token Ring is a demanding networking technology which requires strict adherence to its standards. The operation of the ring introduces a number of limitations based on time and signal strength, as exceeding the limitations or falling below required levels can cause the network to malfunction, activating the fault isolation features. The IEEE 802.5 standard for Token Ring networks places a number of limitations on things such as the total number of stations on a ring, the media which may be used to connect them together, and the maximum lengths of those cables.

3-10 Token Ring

Frames

Technology Basics

Token Ring limitations are very restrictive, and all the restrictions are interrelated. The speed of the network (4 or 16 Mbps), the amount of signal regeneration performed by the hardware, and the type of media to be used (UTP, STP, or ﬁber optics) all act together to determine the total number of stations that a single ring may support and the maximum lengths of the cabling used to connect those stations.

The restrictions of the Token Ring standard are covered in detail in Chapter 6, Token Ring.

A Token Ring frame contains much the same information as an Ethernet frame: a source address, a destination address, and a data ﬁeld which contains the packet that was passed down from the upper layers of the OSI model. The Token Ring frame also contains a Routing Information Field (discussed later in this section) and a number of other control and operation related ﬁelds which have little bearing on the network design process.

Segmentation

Bridges

The most common segmentation device in Token Ring networks is the bridge. Token Ring bridges perform exactly the same function as Ethernet bridges, but the actual mechanics of segmentation work differently. Where Ethernet bridges determine to forward frames based on the correlation between the destination address of a frame and whether that destination address has been seen on the network the frame originated from, Token Ring bridges allow the frame itself to tell them where they should be forwarded.

This bridging method is called Source Route Bridging (SR Bridging). It is based on Token Ring stations sending out a frame that is intended to be received by all stations on the entire network. The frame is copied and transmitted by all the bridges on the network, and information is added to the frame each time it passes over a bridge. The bridge adds its own identiﬁcation to the frame and sends it on.

Once the frame has reached its destination, the receiving station reads the frame and examines the list of bridges that the frame passed through during its transmission. It then builds a routing database that contains the order and identiﬁcations of those bridges. The receiving station then formulates a frame which contains information for the bridges in the Routing Information Field, telling each bridge where to send the frame.

Token Ring 3-11

Technology Basics

This new frame passes through the network to reach the station which originally formulated the request, and it builds its own routing database by reversing the order of the bridge identiﬁcations in the Routing Information Field of the frame. From this point on, the two stations can transmit frames which tell the bridges in the network what route they need to take.

Because this routing information takes up space in a frame and in the routing database of each end station, there is a ﬁnite number of bridges whose identiﬁcations can be included in the Routing Information Field of a Token Ring frame. This translates directly into a limitation on the number of bridges that may be designed into a Token Ring network. An IEEE 802.5 compliant Token Ring network may contain no more than 14 rings, connected by no more than 13 bridges.

Routers

NOTE

contain no more than eight rings and no more than seven bridges in its longest path.

A Token Ring router performs the same function as an Ethernet router. It provides for the interconnection of two or more Token Rings, and transmits frames between them based on more detailed information than is required by bridges. This more in-depth information allows routers to be more discerning and precise with regard to the exchange of frames, but forces them to operate slightly slower due to the time involved in the examination.

The use of a router or series of routers is the only way to “reset” the 13 bridge rule described in the Bridges section of this chapter (above).

In order for a network to be IBM Token Ring compatible, it must

3-12 Token Ring

Strengths and Weaknesses

Token Ring

Performance: Token Ring networks of all kinds are available in two speeds, 4 Mbps and 16 Mbps. The speed selected determines the rapidity of information exchange on the network, and also the number of stations which may be present on the ring (see Chapter 6, under the heading Token Ring Network Rules). Selection of the ring speed will usually tend towards the higher speed, as that provides for faster communications between Token Ring stations. The 16 Mbps speed is half-again faster than the 10 Mbps theoretical maximum of an Ethernet network, and the performance of a Token Ring network will degrade at a controlled and predictable rate as the maximum number of stations on the ring is neared. This provides for a networking technology that is capable of handling a number of stations with high bandwidth requirements on an even and equal basis.

Reliability: Token Ring networks incorporate automatic error isolation and recovery features, which allow a Token Ring network to ﬁnd and eliminate many types of common network problems without requiring human intervention. These capabilities are detailed in Chapter 6, Token Ring. The automatic fault recovery features greatly increase the reliability of a properly conﬁgured Token Ring network.

Technology Basics

Ease of Conﬁguration: Token Ring networks require extensive planning and design before they are conﬁgured. A large number of rules surround the creation of Token Ring networks, and these rules change depending upon the operating speed of the network, the selection of active or passive circuitry, and the selection of media for the network. Attempting to install an improperly-designed Token Ring network can be a very frustrating exercise. Due to the complexity of the network technology, it is not recommended that a Token Ring network be implemented unless at least one person at the facility, preferably the Network Manager, has a reasonably good grasp of the concepts and practices of Token Ring networks.

Troubleshooting: When the automated fault isolation and recovery procedures of the Token Ring network cannot deal with a network problem, the Token Ring network can be difﬁcult to troubleshoot. A fatal error condition on the ring will almost always bring down the entire ring, making a single Token Ring network unusable until the fault is eliminated. Once again, a ﬁrm grasp of the operation of Token Ring networks will be of great assistance to a Network Manager facing a Token Ring error situation.

Token Ring 3-13

Technology Basics

Cost: Token Ring equipment is inherently more specialized, and often more complex, than Ethernet equipment with similar capabilities. This, in conjunction with the relatively smaller number of Token Ring networking hardware suppliers, tends to make the cost of implementing a Token Ring network more expensive than the implementation of an Ethernet network with a similar number of users. As with the complexity of conﬁguration, the cost of Token Ring networking devices is partially dependent upon the choice of media and the selection of active or passive circuitry.

Segmented T oken Ring

Performance: Combining the transmission speed and consistent availability of the Token Ring technology with the segmentation and isolation afforded by bridging creates a very capable network design. The segmentation of Token Rings into workgroups provides for greater network availability and for the conﬁguration of a greater number of workstations and other nodes in the network as a whole.

Reliability: The automatic fault detection and isolation features of the Token Ring technology add to the reliability of the multiple bridged Token Ring network. The incorporation of bridging adds an additional layer of complexity which may be difﬁcult to manage, but the operation of the bridges keeps network problems on one Token Ring from affecting other Token Rings in the network.

Ease of Conﬁguration: The segmented Token Ring is the most complex network technology treated in this Networking Guide. The inherent complexity of design and conﬁguration of a Token Ring network is added to the requirements of segmentation and bridging decisions. For those who fully understand the Token Ring technology, or have some experience designing single Token Ring networks, the move to a segmented Token Ring network is a simple one. For those with little or no experience in Token Ring networking, the design of a segmented Token Ring network is a somewhat intimidating, but by no means impossible, task. As with all bridged technologies, the use of careful forethought, patience, and planning will serve to greatly reduce the difﬁculty of designing a segmented Token Ring network.

Troubleshooting: The use of multiple Token Ring networks provides for superior streamlining of the troubleshooting process. The inherent fault isolation and recovery capabilities of the Token Ring technology, combined with the segment isolation provided by the use of bridging, allows for the quick and automated location and elimination of most common error conditions, and the continued operation of other Token Rings in the event of a fatal error condition on one ring.

Cost: Segmented Token Ring conﬁgurations require that a series of individual Token Ring networks be set up, then connected with Token Ring bridges. These bridges introduce the additional costs associated with the segmented Token Ring conﬁguration. As these bridges are reasonably complex devices, the added cost is not insigniﬁcant. Also, whenever a new ring is created, it is necessary to supply a Token Ring management module to support and initialize the ring.

3-14 Token Ring

Special Design Considerations

• The orderly progression of transmission and reception throughout the Token

Ring allows special fault identiﬁcations and automatic correction features to be built into the technology.

• There are limitations to the number of stations on a ring and the distances to

which cabling can be run. These limitations change according to the type of cable used, the speed of the network, and a number of other factors.

Fiber Distributed Data Interface

Abstract

Fiber Distributed Data Interface, or FDDI, is a high-speed networking technology which provides a maximum available throughput of 100 Mbps. Due to its high speed and fault-tolerance, it is often used to connect slower workgroup LANs to one another.

Technology Basics

Theory

The ﬁrst portions of an FDDI standard were ratiﬁed in 1988 by the American National Standards Institute, or ANSI. The FDDI Working Group, X3T9.5, for whom the FDDI standard is named, have continued to release speciﬁcations and updates to make the FDDI technology more accessible and capable for network operations. Recently, additional speciﬁcations have been ratiﬁed and published which detail the use of transmission media other than ﬁber optics for FDDI networking. For more information on the standard media which may be used in an FDDI network, see Chapters 7 and 10.

FDDI LANs operate under the same basic principles as Token Ring networks. The operating network provides regular opportunities for stations to transmit information, and each station can expect to receive network data at regular intervals. A specialized frame of information, also called a token, is passed from one station to another, traversing the ring in a pre-set amount of time. No station may transmit unless it has received and “claimed” the token.

Fiber Distributed Data Interface 3-15

Technology Basics

Operation

Rings

FDDI operation is based on the movement of data around a series of rings. Like the organization of the Token Ring technology discussed earlier, FDDI data passes from one station to another in a predetermined order. In FDDI LANs, there are two main types of rings: dual counterrotating rings and single rings.

The dual counterrotating ring, hereafter referred to as the dual ring, is the main ring of an FDDI network. It incorporates the most powerful fault-tolerance features of the FDDI technology. The dual ring consists of two rings, one designed to pass data in one direction, one designed to pass it in the opposite direction. If there is a fault in the dual ring, the network will cause transmissions to wrap from the primary ring to the secondary ring and travel back along the network, avoiding the fault and keeping the ring whole.

The dual ring can be connected to a series of single rings, which extend the FDDI network to many stations. A connection to a single FDDI ring does not provide the same level of fault-tolerance as a connection to the dual ring, but typically costs less. Since a series of single rings can be connected to each other through the dual ring, single ring connections are often used to make connections to workstations and end users, while the dual ring handles connections between hubs.

Devices

As there are two types of rings in FDDI networks, there are also two methods that FDDI devices may use to connect to the network. FDDI device may be dual-attached, connecting directly to the dual ring, or single-attached, connecting to a single ring. Dual-attached devices require two FDDI ports, one attaching to the primary ring and one attaching to the secondary ring. Dual attached devices are more expensive than single attached devices, but incorporate greater fault tolerance.

FDDI devices, whether single attached or dual attached, fall into one of two categories. Any FDDI device which is an endpoint on the network, such as a workstation or a ﬁleserver, is referred to as a station. Stations may be dual-attached or single-attached, depending on the connection method they use. If an FDDI workstation is designed to be connected to the dual counterrotating rings, it is called a “Dual Attached Station,” abbreviated DAS.

FDDI devices which provide an access point for other FDDI devices are called concentrators. The typical concentrator connects a series of Single Attached Stations (SASs) to the FDDI ring. If the concentrator is attached directly to the dual ring, it is referred to as a Dual Attached Concentrator (DAC).

3-16 Fiber Distributed Data Interface

Frames

The FDDI technology collects data into “frames” for transmission. A frame is a speciﬁc format for data and control information. There are two basic types of frames: network control frames and data frames. Network control frames are made up of instructions that are intended for the devices on the FDDI network, informing the stations of changes to the ring or problems with the network. An example of a network control frame is the token that is passed around the FDDI ring.

Data frames hold the actual information of the transmission. They are made up of the basic components of any collection of formatted network data: a source address, a destination address, and a data ﬁeld, which contains a packet which was passed down from the Network Layer of the OSI model. A number of additional bits of control information are included in the frame for use by the FDDI stations receiving and transmitting the frame.

