Black Box MDS920C-10BT, MDS921AE-10BT User Manual
MDS920C-10BT RACK-CARD
MDS921AE-10BT STANDALONE
MDS922AE-10BT MINI-RACK
Ethernet Router / Bridge
User Manual
Version 1.0
Revision 11 February 2003
© Copyright ©2002 by BLACK BOX Network Services AG. The contents of this publication may
not be reproduced in any part or as a whole, transcribed, stored in a retrieval system, translated
into any language, or transmitted in any form or by any means, electronic, mechanical, magnetic,
optical, chemical, photocopying, manual, or otherwise, without the prior written permission of
BLACK BOX Network Services AG. All rights reserved.
VERSION CONTROL.......................................................................................................... 5
1 INTRODUCTION........................................................................................................ 6
3 SPECIFICATIONS ................................ FEHLER! TEXTMARKE NICHT DEFINIERT.
4 TECHNOLOGIES....................................................................................................... 9
4.1 xDSL technology, background .......................................................................... 9
4.1.1 Asymmetric DSL (ADSL) technology ..................................................9
4.1.2 ISDN DSL technology .......................................................................10
4.1.3 High bit rate DSL (HDSL) technology................................................ 11
4.1.4 EXTRAns technology ........................................................................ 13
4.1.5 Multispeed DSL (MDSL) technology ................................................. 14
4.1.6 Multispeed DSL (MSDSL) technology............................................... 15
4.1.7 G.shdsl technology............................................................................ 16
4.2 Local area network integration. Access to Internet ......................................... 17
4.2.1 TCP/IP stack structure ......................................................................17
4.2.2 Address assignment in IP networks .................................................. 19
4.2.3 Bridging of local networks .................................................................21
4.2.4 Routing of networks........................................................................... 25
5 DESCRIPTION OF THE DEVICE ............................................................................31
5.1.1 Background ....................................................................................... 31
5.2 Operation mode ..............................................................................................33
5.2.1 G.shdsl line interface......................................................................... 33
5.2.2 Ethernet 10BaseT interface............................................................... 34
5.2.3 ATM interface .................................................................................... 34
5.3 Description of LEDs ........................................................................................35
6 MECHANIC DESIGN ............................................................................................... 36
7 EQUIPMENT INSTALLATION................................................................................. 41
8 PROGRAMMING GUIDE......................................................................................... 43
8.1 Introduction .....................................................................................................43
8.2 Main menu of the bridge mode .......................................................................47
8.2.1 Help command .................................................................................. 47
8.2.2 Home command ................................................................................ 47
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8.2.3 Default command .............................................................................. 48
8.2.4 Lan command.................................................................................... 48
8.2.5 List command .................................................................................... 49
8.2.6 Manage command............................................................................. 50
8.2.7 Mode command................................................................................. 52
8.2.8 Ping command ..................................................................................53
8.2.9 Quick command ................................................................................53
8.2.10 R1483 command ............................................................................... 54
8.2.11 Restart command .............................................................................. 57
8.2.12 Save command .................................................................................57
8.2.13 Shdsl command................................................................................. 58
8.2.14 Show command................................................................................. 62
8.2.15 Ver command .................................................................................... 62
8.3 Main menu of the router mode ........................................................................62
8.3.1 Default command .............................................................................. 62
8.3.2 Dnsrelay command ...........................................................................62
8.3.3 8.3.3 Ipoa command.......................................................................... 64
8.3.4 Lan command.................................................................................... 66
8.3.5 List command .................................................................................... 68
8.3.6 Manage command............................................................................. 68
8.3.7 Mode command................................................................................. 68
8.3.8 Pat command .................................................................................... 68
8.3.9 Ping command ..................................................................................70
8.3.10 Pppoa command ............................................................................... 70
8.3.11 Pppoe command ............................................................................... 73
8.3.12 R1483 command ............................................................................... 74
8.3.13 Quick command ................................................................................76
8.3.14 Restart command .............................................................................. 77
8.3.15 Rtable command ............................................................................... 77
8.3.16 Save command .................................................................................78
8.3.17 Shdsl command................................................................................. 78
8.3.18 Show command................................................................................. 78
8.3.19 Ver command .................................................................................... 78
9 FIRMWARE LOADING ............................................................................................ 79
9.1 Firmware loading guide................................................................................... 79
10 TECHNICAL SPECIFICATIONS ............................................................................. 80
10.1 Interfaces ........................................................................................................80
10.1.1 Monitor interface................................................................................ 80
10.1.2 Network management interface ........................................................80
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10.1.3 SHDSL interface................................................................................ 80
10.1.4 Network interface ..............................................................................81
10.2 Power supply................................................................................................... 81
10.2.1 Protection against dangerous affects................................................ 82
10.2.2 Surge safety ...................................................................................... 82
10.3 10.3 Climatic conditions .................................................................................. 82
10.4 10.4 Guarantee ............................................................................................... 83
10.5 10.5 Physical dimensions................................................................................ 83
11 CONNECTORS’ DESCRIPTION ............................................................................. 84
11.1 SHDSL connector ...........................................................................................84
11.2 Monitor connector ...........................................................................................84
11.3 Ethernet (10BaseT) connector ........................................................................ 85
11.4 Power connector (For MDS922AE-10BT)....................................................... 85
12 DESCRIPTION OF INTERFACE CABLES ............................................................. 86
12.1 «Direct» Ethernet cable................................................................................... 86
12.2 Cross-over Ethernet cable ..............................................................................87
12.3 Monitor connector ...........................................................................................87
13 DELIVERY SET .......................................................................................................88
14 GLOSSARY ............................................................................................................. 89
15 EXAMPLE OF NETWORK CONFIGURATION ....................................................... 92
15.1 Router 1 ..........................................................................................................92
15.2 Router 2 ..........................................................................................................94
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VERSION CONTROL
version Date
0.1 30. Jul. 2001 First version
0.2 9. Sept. 2001 Pre-official version
1.0 11. Feb. 2003 Layout, Small changes throughout the text
Major changes to previous version
Chapter 4.2 is changed
Small changes throughout the text
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1 INTRODUCTION
MDS921AE-10BT, MDS920C-10BT and MDS922AE-10BT network and line termination units are
a part of the Black Box PAM family (hereafter family) constructed for the organization of highspeed communication channels over one pair copper lines (DSL).
This family of units represents G.shdsl modems that have 64 – 2312-kbit/s speed of data transfer.
The modern type of TC-PAM encoding has the best characteristics of long-distance data
transmission and electromagnetic compatibility while working over one pair subscriber lines. TCPAM can be deciphered as Trellis Coded Pulse Amplitude Modulation. The essence of this
encoding method consists of an increase of layer numbers (encoding states) from 4 (as in 2B1Q)
to 16 and use of a special error-correction mechanism.
This family of modems with different network interfaces (G.703, Nx64 (V.35/V.36/X.21) and
Ethernet 10Base-T) can be used as transfer systems between multiplexers, routers and crossconnection devices in different networks, for example:
1. for the organization of E1 (2048 Kbit/s) channels between Public automatic branch
exchange (PABX), Digital Loop Carrier systems, TDM multiplexing and terminal stations
of mobile networks as well as their connection to SDH networks;
2. for the organization of high-speed communication channels (data links) in data transfer
networks and connection of Internet-providers’ access nodes;
3. for the connection of remote working stations (computers) and small Ethernet branches to
the office computer network and for integration of IP and IPX network segments, for
providing Internet access, etc.
The Ethernet 10BaseT interface allows an operator to provide services for the interconnection of
territorially distributed local networks, to provide high-speed access to Internet and to use
MDS921AE-10BT, MDS920C-10BT and MDS922AE-10BT devices in applications that require
high speed data transmission.