Strengths and Weaknesses

Technology Basics

FDDI

Performance: FDDI networks operate at 100 Mbps, ten times faster than the theoretical maximum of an Ethernet network. While this level of performance is exceptional and can greatly reduce network access time required to perform operations, not all workstations are capable of using all 100 Mbps provided by the network. In some cases, the applications or workstations on your network may not be robust enough to get the full use out of FDDI. A careful examination of the amount of networking trafﬁc you expect from the workstations can help you decide if FDDI will be useful for your network.

Reliability: The reliability of FDDI is one of the key features to consider when examining the technology. With automatic fault isolation and recovery features similar to those used in Token Ring networks, but faster and more streamlined, the technology is one of the most reliable and consistently available networking technologies today. Add to this the ability to create automatic failover paths for important devices or connections with a minimum of effort, and FDDI takes the lead for reliability.

Ease of Conﬁguration: FDDI design and conﬁguration is quite similar to the design and conﬁguration of Token Ring networks, but with a reduction in overall complexity. If the connection rules are followed and none of the generous maximums placed on the number of stations, media used, or distance allowable are exceeded, conﬁguring an FDDI network is a simple matter of providing sufﬁcient port count and determining which devices will populate the dual ring and which will be attached through concentrators.

Fiber Distributed Data Interface 3-17

Technology Basics

Troubleshooting: In the event that an FDDI network undergoes a failure, the network will literally bend over backwards in an effort to keep the network operating. The exceptionally fast reaction time on station and ring wrapping heals the FDDI network almost instantly. Multiple faults in the network will eventually be isolated by the normal operation of the technology. This may sometimes lead to some unusual network conditions, but the network will try to remain operational at any cost. Once trouble segments have been located, the relative scarcity of FDDI troubleshooting devices may increase the time needed to diagnose and deal with a problem.

Cost: FDDI is expensive. In part due to the complexity and sophistication of the devices used and in part due to the relatively small installed base of FDDI networks, FDDI networking hardware remains among the most expensive in the networking industry. In conjunction with the expense involved in installing and purchasing ﬁber optic cabling, the default media for FDDI connections, FDDI is easily the most expensive standardized technology addressed in this Networking Guide. This added expense often relegates the FDDI technology to the corporate or facility backbone network, supplying interconneciton for slower workgroup LANs.

Special Design Considerations

• FDDI operates like Token Ring, but faster.

• The technology, designed to use the very noise-resistant ﬁber optic media, supports very long distances and a large number of stations.

• There are generous maximum limitations to the number of stations on a ring and the maximum length of an FDDI cable.

• FDDI is expensive when compared to other networking technologies, and is often used initially as a backbone technology.

3-18 Fiber Distributed Data Interface

Chapter 4

Network Design

This chapter deals with the process of visualizing and planning the basic form and operation of a network.

The network design process is the formation of the network, from initial concept to the plan of implementation. In this Networking Guide, for the sake of brevity, the process of network design is separated from the process of network conﬁguration. Network design is presented and treated as the decisions leading up to the selection of hardware, and network conﬁguration is the process of putting hardware together to create a functioning network.

NOTES

As this Networking Guide is concerned with the decisions made regarding hardware and facility cabling and not with the administration of networks or the speciﬁc uses to which they are put, several aspects of the overall process of network design are not treated in this document, such as the selection of a Network Operating System (NOS), the choice of applications or of workstation types, or other speciﬁc decisions generally out of the purview of Cabletron Systems as a provider of networking hardware.

When designing a network installation or conﬁguration, draw the network. At the very least, make a rough sketch of each

aspect of the network design process. Seeing the v arious parts of your design will help you identify strengths and weaknesses and make it easier for y ou to achieve a grasp of the network as a whole.

Similarly, draw out the network conﬁguration once you begin selecting hardware. While it is not necessary to represent every port, labeling modules and showing the connections made between them can point out potential problems before they are cemented into the conﬁguration.

4-1

Network Design

Workgroup Creation

What Is a Workgroup?

A workgroup is a group of network end stations that are related in some way. The conditions of this relationship are determined by the Network Manager, and can be based on anything from device type to user occupation or even device color. As the workgroups are the operating portion of the network, where information is created and given direction, the workgroup is the portion of the network which creates trafﬁc and network congestion. As such, it is the most complicated portion of the network to design.

A logical, well-thought-out workgroup plan and a skillful execution of the creation of workgroups according to a ﬁrm set of criteria goes a long way toward ensuring that the network which results will be functional, ﬂexible, reliable, and sufﬁciently robust to handle the demands placed on it by users.

The idea of the workgroup in the network roughly translates to the use of segmentation. Ideally, segmentation should be planned between separate workgroups or between collections of related workgroups, not within workgroups. The workgroup concept divides the network according to a cohesive plan in the interests of reliability, efﬁciency, or ease of recovery. While all of these are important factors in the operation of the network, certain choices made in the design of networks, from technology and topology to the organization of stations and the segmentations method used if any, will improve some aspects of the network at a cost to others. Striking the proper balance of these factors is the purview of Network Managers, who must investigate and determine the needs and preferences of the proposed network’s users.

Workgroup Establishment Criteria

This section examines some of the methods that may be used to divide the population mass of end users of a network into cohesive and deﬁned workgroups.

Geographical Proximity

Organizing workgroups by geographical proximity creates workgroups made up of discrete sections of a facility. While in many cases the physical locations of departments may correspond exactly to a facility layout, the geographical proximity criteria of workgroup organization does not take function into account. As the deciding criteria for this type of workgroup organization is location only, geographical proximity is often the least efﬁcient workgroup creation method in terms of performance, reliability, and troubleshooting.

4-2 Workgroup Creation

Workgroups

Network Design

: Service Workstations : Sales Workstations : Research Workstations : Receiving Workstations

Figure 4-1. Geographical Proximity Workgroups

Having well deﬁned rules of geographical proximity as the deciding factor in workgroup design does, however, make the physical act of fault recovery easier in many networks. If an entire location is suffering errors or loss of network operation, there is a deﬁned physical location to begin examining network devices for faults.

Corporate Organization

Corporations, companies, and agencies all separate employees by primary function. No one person “does it all,” and most employees are specialists in the sense that they perform one function or a series of functions that are assigned to them by their job descriptions. These functions dictate what types of information and network utilization they require: manufacturing personnel deal primarily with manufacturing information; accounting personnel deal primarily with sales, proﬁt, and expenditure information; and research personnel primarily perform design and testing operations.

Workgroup Creation 4-3

Network Design

Since most of the time business departments are involved with sharing information among other members of their department or a group of related departments (Accounting, Personnel, and Payroll, for example), the division of the end user population into workgroups based on corporate function and separated by bridges, switches, or routers tends to improve network performance by keeping information passed within each department from impacting the ﬂow of information within other departments. This provides natural divisions within the network for the use of bridging or routing, keeping local trafﬁc from congesting the network where it is utilized by other departments.

: Sales Workstations : Research Workstations : Receiving Workstations

: W orkgroup A : W orkgroup B : W orkgroup C

Figure 4-2. Corporate Organization Workgroups

As the creation of workgroups based on departmental organization mirrors the operation of the company, the expandability of the network is simpliﬁed; since departmental growth can often be predicted in stable or growing companies, the network can be designed to allow for simpliﬁed expansion in the departments most likely to grow.

Without the use of management software to monitor the operation of workgroups determined by corporate organization, troubleshooting and fault recovery can be difﬁcult in a network of this kind. As the end users are not necessarily located in the same area, faults which affect the workgroup must be looked for in several locations.

4-4 Workgroup Creation

Common Function

Network Design

An even trade-off is made in reliability in networks organized in this fashion. While the organization of the network into departmental workgroups increases the inherent complexity of the network by creating several segments based on function, the loss of a workgroup will disrupt the operation of only that workgroup, allowing the operation of other workgroups to continue with no disruption other than the loss of communication with the faulty workgroup/department.

Segmentation by common function is often used to provide further division of the network within larger overall departments, or to facilitate the use of certain network applications by speciﬁc end users common throughout much of the department. An example of this might be the creation of a Documentation workgroup in a corporation within which each department had a dedicated Documentation person handling recording and reporting. This would create workgroups of the members of each department (R&D, Sales, Receiving, etc.) and one workgroup which encompassed only the Documentation personnel of each department, who, although working in different departments, all require access to the same functions through the network.

Workgroup Creation 4-5

: Sales Workstations : Research Workstations : Receiving Workstations

: W orkgroup A

: Documentation Personnel

: W orkgroup B : W orkgroup C : W orkgroup D

Figure 4-3. Common Function Workgroups

Network Design

The creation of workgroups based on common function enhances the performance of those dedicated functions at a cost to the performance of the network as a whole. In addition, the management demands placed on a network by common function networks distributed across an entire facility or corporation are much the same as those of a corporate organization workgroup scheme, but even more intense.

Priority Organization

Priority organization is a ﬂexible term which refers to the Network manager assigning devices to workgroups based on speciﬁc priorities. As such, it is the most ﬂexible scheme for creating workgroups, for it is based solely on the relative importance of certain network characteristics to individual end users and equipment. Priority organization can be used to create high-speed, high-reliability, or rapidly recovering workgroups to those stations which require those characteristics. Unfortunately, it combines some of the worst features of the other methods of arranging workgroups as the cost of this level of control.

An example of priority organization is the common practice of connecting all the ﬁle servers for a particular facility to a high-speed network access device in a single location, regardless of the location of the workgroups which need to access them. This practice is known as “server farming,” and is used, in many cases, to keep network users from attempting to repair, reconﬁgure, or use the servers in imaginative, and often hazardous, ways.

: Sales Workstations : Research Workstations : Receiving Workstations

f/s

: File Servers

Figure 4-4. Priority Organization Workgroups

f/s f/s

f/s

: Priority Workgroup : Standard Workgroup

4-6 Workgroup Creation

Network Design

Priority organization of this manner in a single-segment network involves providing stations in the priority workgroups with qualities of media and network connection based on that priority. The stations in the server farm, to continue with that example, might have redundant connections to the network in the event that one cable failed, or might utilize a media that is resistant to interference, such as ﬁber optic cabling, or might be best served by a centralized location. A priority organization workgroup or sub-section of a workgroup which is located on the same network segment as its most common users is usually an efﬁcient and safe use of resources, and will not impede the operation of the network.

While keeping the users separate from the devices they need to access on a regular basis does enhance the Network Manager’s control over its use and operation, it does reduce network performance in networks utilizing segmentation. The priority organization method, by connecting stations to the network based on their relative importance, makes little or no accommodation for the localization of network trafﬁc which is the purpose of segmentation. If a ﬁle server is located in the server farm workgroup, which is segmented from the rest of the network, every user needing access to any ﬁle server must cross a segmenting device such as a bridge or switch, which introduces access delays as the device reads in the packet, examines the packet, determines whether to send it on or discard it, checks the packet for errors, and acts on its forward or discard decision. The necessity of crossing the segmenting device on a regular basis destroys the network availability that is gained by bridging, as local trafﬁc is no longer kept local.

The use of priority organization also introduces additional troubleshooting complications. If a station in the Sales department cannot access their ﬁle server, is it because the server has failed, the bridge connecting the server to that department is in error, the connection from the Sales department to the bridge is down, or the connection from the employee’s workstation to the rest of the Sales workgroup is faulty?

Selecting W orkgroup Organization

Once the rules for the creation of workgroups have been cemented and the Network Manager has determined which end user stations will be members of which workgroups, the issue of how the workgroups will relate to one another must be addressed. These relations determine the network equipment required for the network to operate and for the workgroups to communicate with one another.

Workgroups can be all grouped together into a single network or separated from one another by various means. In networks with relatively light network demands (few users, long periods of light trafﬁc, etc.) a single, unsegmented network will often provide faster responses than a segmented network of the same technology. As user counts and network trafﬁc increase, however, the utilization of segmentation becomes more and more important.

Workgroup Creation 4-7

Network Design

Before planning the segmentation of a network, there are a number of things which should be considered and noted:

• Any locations which have regular and repeated periods of extremely high trafﬁc (such as computer labs in instructional facilities) cause signiﬁcant increases in overall network load unless they are segmented.

• Situations where two separate groups of users require the network without creating a time overlap can be conducive to the grouping of workgroups without segmentation.

• Remember the 80/20 rule. Keep approximately 80% of the network trafﬁc in a workgroup local, and limit trafﬁc which must pass through a bridge, switch or router to 20% or less.

The organization of workgroups is most effectively made, from an efﬁciency standpoint, in a manner similar to the priority organization method of creating workgroups. Certain workgroups have higher network demands, greater numbers of users, or more stringent reliability requirements. These workgroups must be considered for segmentation on a case-by-case basis.

As an example, a facility to be networked has been examined and a workgroup organization scheme based on common function has been selected. Four workgroups have been identiﬁed: Sales, Customer Service, Records, and Shipping.

The Sales workgroup is extremely large, made up of more than twice the number of stations that compose all the other workgroups combined. The Sales workgroup requires fairly easy access to the network, but does not often need to communicate with groups other than Shipping.

The Customer Service workgroup, while small, is considered the most important segment by the Management and the Network Designer.

The Shipping and Records departments are both relatively small. They communicate with one another often and do not require an extremely high-performance network connection. After every business day, the Records department performs a full backup of all the records for that ﬁnancial quarter, a process which will ﬂood the network with constant trafﬁc for half an hour or more.

As the division and organization of this network into workgroups is planned, a series of illustrations representing the weights of certain network characteristics will be inserted in the text. In these diagrams, the value of the characteristic being examined for each proposed workgroup is denoted by size and by shade. The size of the ﬁeld of the illustrations is directly proportional to the number of end stations in each ﬁeld. When certain characteristics of the network are depicted by the ﬁelds, the darker the shading of the ﬁeld, the more that ﬁeld experiences or embodies the aspect under consideration.

4-8 Workgroup Creation

The Initial Field

Network Design

To begin, we gather the requisite information and determine the number of end stations that the network for this facility will have to support. This gives us the initial ﬁeld of the network before any workgroups have been decided upon.

ALL USERS

Having deﬁned the initial ﬁeld from which workgroups will be built, we then determine the criteria on which the division of the end stations into workgroups will take place. In this simpliﬁed case, the workgroups will be determined solely on the basis of department membership, following the corporate organization workgroup establishment criteria. This produces four workgroups from the initial single ﬁeld:

Workgroup Priority

SALES REC SHP

Having divided the network into initial workgroups, we need to examine the sizes of the workgroups which result. Those workgroups with large numbers of stations on them may require further segmentation later, while small workgroups may be able to be combined with other small workgroups. The decision to attempt further division or combination will have to be made following the examination of the network and the workgroups on the basis of other information.

One of these things to be considered is the relative priority of the workgroups. This priority may be based on network speed and access time, reliability, other factors, or a combination of such factors. This priority must be determined relative to the other departments to be networked, not on an absolute basis. Everyone will consider their workgroup to be the most essential, but detached decisions must be made regarding these priorities if there is to be any consideration of priority whatsoever.

In our example network, company policy dictates that the Customer Service department is the foremost, and requires both fast and reliable access to the network. This top priority segment is followed in access importance by the Sales department, which cannot make sales as efﬁciently without access to the network. Records and Shipping can both operate without network access for at least a business day.

C S

Workgroup Creation 4-9

SALES REC SHP

C S

Network Design

Internal Load

The efﬁciency and speed of a network is dependent upon the trafﬁc load of that network. This is one of the primary reasons for bridging; by keeping local trafﬁc local, the performance of other network segments tends to increase. An examination of the relative amount of internal trafﬁc each workgroup creates can indicate which workgroups can be segmented in order to keep them from affecting the operation of other workgroups. In our example, the Sales department workgroup produces a vast amount of trafﬁc which remains within the workgroup, so it becomes a prime candidate for segmentation.

SALES REC SHP

By examining the extent to which each workgroup displays the characteristics being considered, you can determine which workgroups are candidates for segmentation from the rest of the network and which are candidates for combination with other, preferably related, workgroups. You can also spot those workgroups which may require re-consideration and possible further division due to large numbers of users or high expected trafﬁc. By determining what segmentation level, if any, you wish to employ, you can develop the layout of your workgroups smoothly and effectively.

C S

ALL USERS SALES REC SHP SALES REC SHP

SALES REC SHP SALES REC SHP

C S

Size of Workgroup Required Level of Service Internal Group Trafﬁc

Sum of Network Demand (indicates need for segmentation)

External Load

When determining which workgroups will be separated from or combined with other workgroups, consider the locations where segmentation will be used. Those workgroups upon which a high percentage of the absolute trafﬁc is intended for other workgroups should not be separated from the other workgroups by too much segmentation. If a workgroup which requires constant access to other workgroups is kept apart from those workgroups by a large number of segmentation devices, the access time for that workgroup is signiﬁcantly increased.

4-10 Workgroup Creation

Network Design

SALES REC SHP

Looking at the above example, it makes more sense to keep the Records Department workgroup ‘close’ to the other segments of the network. Since it requires access to other segments on a more frequent basis than the others, keeping the Records workgroup from being segmented an excessive extent will help to increase the performance of Records department network operations.

bad

SALES 2

SALES 1

b b

Access to furthest group through 4 bridges and 3 shared segments

better

SALES 2

C S

better

SALES 2

C S

RECSHP

SALES 1

Access to furthest group through 2 bridges and 1 shared segment

SHP

REC

best

SALES 2

C S

SALES 1

Access to furthest group through

2 bridges and no shared segments

RECSHP

SALES 1

Access to furthest group through 1 bridge and no shared segments

This symbol indicates a bridge

RECSHP

Figure 4-5. Workgroup Proximity Organization

Departments with low network demands, as determined in the previous discussion, can be combined with those workgroups with frequent inter-group communications needs, as shown in the example above where the Records and Shipping workgroups were combined.

Workgroup Creation 4-11

Network Design

Selecting W orkgroup T echnologies

The selection of a network technology at the workgroup level is a very important decision, and one that should be made only after careful consideration and evaluation. Before deciding on a network technology to be used by the workgroups, make sure you are familiar with the operation of each type of technology, the strengths and shortcomings of those technologies, and the special design considerations that each technology imposes on the network. Chapter 3 is a good place to go for initial information, but the text deliberately avoids examining the technologies in great detail. For more detailed treatments of the technologies, you may wish to refer to Chapters 5, 6, and 7. There are also several texts on network technologies available through academic and technical booksellers.

The selection of a workgroup technology is an analysis of functionality. It is the job of the Network Manager or persons designing the network to determine which factors of the network design are the foremost requirements.

The most common determining factors in selecting a network technology are performance (speed of operation), reliability, ease of conﬁguration, troubleshooting, and cost. Cost is a separate issue from price, as cost is based on the inherent expenses of the technology, whereas price is highly dependent upon the vendor supplying the products and the quality of the products and service associated with them.

Each of these factors in deciding upon a network technology are treated in Chapter 3, Technology Basics.

This information is not intended to be the only guide for deciding upon a networking technology. The selection of a technology determines the capabilities and characteristics of the entire network, and is one of the most important and long-term decisions you make when designing a network.

For this reason, once you feel you have selected a suitable technology, do further research on that technology if you have any questions about its operation or the means by which a network is created using that technology. Contact your Cabletron Systems Sales Representative for information, or read any of the technical books available on the subject matter.

4-12 Workgroup Creation

Backbone Planning

What Is a Backbone?

A backbone is a network segment or cable which is used to provide for the interconnection of a number of smaller workgroups or self-contained networks. The outlying networks, workgroups, or hubs communicate with one another through the backbone network.

The use of a dedicated network acting as a backbone, tying all the separate networks together, is of beneﬁt for several reasons.

• Using a single network to handle the extremely important connections between networks allows Network Designers to utilize highly reliable technologies and cables. These designs are frequently expensive, and using them, initially, in the backbone network provides the beneﬁts of these technologies or media without requiring the expense of providing that level of service to all points of the network.

• A backbone network can be migrated out to the workgroups as the facility-wide network grows. As more users are added, it is often much easier to attach a concentrator or hub to a small backbone network than to continue expanding workgroups that may be already quite congested. In addition, the backbone can provide a point from which a higher-speed technology can be ‘painted out’ to the rest of the network as needs dictate and as money becomes available.

Network Design

• Since the amount of communications passing between several workgroups or hubs in an entire facility or campus is often quite large, backbone networks often use higher-speed networking technologies than the technologies used by the workgroup networks. A very common workgroup and backbone scenario involves several Ethernet workgroups in a building or campus connected to an FDDI backbone. This offers the communications passing between the separate Ethernet networks, operating at under 10 Mbps, to access a highly reliable and available 100 Mbps network for communications between workgroups.

Backbone Planning 4-13

Network Design

Methods of Conﬁguring Backbones

Backbone networks can be set up in a number of different ways. This Networking Guide will present three of the most common means of conﬁguring backbone networks. From these three basic types; the distributed backbone, the collapsed backbone, and the device collapsed backbone, nearly any backbone network implementation required may be designed.

The Distributed Backbone

One method of creating a backbone network is to sequentially string all of the workgroup networks or hubs together. Cabling is run from one workgroup hub to the next, providing the necessary connections. This method of conﬁguring a backbone network may be used with any technology except ATM, which requires a device backbone conﬁguration (detailed later in this chapter).

hub

1706n16

Figure 4-6. Distributed Backbone

A distributed backbone is usually the least expensive backbone network selection, as the only cabling required is that which runs from one workgroup network to another. The problems inherent in the distributed backbone network are somewhat limiting, however:

• Connectivity Requirements - FDDI and Token Ring networks must form a complete, unbroken ring. Ethernet backbones are most effective if all workgroup networks are attached to a shared bus, such as a thick coaxial cable.

• Limited Expandability - While it is possible to simply add stations to a thick Ethernet backbone, the ring-dependent technologies (Token Ring and FDDI) require that existing cable be cut and terminated or replaced with additional cable runs when new workgroup networks are added to the backbone.

• Troubleshooting Complexity - If a distributed backbone suffers an error or a faulty cable, locating the fault in the network often takes up much of the total troubleshooting time. If a cable is at fault, the Network Manager may end up spending a lot of time pulling and testing new cabling.

4-14 Backbone Planning

• Limited Control - The use of a distributed backbone makes the isolation of

workgroups from the rest of the overall network somewhat time-consuming. If a workgroup in a distributed backbone needs to be disconnected from the other networks physically, for whatever reason, the distributed backbone requires that a Network Manager go out to the physical location of the workgroup network and disconnect the required cables, making any additions or changes necessary to keep the backbone network whole and operating.

The Collapsed Backbone

It is also possible to run cables from a central point, often a network management ofﬁce or central wiring closet, out to each workgroup network and back. These cabling runs are then terminated at a central point such as a patch panel. The patch panel ports for each of the cable runs can then be connected to one another using jumper cables. In this fashion, provided that no technology restrictions are exceeded, chains and rings of workgroup networks can be created.