The use of ATM technology makes it possible to connect MDS921AE-10BT, MDS920C-10BT and
MDS922AE-10BT units to DSLAM devices of different manufacturers.
The units of this family, subdivided into network termination (NTU) and line termination units
(LTU), can be installed at the customer (user) premises and at the operator (provider) nodes,
respectively. NTU–NTU connections (for instance for the connection of two local networks) or
LTU–LTU connections (for the connection of large nodes) can be used to organize “point-topoint” connection.
It is possible to power units locally with 48 V
(telephone station batteries) and with 220 VAC.
DC
This family of devices has three mechanic designs:
1. Sub-Rack (MDS920C-10BT) is a unit to be mounted in 19’’ chassis (MDS920AE-RMDC)
2. Mini-Rack (MDS922AE-10BT) is a unit of 1U height (44.5 mm) to be mounted in 19’’ rack
cabinet;
3. Stand Alone (MDS921AE-10BT) is a compact unit to be mounted on the tabletop/desktop
or another horizontal surface.
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The units have all the possibilities for monitoring and management. Different management
protocols that are used in the firmware of the family allow one to implement:
1. local management using a computer, which supports the VT 100 type emulation of the
terminal;
2. remote monitoring and configuring over Telnet protocol;
3. remote monitoring and configuring over HTTP protocol;
4. support of the CMU SNMP-agent for remote monitoring and configuring while working as
a part of sophisticated networks under Simple Network Management Protocol (SNMP).
The use of Flash-memory chips as read only memory (ROM) facilitates the loading of new
firmware versions.
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3 SPECIFICATION
• High-speed symmetric data transfer over a 135-ohm physical twisted copper pair according to
G.shdsl ITU G.991.2;
• TC-PAM line encoding;
• line speed from 72 Kbit/s to 2320 Kbit/s;
• automatic and manual line speed adjusting;
• Ethernet 10Base T interface;
• bridge and router function;
• built-in 4-port HUB on;
• AAL5 for ATM over SHDSL;
• function of traffic priorities;
• DNS support;
• built-in DHCP server;
• NAT support;
• static and dynamic routing, RIP;
• built-in function of diagnostics and self-testing;
• low power consumption;
• console port for local management;
• TELNET and HTTP management;
• built-in SNMP agent;
• possibility of remote firmware loading through TFTP protocol;
• different types of mechanic design.
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4 TECHNOLOGIES
4.1 xDSL technology, background
xDSL technology appeared due to the growing user’s demand to high-speed digital stream
transfer over telephone copper pairs. Operators had to organize the interconnection of backbone
stations of cellular networks, Digital Loop Carrier systems, interstation connection and to provide
high-speed Internet access at minimal expenses. In these circumstances, it was reasonable to
use the existing telephone cables. The new technology acquired the name – xDSL (Digital
Subscriber Line).
“x” key
x is a variable in the DSL technology, where every word has its own meaning. Thus the term
“Digital” means, that not an analogue but digital signal, that was processed by one of line
encoding methods, is transmitted over pairs. In fact, the term xDSL points at this or that line code
the distance of data transfer and maximal connection speed depend on it. However, some
technologies, for example ADSL, can use one of the two line codes: either Discrete Multi-Tone
(DMT) or Carrierless Amplitude/Phase (CAP).
The term “Subscriber Line” is referred to physical copper pairs of telephone cable, or in simple
words, to “direct wires”. The term DSL was originally referred to the ISDN technology, but later
was borrowed by the developers of xDSL technologies.
4.1.1 Asymmetric DSL (ADSL) technology
The most popular DSL technology, ADSL, was developed in Bellcore laboratory in late 1980s.
Standards Institutes assigned the use of carrier set modulation, which acquired the name DMT,
to ADSL, while another leading method received the name of Rate-Adaptive DSL (RADSL).