Network Design

hub

Cross-Connected

Patch Panel

1706n17

Figure 4-7. Collapsed Backbone

Having the individual cable runs of the backbone connected to one another at a single point can make this conﬁguration more expensive than the distributed backbone, but the added conﬁguration and control options provided by the collapsed backbone often outweighs the associated costs.

• Connectivity Requirements - The collapsed backbone implementation brings

all cables of the backbone to a central point, and the requirements of the Token Ring and FDDI technologies for an unbroken ring still apply.

Backbone Planning 4-15

Network Design

• Ease of Expandability - Since the cables of the collapsed backbone originate from a patch panel in one location, adding new cable runs to accommodate new workgroups or to bypass outmoded ones is a simple matter of changing a few jumper cables. If the network cabling was planned far enough in advance, the facility cabling required to add new workgroups to the backbone network may be already in place, requiring only a set of jumper cables and a short amount of time to connect. The use of a collapsed backbone can ease the transition from a backbone network with no controlling hardware to a device collapsed backbone in the future.

• Simpliﬁed Troubleshooting - Workgroups can be bypassed by simply re-conﬁguring a single patch panel. This can easily isolate a problem segment for troubleshooting, and keeps the backbone network from being kept in a fault condition.

• Moderate Control - The isolation of workgroups and the reorganization of the backbone network is simpliﬁed with the collapsed backbone, but the system does not incorporate any management features beyond the physical connections of facility cabling. For advanced and detailed network control operations, the device collapsed backbone (discussed below) is superior to the collapsed backbone alone.

Devices as Backbones

Once a collapsed backbone has been designed, it is a simple matter to connect the multiple backbone cables together through a device. Often this device is a multiport router, network switch, or a modular chassis. The use of a device of this type to make the connections between workgroups greatly increases the control that Network Managers have over the network, and may improve performance by streamlining the communications between networks.

hub

Ethernet Switch

1706n18

Figure 4-8. Device Collapsed Backbone

4-16 Backbone Planning

Network Design

The device collapsed backbone is the most expensive backbone choice, simply due to adding the cost of sophisticated, high-performance hardware to the costs of a collapsed backbone cabling layout. In many cases, the additional control and functionality provided by the device collapsed backbone conﬁguration are so valuable that the cost is well worth it.

• Connectivity Requirements - The device collapsed backbone implementation

brings all cables of the backbone to a single device, which takes care of the interconnection issues.

• Ease of Expandability - As all the workgroups of the network are connected

through the backbone device, the expandability of the network is limited by the amount of expandability that the device is capable of. As with the design of facility cabling, planning for future needs will go a long way toward reducing future expenses and possibly avoiding a costly “forklift” upgrade.

• Simpliﬁed Troubleshooting - The device collapsed backbone, by connecting

the workgroups through a manageable device, provides not only simpliﬁed troubleshooting, but the ability to detect some backbone faults before they become network failures.

• Extensive Control - The device collapsed backbone provides the highest level

of network control. Workgroups and devices on the backbone can be included or bypassed with the click of a mouse or through the use of a terminal session. Physically adding workgroups to the network will still require the connection of facility cabling and jumper cables, but, as with the standard collapsed backbone, the usefulness of planning ahead cannot be over-emphasized.

One danger of the device collapsed backbone is the existence of a single point of failure: the backbone device. If the backbone device fails, the backbone network will not operate. For more information on single points of failure and avoiding their creation in a network, refer to the Fault Aversion section of this chapter.

Choosing Backbone Technologies

The selection of a backbone technology is a similar process to the selection of workgroup technologies. As with the selection of a workgroup technology, make sure you are familiar with the operation of each type of technology, the strengths and shortcomings of those technologies, and the special design considerations that each technology imposes on the network. You may, again, wish to refer to the training information of this Networking Guide for initial instruction.

The selection of a backbone technology requires a careful examination of the needs of your facility and the ways that the various technologies and organization styles can ﬁt those needs. It is the job of the Network Manager or persons designing the network to determine which factors of the network design are the foremost requirements.

Backbone Planning 4-17

Network Design

The determining factors in selecting a backbone network technology are the same as those used in selecting workgroup technologies - performance (speed of operation), reliability, ease of conﬁguration, troubleshooting, and cost. In the backbone network, it is quite common to plan far ahead, providing more bandwidth than you think you will need. If this is done correctly, you will be able to upgrade the technologies of the outlying workgroup networks without requiring an immediate rebuilding of the backbone network.

Creating a Manageable Plan

A well thought-out and carefully designed network is still difﬁcult to troubleshoot if no one else knows how it is put together. There may come a time when the designer of the network is not available, for whatever reason, and troubleshooting or re-conﬁguration needs to be done. It may also become necessary to expand the network to accommodate a growing use of workstations or increases in personnel. It is at these times that a properly thought-out, implemented, and recorded network plan becomes a life saver.

The network plan is the “concept” behind the entire network. It deals with everything from where devices will be located and where cables will be run to the advanced or future technologies that the network may incorporate as it grows or changes.

A good network plan can go a long way to eliminating headaches during the conﬁguration and implementation stages. Have an overall design in mind when you begin planning individual conﬁgurations, and the network will be much easier to see as a whole. The network plan, in the design stages of networking, can point out areas that need additional work, help you locate possible trouble spots, and allow you to make the network more capable, more reliable, and more expandable than a haphazardly-assembled collection of cables and hardware.

4-18 Creating a Manageable Plan

Logical Layout

Component Location

The actual locations of the networking hardware is an important aspect of logical layout. As a network designer, you should determine how you want to treat the placement of devices and hold to that decision whenever possible.

Some of the commonly considered aspects of logical layout are as follows:

• Workgroup Location - If a workgroup is centered in a particular area of a

• Security - This is related to Centralization and Control (see below). In some

• Centralization and Control - If you require more control over the networking

Network Design

facility, you may wish to locate the networking hardware directly related to that workgroup in the same physical area as the workgroup.

cases, for security reasons, you may wish to place networking hardware in locations where they are not easily accessed by unqualiﬁed personnel. The usual course of action for security is to place networking equipment in an enclosed equipment cabinet or a locked wiring closet.

hardware than you can get from locking it away, you can place many devices in one central location such as a Network Management ofﬁce. For a small facility, it is entirely possible that all the networking hardware except end user workstations will be located in an ofﬁce such as this. An arrangement of this sort provides total control over the use and conﬁguration of the hardware in the hands of the Network Manager. This centralization also makes the location and isolation of faults much faster, as several problems may be identiﬁed and eliminated without the Network Manager ever having to leave the room.

Cabling

The method by which cabling is run from devices to end user stations is an important part of a manageable, expandable plan. Logically deﬁning a workable, ﬂexible, and expandable cabling system for a facility goes a long way toward making repairs and expansions to the network less difﬁcult. This Networking Guide will treat the issue of cabling plans brieﬂy, but other Cabletron Systems documentation and speciﬁc product Installation Guides treat cabling requirements in greater detail.

The most important thing to remember when planning cabling installations is that attention to detail will pay off. You may save a few hours at installation by not labeling your cables, but those saved hours will be more than spent later when you are attempting to locate the cable connected to John Doe’s workstation so that he can be moved to a different workgroup.

• Design cable installations with the future in mind. It is less expensive to install

an extra 40 or 50 cables during the initial installation than to have to go back and pull 10 cables on two different occasions because a department grew.

Creating a Manageable Plan 4-19

Network Design

• Keep cabling neatly organized. Bundle several cables together and secure them to places where they may be easily accessed. If one bundle of cables is associated with a speciﬁc workgroup or facility location, label that bundle periodically to eliminate any later confusion.

• Don’t connect raw facility cable to equipment ports. Facility cable should be connected to punchdown blocks, patch panels, or distribution boxes. These are simple wiring devices which allow you to use small jumper cables to connect the networking hardware to the facility cabling. These devices make labeling cables and changing connections much easier.

• Label everything. Every cable installed should be identiﬁed in at least two places (each end) by a numerical code. Every patch panel or distribution box port should be labeled as well. Many network wallplates have spaces where wallplate numbers can be displayed.

Use a standard, decipherable labeling code for cable and hardware. A label reading L2N5W2C1S243 may look like gibberish now, but if you know that the letter codes indicate locations or conditions of installation, it can be quite helpful. Table 4-1, below, shows the meanings of the codes and numbers of this example.

Cable Label: L2N5W2C1S243

Table 4-1. Cable Code Key

Code Code Deﬁnition Meaning

L2 Location 2 Engineering Building

N5 Network 5 Network Map #5

W2 Workgroup 2 Production Controls Workgroup

C1 Closet 1 Wiring Closet #1

S243 Station 243 Wallplate #243

The code key depicted above is only an example, and is not indicative of any industry standard or generally accepted cable marking practices.

4-20 Creating a Manageable Plan

Fault Aversion

A good network design strategy realizes the importance of avoiding future trouble spots. It is possible to design a network such that the most dangerous of these trouble spots are either eliminated, covered by contingencies, or their effects are minimized. This aspect of network design is called “fault aversion.”

A fault averse network uses the capabilities of available hardware and the fault-tolerance or recovery features of the technologies of the network to provide for three things: the elimination of single points of failure, the availability of redundancy, and the quick and easy isolation of and recovery from errors or problems.

Single Points of Failure

A single point of failure is any one device, cable or connection that, if it should fail or be removed from the network, would disable all or a sizable part of the network.

Most Cabletron Systems hardware seeks to eliminate single points of failure from within the device, by providing for redundant links or the distribution of essential functions among several related devices. Using devices in accordance with their fault-tolerant designs makes the network more able to continue operations automatically in the event of a component or cable failure.

Network Design

Redundancy

An example of a very obvious single point of failure is a shared segment of thick coaxial cable in an Ethernet network. All of the stations rely on the availability of the one coaxial segment. Should the segment fail, due to a break in the cable or the removal of a terminator, the network fails. A design eliminating the cable as a single point of failure might use several thin coaxial cable segments attaching to a repeater or modular chassis. Any one coaxial cable segment may fail without bringing down the other coaxial cable segments. The repeater can be seen as a single point of failure, but only from the point of view of the connection between segments, as the segments themselves will continue to operate without the repeater.

The location and elimination of single points of failure is a very difﬁcult step in network planning. It is important to set realistic limits on the elimination of these single points of failure. A network that completely eliminates single points of failure will be more expensive and complex than a network that eliminates only the most dangerous single points of failure.

Redundancy is the provision of or availability of backup systems. Redundancy is designed into a fault-averse network to allow a system or connection to quickly be activated to take the place of a failed system. Redundancy features are most often inherent parts of the networking technology being used, but the network must be designed to take advantage of those features.

Creating a Manageable Plan 4-21

Network Design

When designing a network, check the descriptions of the products to see if they support the creation of redundant links to devices. It is often a good idea to have some form of back-up capability for the network. For example, an Ethernet network can be designed using only standard Ethernet A channel Media Interface Modules, which will stop operating if the management module for their chassis fails. The network could also be conﬁgured with all Repeating Media Interface Modules, which provide their own repeating functions and can operate, at reduced functionality, without the management module present.

Isolation and Recovery

No matter how much redundancy is designed into a network, and no matter how much single points of failure are eliminated, the law of averages eventually catches up to any network, and a failure will occur. Once the failure does occur, the isolation and recovery process begins. If a network is designed to eliminate confusing layouts and make the troubleshooting procedure efﬁcient and effective, the amount of time a network is down is reduced. Comprehensive planning of workgroups and backbones is the most directly effective way to design isolation and recovery features into the network. Additionally, the use of built-in diagnostic systems, such as LANVIEW LED indicators, can provide quick and easy gathering of network operation information.