ADSL transmits downstream to the end user and upstream to the net. The ADSL technology
does not use 25–30-kHz frequencies, used for a subscriber’s access to public switched telephone
network (PSTN). This provides simultaneous subscriber’s access to data transfer networks and
PSTN over the same copper pair. The original ADSL implies the presence of splitters both on
LTU and NTU. However, ADSL without splitters found its use and acquired the name of ADSL
Lite or G.lite. Later, it was standardized by ITU-T. This standard supports downstream speeds up
to 1.5 Mbit/s and upstream speeds up to 512 Kbit/s.
4.1.1.1 ADSL in brief
Standard
• G.lite G.992.1 (G.DMT)
• T1 413-1998
• Interoperability between equipment of different manufacturers
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Transmission rate
• Downstream
• up to 6–8 Mbit/s
• up to 1.5 Mbit/s for G.lite
• Upstream
• up to 640 Kbit/s
• up to 512 Kbit/s for G.lite
Line code
• DMT
• CAP
Number of pairs
• one pair
Usage
• public network operators (PNO) and Internet service providers
Restrictions
• Asymmetry
4.1.2 ISDN DSL technology
The IDSL technology is based on the ISDN technology, but without switching. IDSL uses 2B1Q
line encoding and has two B and one D channel capacity, which allows transmitting data bidirectional at 144 Kbit/s. The necessity to transmit simultaneously voice and data served as a
powerful spur to the further development of IDSL. Thus, the channel capacity is divided between
voice unit and digital interface. NTU-128 Voice is an example of a device implementing this
mode,
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4.1.2.1 IDSL in brief
Standard
• T1.601
• interoperability between equipment of different manufacturers at the U interface layer
Transmission rate
• up to 144 Kbit/s
• up to 64 Kbit/s + voice channel in NTU-128 Voice
Line code
• 2B1Q
Transmission medium
• one pair
• possibility of the regenerator’s installation
Usage
• PNOs and providers of Internet services
• commercial operators
• integration of LANs
Restrictions
• low speed
• impossibility of transmission rate adjusting
4.1.3 High bit rate DSL (HDSL) technology
The HDSL technology allows transmitting synchronous digital data at 1.54- or 2.048-Mbit/s speed
over two copper pairs. This standard was accepted by European Telecommunication Standards
Institute. 2B1Q or CAP 64 are used as line codes. The data transmission rate over each pair is
1168 Kbit/s, the decrease of linear rate is not provided. The 2B1Q encoding HDSL technology
allows to connect up to 3 remotely powered regenerators.
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4.1.3.1 HDSL in brief
Standard
• ETSI TS 101 135
• interoperability between equipment of different manufacturers is not provided
Transmission rate
• 1168 Kbit/s over each pair (2 Mbps over two pairs)
Line code
• 2B1Q
• CAP 64
Transmission medium
• two pairs
• possibility of the regenerator’s installation (up to 3)
Usage
• PNOs
• long-haul E1 transmission
• organization of trunk lines between PABX
• increase of capacity of subscribers’ lines with the help of Digital Loop Carrier systems
• high-speed access to SDH networks
Restrictions
• two pairs are used for stream transmission
• impossibility to regulate transmission rates
• increased influence on analog systems with frequency division multiplexing/demultiplexing
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4.1.4 EXTRAns technology
xDSL solutions are widely used for the organization of interstation trunk lines, creation of routes
for multiplexers and routers. But the wide spread of analog systems with frequency division
multiplexing/demultiplexing of the K-60, K-24, K-12 types makes it difficult to use standard xDSL
solutions over backbone (trunk lines)
and zone cables of the different types with the wire
diameter of 0.9–1.2, if one of the analog systems works over cables. To digitalize local and zone
lines, NTC NATEKS engineered the EXTRAns technology based on asymmetric adaptive
multimode CAP modulation with a regulated level. This technology allows transmitting
synchronous digital stream with a changeable line speed from 144 to 2064 Kbit/s over two copper
pairs. The technology stipulates the installation of up to 6 remotely powered regenerators. The
number of regenerators can be doubled if they are remotely powered from two-manned repeater
station, where power source is available.