An example of this is the automatic wrapping of the dual ring structure of FDDI networks. If a station on the dual ring is lost, the ring wraps back upon itself at the two points between which the signal was interrupted or lost, closing the ring back up and allowing trafﬁc to continue passing. A good FDDI network design takes advantage of this recovery feature by placing the most essential devices, ones which are not intended to fail often or be shut down, on the dual ring, where they will beneﬁt from the automated recovery feature.

Network Maps and Record Keeping

A large portion of the process of expanding an existing network or troubleshooting faults and problems is determining what the current state of that network is. Keeping a running record of the status of the network, its conﬁguration, and any changes made to that conﬁguration, can go a long way toward simplifying the expansion of the network or migration to new technologies.

Tracking Functions

Networks are inherently complex things. There is a large amount of detailed information that needs to be recorded, and there are many different people who need differing levels of information about the network. Since the layers of complexity required by different people cannot always be crammed onto one network map, it may be very useful to keep a series of maps, each showing differing levels of complexity.

4-22 Creating a Manageable Plan

Tracking Changes

Network Design

For example, a network map set might include a facility map showing the division of areas into workgroups, a map showing the location, layout, and type of physical cabling, one showing the locations of networking hardware, and individual maps showing the locations and types of physical devices.

If you are using a network management package, such as Cabletron Systems SPECTRUM Element Manager, it is helpful to have a network map which shows the MAC addresses and IP addresses of the devices on the network.

Your network maps will be used for keeping track of a large amount of information, which will naturally change over time. As the network grows or is altered, the devices that make up the network will change, new workgroups will be added, segmented off from larger workgroups or combined with smaller ones. It is, therefore, important to keep track of the changes made to the network, and the network map is a good place to do this.

A network map that indicates a patch panel, punchdown block, or breakout box should identify that patch panel by a numerical or alphabetical code. This code should indicate a patch panel chart which can be referred to for connection information.

Any network device which appears on the general network map should be identiﬁed by some short and easily read code. This code refers to a separate list of the actual type of device. For example, the network map might show a diamond shape with “B882” written in it. A look at the chart or table of devices associated with this map indicates that the “B” in the code indicates a bridge, and bridge “882” is a standalone 2-port Cabletron Systems Ethernet bridge, NBR-220. If in the future this device is upgraded, the map can remain the same, but the device code table or chart can be changed. If, for example, the NBR-220 was upgraded to an Ethernet switch in a small chassis, the chart entry for “B882” could be changed to read “Cabletron Systems ESXMIM 6-port Ethernet switch in MMAC-M3FNB small modular chassis” without requiring any changes to the overall network map.

Creating a Manageable Plan 4-23

Network Design

Network Expandability

Networks tend toward growth. As businesses change and networking capabilities become more and more a part of the business process, networks grow in size or complexity and capability. For this reason, it is important, in any network, to plan for future expansion.

One of the easiest ways to plan for growth when designing a network which uses Cabletron Systems modular hub products is to buy a chassis with more capacity than you need, allowing you to simply add the modules you require later. If a design you have come up with requires ﬁve module slots, you could select the MMAC-M5FNB as a chassis. This would fulﬁll your module needs, but not provide any open slots for expansion. For a small amount more, you could build the design around the MMAC-M8FNB chassis, which provides the same functionality as the MMAC-M5FNB, but also supplies a total of eight module slots, giving you room to grow without needing to purchase a new chassis.

Expansion does not only mean being able to increase the total port count; expandability includes the later incorporation of new and future technologies, increasing the power, speed, and reliability of the network.

Cabletron Systems’ PLUS architecture, a key component of the design of the MMAC and its modules, is an effort to make planning for the future easier. By providing the capability for advanced functionality to be included as it is needed, the PLUS architecture smooths the upgrade and expansion path. For information on the various aspects of the PLUS architecture, contact your Cabletron Systems Sales Representative.

Network Migration

The MMAC-FNB series of modular hubs allow Ethernet, Token Ring, and FDDI technologies to be run simultaneously within one hub. The hub is fully protocol and network technology independent, providing connectivity to both existing and emerging technologies. This ability to support multiple network technologies is a feature of Cabletron Systems’ patented Flexible Network Bus, which allows hub modules to utilize the hub’s independent Multichannel Ethernet, Token Ring, and FDDI buses without requiring the artiﬁcial segmentation of the backplane. This means that every technology may operate on its own bus without requiring the customer to invest in an expensive, time-consuming, and network-disabling backplane upgrade.

4-24 Network Expandability

Cabletron Systems has provided a seamless migration path to high speed technologies from the Multi Media Access Center through the use of Bridge Router Interface Modules (BRIMs). Called a “module within a module,” BRIM modules can be plugged into an EMM-E6 or ESXMIM, or any other BRIM-capable device, including stackable hubs and standalone devices. The module containing the BRIM then has a bridged connection from the technology of that module to the technology of the BRIM, be it Ethernet, Token Ring, FDDI, or ATM. The BRIM offers great media and topology ﬂexibility, is customer installable and provides a migration path to future technologies.

The use of BRIMs, in conjunction with the FNB backplane, allows new or enhanced capabilities to be added to the chassis slowly, and only as needed. This capability needs to be considered in any network design, as the incorporation of more and more powerful and reliable technologies is part of the growth of a network. By allowing for the use of BRIMs and taking advantage of the versatility of the FNB, your network can incorporate the technology it needs, without having to pay for more than it needs.

Designing with the MMAC

Network Design

All of the designs in this Networking Guide are based on the use of the Cabletron Systems Multi Media Access Center, or MMAC. The Multi Media Access Center is a modular networking chassis that allows network designers to create multifunction, ﬂexible networks for today and provides expansion capabilities for the needs of tomorrow. With the ability to support multiple Ethernet, Token Ring, and FDDI networks simultaneously, all with the out-of-the-box chassis, the Multi Media Access Center offers excellent performance, reliability, and ﬁtness for a vast array of network conﬁgurations and needs.

Modular Chassis

The MMAC is an intelligent modular networking chassis, also referred to as a concentrator, distribution frame, or hub. Its function is to provide a single, central point to which several network stations and devices may be connected. In effect, the modular chassis acts as the conﬂuence of network activity; it is the point to which stations are connected, and it is the function of the modular chassis to ensure the reception of data from a network station and the propagation of that data to destination stations on the network. The chassis acts as a giant intersection or crossroads of information, facilitating the smooth operation of the network.

Designing with the MMAC 4-25

Network Design

A beneﬁt of the centralization of network connections into a modular chassis is the simpliﬁcation of management functions. Network management is an essential part of the operation of any network, no matter if that management is simple troubleshooting or advanced virtual workgroup creation. By having the vast majority of network connections made between stations at a single point, management tasks become simpliﬁed. Instead of strewing troubleshooting functions across an entire facility through vast runs of cabling and hidden connections, the modular hub provides one point where network difﬁculties can be identiﬁed, examined, and rectiﬁed. In addition, the use of a modular chassis for networking allows the network manager to monitor the performance and operation of the network, locating and addressing difﬁculties before they grow to become major problems.

The MMAC is available in three different sizes, to allow it to be utilized in several different network applications. These sizes of MMAC are identiﬁed by their capacity. The smallest MMAC is the MMAC-M3FNB, which supports three networking modules. The next size up is the MMAC-M5FNB, supporting ﬁve modules, and ﬁnally, the MMAC-M8FNB which supports up to eight modules. The capacities and capabilities of the three different MMAC chassis are described in detail later in this chapter. Additional information and illustrations may be found in Chapter 11, Product Descriptions.

Reliability and Recovery

As the modular chassis acts as the ‘nerve center’ of the network, it is an extremely important component of the network. Should the chassis be rendered inoperative by disaster or malfunction, it is likely that the stations which are connected to that chassis will be cut off from one another and unable to access the network. By incorporating features which enhance reliability and minimize the time necessary to bring a chassis back to an operational state, most networking device vendors have realized and addressed this potential weakness of the chassis-based network solution.

One primary concern in the operation of networks is the failure of components. If, for example, an Ethernet network’s repeater suffered a catastrophic failure, the network stations would no longer be able to pass packets to one another. The quickest solution to the problem is the removal of the old, malfunctioning repeater from the network and the immediate substitution of an operational repeater. If this can be done without requiring that the chassis be turned off, reconﬁgured, and turned back on again, the process is called hot-swapping.

4-26 Designing with the MMAC

The MMAC, as a modular networking chassis, makes this substitution and replacement a simple procedure. In order to make the Ethernet repeating functionality of the network simple and easy to upgrade, replace, or remove, the Ethernet repeater is often a discrete device on the network. In the MMAC chassis, the Ethernet repeater is a module which may be inserted into, and removed from, the modular chassis. When an Ethernet network is designed, a repeater module is placed in the chassis to perform that function. Should the repeater fail, that module can be removed from the chassis and a new module can be slid into the chassis to replace it. As the backup repeater is in the chassis, the original can be tested and repaired. In this way, the network is unavailable for only a fraction of the time that it would be if all the functions of the network were held in one device.

This use of discrete, easily incorporated and replaced devices, called modules, cards, or boards, provides the modular chassis with a level of ﬂexibility to support many concentrations of users and different types of physical media or cabling through the proper selection of modules.

Technology Flexibility

Network Design

The ﬂexibility of a chassis is also dependent upon the number and types of networks it supports. The use of a patented inter-module connection system allows the MMAC to be conﬁgured with modules for Ethernet networks, Token Ring networks, and FDDI networks, and allows these networks to operate in the same chassis and communicate with one another without requiring external bridges or routers. This connection system is based on the Flexible Network Bus, or FNB technology that is the heart of the MMAC chassis.

The Flexible Network Bus is made up of communications channels which connect modules in one MMAC chassis to each other. Due to the design of the FNB, it is capable of supporting three contiguous Ethernet networks across the entire backplane, four contiguous Token Ring networks across the entire backplane, and one or more FDDI rings. The FNB allows the MMAC chassis to be utilized in a number of network conﬁgurations.

Designing with the MMAC 4-27

Network Design

Power & Management

Ethernet A

Ethernet B

Ethernet C

FDDI

To reduce the costs of chassis failure, Cabletron has designed the MMAC chassis to be as modular as possible. Networking modules, power supplies, the cooling system, any device that the chassis requires for proper operation, is modular in design, and can be removed and replaced with a minimum of effort and in a very short amount of time. The best way to recover from a fault is not to have the fault occur, however. Cabletron Systems has designed the MMAC chassis to be fault tolerant.

The passive backplane architecture of the MMAC chassis is a prime example of this design philosophy. The backplane of a modular chassis is its heart. It is the single point at which all modules in that chassis communicate, receive power, and are managed by the chassis. If the backplane of the chassis fails, every module in that chassis that is connected to the backplane fails as well. To avoid these fatal failures, Cabletron Systems’ MMAC backplane incorporates absolutely no active electronic components. There are no resistors or capacitors that could critically fail on the MMAC backplane. All of the active electronics required for the proper functioning of the MMAC backplane are built onto the chassis modules themselves. In the event of a failure of one of these components, the replacement of the failed module allows the network to be rapidly returned to operation.

Token Ring R1 Token Ring R2

Token Ring R3

Token Ring R4

1706n19

Figure 4-9. Flexible Network Bus Channels

4-28 Designing with the MMAC

MMAC-M8FNB

ETHERNET

RESET

CPU

RJ45

PRT1

CH A

XMT

RCV

XMT

RCV

XMT

RCV XMT

RCV

XMT

RCV

XMT

RCV

C O M 1

C O M 2

Network Design

Module

Module "slot"

Power Redundancy

Fault-tolerance is also necessary in the supply of electrical power to the modular chassis. The chassis supplies all modules with electricity. The supply of power in the correct amperages and voltages for the modules in the chassis is performed by modular power supplies. As power supplies have high demands placed upon them by their operation, they are perhaps more susceptible to failure than other components. Since they are also a key system of the chassis, the loss of a power supply can mean a catastrophic chassis failure in the same way that a backplane failure could. To reduce the danger of this occurrence, Cabletron Systems’ larger MMAC chassis, the MMAC-M5FNB and MMAC-M8FNB, allow for the conﬁguration of redundant, load-sharing power supplies.

The use of the redundant power supplies gives additional tolerance to the system. Simple redundancy means that two power supplies are available to the modules in the chassis. One acts as the primary power supply, while the other remains in standby. In the event that the primary power supply fails, the secondary power supply senses its failure and takes over. While this is a valuable feature, it is a ﬂawed treatment of the problem. If the primary power supply is active while the secondary is inactive, there is no way of knowing that the secondary power supply is indeed functional. It also creates a short but signiﬁcant length of time that the chassis is without power between the failure of the primary power supply and the warm-up and activation of the secondary power supply.

Module Guide Channels

Figure 4-10. Chassis, Slots, and Modules

1706n20

Designing with the MMAC 4-29

Network Design

Cabletron Systems’ treatment of this issue is the utilization of load-sharing capabilities in the power supplies. Load-sharing power supplies examine the chassis they are in, and if they detect another power supply operating in the chassis, they each supply half of the power requirements of that chassis. In this way, both supplies are operating at all times at less than half capacity. In the event that one power supply fails, the other senses the failure and “ramps up” to full capacity, providing power to the chassis without requiring switchover time.

All of the designs that follow utilize the MMAC-FNB chassis exclusively. While some external standalone products may be mentioned in the course of describing a design, this Networking Guide only examines the use of the MMAC chassis in network designs. As the MMAC chassis cannot operate without power being supplied to it, all MMAC chassis designs must incorporate at least one power supply module designed for that capacity MMAC chassis.

NOTE

Every MMAC chassis is shipped conﬁgured with some modules:

- All MMAC chassis are shipped with cooling fan trays and slot covers for empty module slots.

- The MMAC-M3FNB is shipped with the above as well as one modular power supply.

4-30 Designing with the MMAC

Chapter 5

Ethernet

This chapter examines the Ethernet network technology in detail and provides step-by-step design instructions for the creation of Ethernet networks using Cabletron Systems networking products.

Description

Ethernet is what is called a contention-based network technology that provides for a maximum of 10 megabits per second (Mbps) of throughput under ideal conditions. In order to understand how Ethernet devices work, it is ﬁrst necessary to get a basic understanding of how Ethernet networks as a whole operate.

CSMA/CD

Ethernet is based on a network access scheme called Carrier Sense Multiple Access with Collision Detection. This scheme is abbreviated as CSMA/CD. Before we examine what CSMA/CD means to us as designers of networks, let’s take a look at the foundations of Ethernet.

Ethernet is one of the elders of networking, dating back to when the primary networking cabling was coaxial cable. When an Ethernet station had a packet to transmit, it would do so, no permission needed. When a packet was sent, it was transmitted onto the coaxial cable, where it would spread in either direction. Stations attached to the cable would all receive the signal and determine if it was intended for them. The basics of Ethernet communication: transmissions go to all stations on the network, and any station may transmit at any time. This is the MA, or Multiple Access portion of the CSMA/CD technology.

5-1

Ethernet

If the technology was left at that, it is easy to see that it would not function with more than one station on the network not a very efﬁcient networking solution. If two stations, which can transmit without requiring permission, both transmitted different packets at the same time, the electrical impulses that make up their signals would get combined into a long string of gibberish. The more stations you added, and the more they needed to communicate, the less the number of non-mangled packets on the network.

Rather than having each station take a pre-determined turn transmitting, the ﬁrst part of CSMA/CD, Carrier Sense, was incorporated. Carrier Sense is the method by which a station takes a look at the cable to which it is attached. If the cable is being used, the station will wait and try again later. If the cable is without signals, the station will transmit. By using Carrier Sense, Ethernet stations greatly reduce the number of collisions a network experiences.

ACDB

Station C examines network for availability. No signals noted.

ACDB

Figure 5-1. CSMA/CD Operation

In Figure 5-1, above, stations B and D receiv e the tr ansmission

NOTE

before station A. This is due to the fact that signals in a cable or media travel through that media at an appreciable fraction of the speed of light. The differences in reception time are called propagation delay.

Station C transmits. Stations B and D receive transmission.

Station C monitors network for error signals. Stations B and D discard transmission. Station A receives transmission.

1706n21

5-2 Description

Ethernet

Still, two stations may both listen to the line at the same time, and both transmit a short time after, causing a collision. That is where the CD portion of CSMA/CD comes into play. Collision Detection means that if a station transmits and causes a collision, it will notice that a collision has occurred. When this happens, the transmitting stations that were involved in the collision back off for a random amount of time, then begin all over again from the Carrier Sense stage with the same packets.

ACDB

Stations A and C examine network for availability. No signals noted.

ACDB

Both stations A and C transmit. Collision occurs, creating collision signal.

collision

ACDB

Detecting collision, stations A and C wait a random amount of time before attempting retransmission.

1706n22

Figure 5-2. Collision Detection

Although all of these portions of CSMA/CD are important to network design, the most important feature for now is the propagation of signals to every point on the network. It is important to remember that every point on an Ethernet network is connected to every other point on that Ethernet network. There are some devices that allow us to break large networks up into smaller ones, or extend networks to new Ethernets, but we deal with those later.

Description 5-3

Ethernet

Media

While Ethernet was originally designed to utilize coaxial cable, the vast majority of newly designed Ethernet networks operate over Unshielded Twisted Pair (UTP) cabling. Fiber optics runs a distant second, followed by the two types of coaxial cable, thick and thin.

The distance limitations to each media type are as follows:

Table 5-1. Ethernet Distance Limitations

Media Max Distance

Thick Coaxial Cable 500 m

Thin Coaxial Cable 185 m

UTP 100 m

Fiber Optics (Multimode) 1000 m

Fiber Optics (Single Mode) 1000 m

These distances are decided by the IEEE, a standards-making body which has created boundaries within which networks may be designed. This set of rules and boundaries is called the IEEE 802.3 standard.

Connectivity/Transceivers

Workstations and computers require intermediaries to allow them to connect to an Ethernet network. The purpose of these intermediary devices is to translate the electrical signals of the workstation into the electrical signals utilized by the Ethernet network and its selected media and vice versa. Since these devices allow the workstations and network to TRANSmit and reCEIVE information, they are called Transceivers.

The basic function of a transceiver is the conversion of Ethernet signals from a Male AUI port to another type of media. Media adapters, which are related to transceivers, are also available. Where a transceiver only converts signals to or from Male AUI ports, media adapters convert from any standard media to another standard media. There are media adapters that convert the signals used on UTP cabling to the pulses of light utilized on ﬁber optic cabling, and there are transceiver-like devices that convert the signals of external media (such as UTP or coaxial cable) into signals used by the Ethernet bus of a chassis. As these adapting devices allow workstations and devices to connect to the overall Ethernet network, they are often referred to as Connectivity devices.

5-4 Description

The job of a transceiver in chassis design is to provide a connection to a network that has already been established. Once a network has an established repeater in the chassis, the addition of chassis modules which allow Ethernet workstations to connect to the chassis may begin. These connectivity modules may be added to the chassis until the chassis has no more available module slots remaining or until the single network has reached the theoretical maximum population for an Ethernet network, 1,024 stations.

Cabletron Systems connectivity modules are denoted by the sufﬁx MIM, which stands for Media Interface Module. The MIM provides an interface between the backplane of the chassis and the external media for which it is intended. The name of each individual MIM also identiﬁes the external media to which it is intended to connect. For example, MIMs which are intended to provide a backplane connection for Unshielded Twisted Pair media are called TPMIMs, for Twisted Pair Media Interface Module. Those MIMs which provide connections to ﬁber optic cabling are called FOMIMs, for Fiber Optic Media Interface Module. The Example conﬁgurations section of this document provides detailed tables listing the product names, media connectivity, and connector style of each MIM.

Rules and Regulations

Ethernet

Ethernet has a large number of rules associated with it. There are the distance rules, for one, and the maximum population, for another. The maximum population (number of devices) on a single Ethernet network is 1,024. Usually, long before we reach 1,024 devices, we have moved to a new Ethernet network. How we do that is covered later.

Another maximum associated with Ethernet deals with the maximum number of devices per segment of cabling. The original Ethernet media, thick and thin coaxial cable, may each be tapped at multiple points to provide connections for stations. Each tap must be a certain minimum distance away from other taps, to keep them from interfering with each other.

And there are more rules. Each cable has a maximum number of places where it may support a connection. Thick coaxial cable, for example, may have no more than 100 taps. Thin coaxial cable, which is not shielded against interference as well as thick coaxial cable, may have no more than 30 taps. UTP and ﬁber optics have no taps, stations must be connected at each end of the cable.

But what about the 1,024 maximum number of devices? If the absolute maximum you can have on any of these media is 100 (over thick coaxial cable), how can you ever get to the maximum number? And, along the same lines, what use is a UTP link if you can only network two devices? A total of two stations does not seem like a very efﬁcient LAN.

Description 5-5

Ethernet

Repeating

When Ethernet was created, the general philosophy was “one cable, one LAN.” Signals would be transmitted to every point on a single cable. This transmission means is ﬁne for the multipoint media, like thick and thin coaxial cable, but is useless on a media that you cannot tap into, like ﬁber optics or UTP. Also, networks were very limited by distance and the maximum number of stations, as we have seen. In order to extend Ethernet networks, devices called repeaters were developed.

Repeaters take in an Ethernet signal from one connection and send it out to the other stations connected to that repeater. The earliest repeaters were used to connect one length of coaxial cable to a second length of coaxial cable, so you could, for example, use two lengths of thick coaxial cable to reach a maximum end-to-end distance of 1000 meters with a maximum of 200 stations on the network. Later repeaters allowed you to attach multiple segments together. These repeaters are known, logically enough, as multiport repeaters. This changed the Ethernet general philosophy from “one cable, one LAN” to “many cables, one LAN.”

Since it was not always desirable to have thick coaxial cable running through all your ofﬁces, repeaters took the logical step of repeating Ethernet signals to different media. These repeaters could be useful for connecting a network of thick coaxial cable to a network of thin coaxial cable or a section of ﬁber optics. Eventually, these repeaters began to connect large numbers of thin coaxial cables, used commonly in ofﬁces and laboratories, to tap points on thick coaxial cables known as “backbones,” which provided interconnection for the repeaters and were usually kept out of sight in cabling conduits. These multiport repeaters were the forebears of today’s intelligent hubs.

Repeaters/Hubs

Repeaters are one of the building blocks of Ethernet, and are the foundation upon which all Ethernet hubs are based. In its simplest form, a hub, populated with modules, is basically a huge multiport repeater. While additional functions such as bridging and routing may be added, the basic job of a hub is to repeat signals coming in through one port to all the other ports in the hub. By placing a repeater in a modular chassis, you allow the chassis to accept and utilize connectivity modules.

The number of repeaters a given conﬁguration needs depends entirely on the number of Ethernet networks that are being created. For a conﬁguration providing only one Ethernet network, only one device supplying the repeater functionality for that network is required. It is the job of this repeater to support any Media Interface Modules (above) that are conﬁgured to operate on its network. Theoretically, one repeater module could support any number of MIMs providing up to the theoretical maximum number of Ethernet station connections, 1,024. In reality, the modular intelligent chassis will likely be ﬁlled to capacity with MIMs before that maximum is reached, requiring that the expansion of the network be made through the use of a second modular chassis.

5-6 Description

Ethernet

For each new chassis that is to support an Ethernet network through the use of MIMs, repeating must be supplied anew. So, if one network had outgrown its chassis and was being extended by way of a connection to a second modular chassis, repeating would have to be supplied all over again to allow the addition of new MIMs to the second chassis.

Cabletron’s basic repeater module is the IRM, or Intelligent Repeater Module, series. The IRM series of management modules provides repeating for a single Ethernet network within the MMAC chassis and also provides management functions for that network. Other modules which provide repeating functions that support the conﬁguration of MIMs in the chassis include the EMME and EMM-E6 Ethernet modules. A chassis which is intended to contain a single Ethernet network, therefore, would be conﬁgured ﬁrst with some IRM series module or EMME module to provide repeating. This would create the Ethernet network, to which MIMs would be added to supply connectivity.

By the same token, if multiple Ethernet networks are to be conﬁgured in a single MMAC chassis, each separate Ethernet network must receive repeating functions for it to operate. All of the Cabletron Systems IRM modules provide Ethernet repeating for one Ethernet network only, however. Since Cabletron Systems’ MMAC chassis has the capability to support up to three Ethernet networks (Ethernet channels A, B, and C) across the single chassis backplane, how do we provide repeating functions to the second and third networks?

The answer lies in the use of combined function modules. Rather than requiring that a chassis supporting multiple networks consists of a repeater for one network and its associated MIM modules, then a repeater for a second network, followed by its MIM modules, followed by the third repeater and its MIM modules, Cabletron Systems combined the functions of a Media Interface Module with those of a repeater to create MIMs that perform their own repeating. These Repeating Media Interface Modules, or RMIMs, are made speciﬁcally for connection to the B and C Channels of the MMAC chassis backplane. Since the repeater functionality for these additional networks is included in the RMIMs themselves, no additional IRM-style modules are required for the functioning of multiple Ethernets in the chassis.

For example, to create two networks in an MMAC chassis, a module would be required to provide repeating for the MIM modules of the ﬁrst Ethernet, then RMIM modules, which connect to the second Ethernet, are added to the chassis. As these modules supply their own repeating functions for the second Ethernet network, the RMIM modules are able to function without an additional dedicated repeater.

Any MIM which supports a connection to the Ethernet B or C Channels of the MMAC chassis will provide the repeater functionality for those connections. The Ethernet Port Assignment modules, or TPXMIMs, allow ports to be connected to the A, B, or C Channels of the MMAC chassis. The TPXMIM, like the Repeating Media Interface Module, includes repeater functions for the B and C channels of the chassis, and will require the functions of an IRM, EMME, or other A Channel repeater to be able to assign ports to the A Channel Ethernet network.

Description 5-7

Ethernet

Repeater Rules

Naturally, there are rules surrounding repeaters, just as there are rules surrounding cabling. A set of guidelines known collectively as the repeater rule make plain the maximum limits of repeaters. The rule is based on the numbers 5-4-3.

5: Maximum of ﬁve (5) Ethernet segments. No more than ﬁve repeated Ethernet segments may be connected in the longest repeater path. This could be ﬁve segments of thick coaxial cable, three ﬁber optic and two thin coaxial cables, ﬁve interconnected hubs, etc. The way to look for a violation of the 5 rule is to go through your design marking any segments that are connected to repeaters. Then follow those to determine the longest possible path that data would have to travel. If your design has any paths crossing more than ﬁve network segments, you have broken the ﬁrst of the repeater rules.

4: Maximum of four (4) repeater hops. Each time a signal passes through a device that performs repeating, be it a standalone repeater or a hub module, you add one “repeater hop”. To check for 4 rule violations, ﬁgure out the path that passes through the largest number of repeaters. If a signal from one end to the other must pass through more than four repeaters, the design breaks the rule.

Arrow shows path of signal

"Repeater Hop"

Repeater

Network A

Network B

1706n23

Figure 5-3. Repeater Hop

3: The three (3) rule speciﬁes the number of segments connected by repeaters that may have more than two stations on them. If you have three segments connected by repeaters, they may all have users. If, however, you have four segments connected by repeaters, only three of them may have user stations on them, beyond the connections to the repeaters between them. Note that

device other than the repeaters on either end of the path classiﬁes as a

any station for this rule, not just workstations.

The example in Figure 5-4 shows a proposed Ethernet network being examined for repeater rule violations. The ﬁrst step is identifying the longest signal path. A quick count of repeater hops indicates that the longest signal path travels through R1, R2, R4, R5, and R6. With six Ethernet segments in the longest signal path, this network is already out of speciﬁcations, violating the “5” rule.

5-8 Description

Arrow shows path of signal

Repeater Repeater Hop

Ethernet

R1 R2 R3

R4 R5 R6

1706n24

Figure 5-4. Repeater Rule Violation

Furthermore, the number of repeater hops in the longest signal path is ﬁve, one in excess of the four allowed by the repeater rule. The network shown in Figure 5-4 would not function properly, due to the errors introduced by the violations of the repeater rule. This network will have to be redesigned using some form of segmentation (see Segmentation, Chapter 3, or refer to the Segmented Ethernet section later in this chapter).

Description 5-9

Ethernet

Simple Ethernet Conﬁguration

The term “simple” in this deﬁnition is used to indicate that the network is conﬁgured as a single repeated network and does not incorporate any bridging, routing, switching or other segmentation between stations. All of the Ethernet stations in a simple Ethernet network are treated as if they were stations located on one segment of coaxial cable.

Design Philosophy

A simple Ethernet operates on the basic principles of IEEE 802.3 LANs: a transmission by any station on the network is received by every other station, and may transmit at any time that the station senses that the common media (in this case, the modular chassis) is not being utilized.

The simple Ethernet network design is a relatively inexpensive means of networking a limited number of workstations that do not have particularly demanding network availability needs. A small company or educational facility with anywhere from 20 to 180 workstations could utilize a simple Ethernet conﬁguration such as the one described below.

Design Example

This design provides Ethernet networking services for three departments within a small personal accounting ﬁrm with 63 workstations. Two of these workstations are dedicated ﬁleservers and one provides access to a tape drive system used for backups. The cabling used for connections from the workstations to the central network chassis is Category 5 UTP, which is terminated with standard RJ45 patch panels and wallplates.

Since the vast majority of applications and uses for the network are not particularly time- intensive at this location, the cost savings provided by using the simple Ethernet conﬁguration is considered to outweigh the functionality and additional performance that could be gained by utilizing other network technologies and methods. The foundation of this network is a Cabletron Systems MMAC-M8FNB modular networking chassis, outﬁtted with the correct modular power supplies (M8PSM-E). This provides the basic concentrator capability into which the connectivity and functionality modules are to be added.

Select Chassis

5-10 Simple Ethernet Conﬁguration

Ethernet

To provide the starting point for building the Ethernet network, a repeater module must be placed in the chassis. This module will provide repeating functions for the Ethernet modules in the chassis that are conﬁgured to use the Ethernet channel A of the backplane. In addition, all of Cabletron Systems’ Ethernet channel A repeaters provide management functions for the modules in the chassis. To provide Ethernet repeating and supply SNMP management capabilities, this network will utilize an IRM3 module. The IRM3 repeats any Ethernet signals that are transmitted by stations on the A Channel of the MMAC chassis.

Select Chassis

Multi-segment?

yes

IRM Family

To provide user connections, the MMAC chassis needs to utilize transceiver or “connectivity” modules, commonly called Media Interface Modules, or MIMs, as they provide the physical interface between the MMAC backplane and the cabling media that connect to the end-user stations.

IRM Family

Select MIMs

Simple Ethernet Conﬁguration 5-11

Ethernet

This location utilizes UTP cabling with RJ45 connectors, narrowing the choices of modules to the TPMIM-22 and TPMIM-24. Since there are 63 stations to provide connections for, the chassis utilizes three TPMIM-24 Ethernet Media Interface Modules, supplying a total of 72 RJ45 connectors. The chassis that we are using to network this facility now looks like Figure 5-5:

1706n25

Figure 5-5. Example Ethernet Conﬁguration 1

The networking device families used in this network design are the Ethernet repeater and the Ethernet transceiver/Connectivity module. Since there are several different products manufactured by Cabletron Systems which meet these networking device families, the actual conﬁguration of the network can be altered by substituting different products of these families in the modular networking chassis. The product families and their individual members are listed in the series of tables that follow:

5-12 Simple Ethernet Conﬁguration

Modular Intelligent Chassis

The modular intelligent chassis provides the basic platform into which management and networking modules are inserted. It provides the physical interconnection of modules through the backplane bus. It is important to note that modular power supplies are required for the operation of the hub and the modules within it. The power supply for the MMAC-M3FNB is included with the purchase of the chassis. The MMAC-M5FNB and MMAC-M8FNB may both be conﬁgured with one or two power supplies which are purchased separately. The use of two power supplies provides redundant power capability, allowing one power supply to satisfy the power needs of the hub in the event of a power supply failure. See the Cabletron Systems Networking Solutions Catalog for detailed information on available power supplies.

Ethernet

Table 5-2. MMAC Chassis

Product Capacity

MMAC-M3FNB 1 Half Slot 2 Full Slots

MMAC-M5FNB 1 Half Slot 4 Full Slots

MMAC-M8FNB 1 Half Slot 7 Full Slots

a. Capacity is additive; Each chassis has the listed number of full slots in addition to the single, dedicated half-slot utilized by management modules (i.e: the MMAC-M5FNB has a total of 5 module slots).

Ethernet Repeater and Management Module

The Ethernet repeater and management supplies the IEEE 802.3 repeating functions which make it possible for individual connections to share the backplane as a common connection medium. One repeater module is necessary for the operation of the Ethernet modules.

Table 5-3. Ethernet Repeaters

Required Functions Compliant Modules

“A” Channel Ethernet Repeating and Management Functions Only

“A” Channel Ethernet Repeating, Extended Management, and Bridging or Routing to B, C, and External Ethernet Channels

IRM3

EMME/EMM-E6

a. The EMM-E6 utilizes the rightmost two slots in the MMAC chassis, the Management Module slot and one adjacent full Media Interface Module slot.

Simple Ethernet Conﬁguration 5-13

Ethernet

Media Interface Modules

To provide connections for the physical cabling, Media Interface Modules are needed. The number of Media Interface Modules the network must utilize is dependent upon the number of stations to be connected to the network. Before selecting modules you must identify the type of physical cabling media and the connector type to be utilized.

Consult the table below to determine which modules may be used to connect to the physical cabling plant and calculate the number and exact type of modules that will be required to fulﬁll the needs of the network.

Table 5-4. Ethernet MIMs

Media Type Connector Type

AUI (to

DB15 (female) 8 1 MT8-MIM

Number of

Connectors

Max # of

Stations/Connector

Product Name

transceivers)

Thin Coaxial

RG58 12 29 THN-MIM

Cable

UTP RJ45 12 1 TPMIM-22

24 1 TPMIM-24

RJ21 (Telco) 1 12 TPMIM-32

212 TPMIM-34

Multimode Fiber Optics

Sub-Miniature Assembly

1 FOMIM-16

12 1 FOMIM-12

18 1 FOMIM-18

Straight-Tip 6

1 FOMIM-26

12 1 FOMIM-22

18 1 FOMIM-28

Single Mode

Straight-Tip 6

1 FOMIM-36

Fiber Optics

12 1 FOMIM-32

UTP/STP DB9 12 1 TPMIM-T1

a. Note: All Fiber Optic MIMs provide tw o connectors (transmit and receive) for each Ethernet connection. Number of connectors given is the maximum number of transmit and receive pairs

5-14 Simple Ethernet Conﬁguration

18 1 FOMIM-38

available on the module.

Ethernet

As all of the modules listed in Table 5-4, above, are Ethernet Channel A modules, they are fully interchangeable in conﬁguration terms. A conﬁguration like the example given previously could have utilized three TPMIM-34 modules if RJ21 connectors were to be used, or included one FOMIM-26 to connect six stations with ﬁber optic cabling.

Even though many simple Ethernets will occupy only one MMAC chassis, it is important to recognize that any links made from one simple Ethernet chassis to another that is not made through a bridge or router will expand the one network. Without the bridge or router present to perform the ﬁltering or forwarding services for trafﬁc between the two chassis, the modules and stations will act as though they were conﬁgured in a single MMAC. Although the two chassis and their associated modules are operating on the same network, each modular chassis to be conﬁgured in simple Ethernet fashion requires an Ethernet repeater module. Without the module, the stations attached to the connectivity modules will not be able to transmit and receive signals to and from the network.

To illustrate, if a location required two MMAC chassis conﬁgured as in the previous example, connected to one another through a ﬁber optic connection, we would begin by placing an IRM3 in each of the MMAC chassis. These IRM3 modules would provide the necessary Ethernet repeating functions to allow the connectivity modules, TPMIM-24s once again, to operate and communicate with one another. The ﬁber optic link between the two hubs (assuming multimode ﬁber optics and ST connectors) would be made using the ﬁber optic connectors on the front panel of each IRM3. While the stations connected to the TPMIM modules of each hub would be located in physically distinct areas, the network would treat all the stations as being on the same logical network, sharing the same 10 Mbps of bandwidth available in an Ethernet network.

Segmentation

Segmentation is the incorporation of bridging or routing between individual networks or segments. The segmentation of a network is something that must be approached cautiously, and extended discussions of this planning are presented in Chapter 4, Network Design. One of the important design guidelines for segmentation is to try and follow the 80/20 rule. You want, ideally, to keep 80% of your trafﬁc local and pass about 20% of your trafﬁc across segmentation devices. It makes perfect sense, therefore, to put departments that share ﬁleservers on the same segment. That way users needing access to the ﬁle server in question do not have to cross the bridge, cutting into the speed of the bridge and reducing the speed of network operations.

This segmentation is based on the extent to which distinct groups of users or departments are related and require transmission of information betweenone another. For example, the Payroll, Personnel, and Accounting departments of most businesses transfer information on a very regular basis. It makes sense from a network design perspective, therefore, to attempt to keep these departments from having to pass more than 20% of their trafﬁc across a bridge or other segmentation device.

Segmentation 5-15

Ethernet

Bridges

We may also segment the network logically by geographic location. This makes it easier to troubleshoot and expand existing networks. Geographic segmentation is usually an inferior segmentation solution in comparison with segmentation by related function or access. The increase in logical layout that can be accomplished using this method is frequently overshadowed by decreased network performance as a higher percentage of trafﬁc is sent through bridges.

Now that you have learned the basics of single-level repeated Ethernet, we can progress to the next level of expertise, Bridged Ethernet. Bridges are network “trafﬁc control” devices which affect the way that information moves from one network to another. Bridges segment networks, while still allowing trafﬁc that needs to pass between two networks to pass. By doing this, bridges keep trafﬁc local, freeing a bridged network from the effects of trafﬁc on its distant neighbors. A bridge connects networks in a similar fashion to a repeater, but a bridge goes a few steps further, monitoring the individual data packets to determine if they need to be sent to other segments or not, and regenerating, retiming, and reclocking the signal to increase its ability to be retransmitted.

A bridge is a device that can be added to a network to allow expansion beyond the limitations of IEEE 802.3. If an Ethernet network has a repeater hop of four repeaters or a round trip propagation delay near the 51.2µs maximum, a bridge can be used to build an extended network. Ethernet bridges read in packets and decide to ﬁlter, discard, or forward based on the destination address of the packet. The simple forward/ﬁlter decision process allows a bridge to segment trafﬁc between two networks, keeping local trafﬁc local. This process increases the availability of each network while still allowing trafﬁc destined for the opposite side of the bridge to pass.

When multiple Ethernet networks are created within a single chassis, they all operate as separate networks. Although a conﬁguration may support three Ethernet networks in a single modular chassis, they are considered separate from one another and do not exchange information. This effectively means that an Ethernet station on Ethernet Channel A is not able to communicate with an Ethernet station on Ethernet Channel B in the same chassis.

It is the function of bridges to connect separate networks while maintaining the separation between them. An Ethernet bridge acts in much the same way as a bridge in the physical world. It provides a single point of access between both networks, allowing trafﬁc which must cross to the other network to do so, while keeping unrelated network activity on one side of the bridge from impacting the operation of the network on the other side of the bridge.

5-16 Segmentation

Ethernet

Network A

Bridge

Network B

1706n26

Figure 5-6. Bridges

The bridge is considered a node on the network and performs store and forward functions for packets on each network. This contrasts with a repeater which repeats the signal bit by bit from one side of the network to the other. The bridge actually reads each packet, checks the packet for accuracy, then decides whether the packet should be sent to the other network based on the destination address. If the other network is busy, it is the responsibility of the bridge to store the packet, for a reasonable time, until the transmission can be made.

The bridge is also responsible for handling collisions. If a collision happens as the bridge is transmitting onto the second network, the bridge is responsible for the back off and retransmission process. The original sending node is not made aware of the collision. It assumes the packet has been sent correctly. If the bridge is unable to send the packet to its ﬁnal destination, the original sending station, expecting some response from the device it was attempting to contact, will “time out” and may attempt retransmission.

The bridge decides whether to forward or ﬁlter a packet based on the physical location of the destination device with respect to the source device. A bridge dynamically learns the physical location of devices by logging the source addresses of each packet and the bridge port the packet was received on in a table called the Source Address Table (SAT).

Segmentation 5-17

Ethernet

Cabletron produces two different types of Ethernet bridges; those that are conﬁgured to operate within the MMAC chassis and those which are standalone external bridges. Cabletron’s main internal bridge module for Ethernet networks are the EMME family of repeater/bridge/management modules, the EMME and

EMM-E6. These modules provide bridging functions for the three Ethernet

channels of the MMAC backplane, bridging from these channels to one (in the case of the EMME) or three (in the case of the EMM-E6) external networks, repeating functions for Ethernet Channel A, and high-level management of the modules within the chassis. Any chassis intending to use the Multichannel Ethernet architecture of the MMAC chassis, therefore, requires the use of an EMME or EMM-E6 module to supply the repeating and bridging functions required.

In addition, Ethernet bridges can connect an Ethernet network to another technology, provided that the networks are similar. Similar technologies only differ in the lower level network control operations such as the formulation of packets and the transmission of signals. Since bridges operate at a relatively basic level, they can perform the translation or encapsulation required to connect two different technologies such as Ethernet and FDDI, or Ethernet and Token Ring.

Cabletron manufactures several multiple-technology bridges. An example of a multi-technology bridge is the FDMMIM. The FDMMIM provides FDDI management for the creation of FDDI rings within an MMAC chassis, as described in Chapter 7, FDDI. In addition to its FDDI ring control functions, the FDMMIM is also an Ethernet-to-FDDI bridge. The FDMMIM connects to Ethernet Channel A of the MMAC backplane and to the FDDI portion of the MMAC’s FNB. By placing an FDMMIM in a chassis containing Ethernet modules with a connection to Ethernet Channel A, whether through an IRM repeater or an EMME bridge, the FDMMIM converts any FDDI frames intended for those Ethernet modules into Ethernet packets, and converts Ethernet packets from Ethernet Channel A which are intended for the FDDI ring into FDDI frames.

This bridging functionality is the basis of creating backbones which utilize a different technology than workgroup networks. The most common backbone strategy is to interconnect several Ethernet workgroups, which operate at a maximum of 10 Mbps, to each other with an FDDI backbone, which operates at 100 Mbps. An FDMMIM conﬁgured in an Ethernet chassis provides the bridging functions necessary to allow the chassis to connect to such a backbone.

5-18 Segmentation

Ethernet

FDDI Backbone

EthernetEthernet

1706n27

Figure 5-7. FDDI Backbone

Bridge modules which connect an Ethernet network to Token Ring networks or Wide Area networks, serial terminals, AppleTalk networks, and Systems Network Architecture (SNA) are also available.

One special case of Ethernet bridging is the use of Bridge/Router Interface Modules (BRIMs). These are small modules which are placed within another MMAC module and which can be used to provide a bridged or routed connection to another network or another similar network technology. Cabletron Systems produces a number of products which support BRIMs, such as the EMM-E6 and the ESXMIM Ethernet modules. The BRIM module performs technology-to-technology interconnection without reducing the ﬂexibility of the module it is conﬁgured in.

For example; an EMM-E6 module could be conﬁgured initially without any BRIM connections. As a facility added more users and another modular chassis to the corporate network, perhaps a BRIM-E6 could be added to the EMM-E6 management module of the chassis. This Ethernet BRIM would provide one external bridged Ethernet connection which could be used for the chassis-to-chassis connection. As the network continued to grow, and more MMAC chassis were added to the network, the trafﬁc on the chassis-to-chassis links increased, until it was decided that an FDDI connection between the separate chassis would help reduce network delays. In this case, the BRIM-E6 modules which had been providing Ethernet to Ethernet bridging in the EMM-E6 module could be exchanged for BRIM-F6 modules, which would supply Ethernet to FDDI bridging for these chassis.

Segmentation 5-19

Ethernet

The table which follows lists the available MMAC chassis modules which perform bridging between the Ethernet technology and other similar network technologies.

Bridging: This indicates what technologies the module bridges. In the case of the ETWMIM, the module is capable of bridging Ethernet to both Token Ring and Wide Area networks. For this reason, the ETWMIM is treated in two separate entries in the table.

Ethernet Subscription: This entry refers to the means by which the bridge module connects to the Ethernet network it is bridging to or from. In some cases, the bridge may be conﬁgured to use one of a series of subscription methods.

Bridged Technology Subscription: This ﬁeld details where the physical connection to the non-Ethernet bridged technology is made. Entries indicating connections made through a BRIM slot, by nature, indicate a front panel connection scheme.

Table 5-5. Multi-Technology Bridges

Module Bridging Ethernet Subscription

FDMMIM/

Ethernet-FDDI Channel A FDDI/FNB

Bridged Technology

Subscription

FDMMIM-04

ETWMIM Ethernet-TR Channel A/External TR/FNB

ETWMIM Ethernet-WAN Channel A/External Front Panel

CSMIM2-T Ethernet-TCP/IP

Channels A/B/C Front Panel

Terminals

CSMIM2-LT Ethernet-TCP/IP and

Channels A/B/C Front Panel

DEC LAT Terminals

GatorSTAR/GX-M Ethernet-AppleTalk Channel A Front Panel

EMM-E6 Ethernet-BRIM Channels A/B/C/D BRIM Slot

ESXMIM/

Ethernet-BRIM Channel A/External BRIM Slot

ESXMIM-F2

SNACMIM/E Ethernet-SNA Channel A Front Panel

Bridging Rules

As with repeaters, there are rules associated with the maximum number of bridges that may be used on any signal path. With bridges, you may conﬁgure an Ethernet signal path which contains eight (8) segments and seven (7) bridges.

5-20 Segmentation

Cabletron Systems MMAC-5FNB, MMAC-FNB Networking Manual

Specifications and Main Features

Frequently Asked Questions

User Manual