4.1.4.1 EXTRAns in brief
Standard
• Patent № 2001104235/20(004956) of the Federal Institute of Industrial Property
• interoperability with the equipment of other manufacturers is not provided
Transmission rate
• 144–2064 Kbit/s
Line code
• CAP-EXTRAns
Transmission medium
• two pairs
• possibility of the regenerator’s installation (up to 6)
Usage
• PNOs, Competitive Local Exchange Carriers (CLECs)
• creation of long-haul digital routes with many regenerating segments
• organization of trunk lines between PABX
• increase of capacity of subscribers’ lines with the help of Digital Loop Carrier systems
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• high-speed access to SDH networks
Restrictions
• two pairs are used for full stream transmission
4.1.5 Multispeed DSL (MDSL) technology
The term SDSL was used for several years on the market. It was referred to all solutions meant
for the synchronous digital stream transmission over one pair. This technology supports the
possibility of line speed regulation over long-haul distances. The technology is implemented in
MDSL and MSDSL, the latter one having a different line code. The MDSL technology uses 2B1Q
line code. The transmission rate varies from 144 to 2320 Kbit/s. Regenerators are not used here.
4.1.5.1 MDSL in brief
Standard
• ETSI TS 101 135
• interoperability between equipment of different manufacturers at the level of DSL chips
Transmission rate
• 144–2320 Kbit/s
Line code
• 2B1Q
Transmission medium
• one pair
• impossibility of the regenerator’s installation
Usage
• Internet service providers
• access to Internet
• integration of LANs
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Restrictions
• the shortest distance of data transfer over one wire compared to other technologies
4.1.6 Multispeed DSL (MSDSL) technology
The MSDSL technology is a further development of the MDSL technology. It allows to run over
longer distances because of using a more progressive line code – CAP. In addition, it is possible
to install a CAP-splitter, which allows using the copper pair for both data transfer and telephoning
connection. The technology supports the installation of a line regenerator. However, the spectral
characteristics of the CAP code interfere with other xDSL systems, running over the neighboring
pairs in the same cable. The ADSL technology is exposed to the greatest influence.
4.1.6.1 MSDSL, in brief
Standard
• ETSI TS 101 135
• interoperability between equipment of different manufacturers is not provided
Transmission rate
• 144–2064 Kbit/s
Straight-line code
• CAP8…CAP128
4.1.6.2 Transmission medium
• one pair
• one regenerator
Usage
• PNOs, Internet service providers
• access to Internet
• integration of LANs
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• creation of trunk lines between PABX
• increase of capacity of subscribers’ lines with the help of Digital Loop Carrier systems
• high-speed access to SDH networks
Restrictions
• absence of compatibility with the equipment of other manufacturers
• interference with other xDSL services
4.1.7 G.shdsl technology
The G.shdsl technology was engineered as a universal technology of synchronous digital data
transmission. It became an international standard for symmetric systems. The technology
supports the transmission over one and two pairs. Special stress, while developing the
technology, was laid to provide spectral compatibility with other technologies such as ADSL,
IDSL, MDSL, MSDSL.
4.1.7.1 G.shdsl, in brief
Standard
• ITU-T G.991.2
• compatibility with the equipment of other manufacturers
Transmission rate
• 192–2360 Kbit/s
Line code
• TC-PAM
Transmission medium
• one or two pairs
• possible installation of up to three regenerators
Usage
• PNOs, Internet service providers
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• access to Internet
• integration of LANs
• creation of trunk lines between PABX
• increase of subscribers’ lines with the help of Digital Loop Carrier systems
• high-speed access to SDH networks
4.2 Local area network integration. Access to Internet
Local area networks facilitate documentation-processing, access to data in modern companies
but the Ethernet technology does not allow to transfer data at long distances and create Wide
Area Networks (WANs). xDSL can be used for the solution of this problem.
4.2.1 TCP/IP stack structure
TCP/IP became widely practiced with the development of the Internet all over the world. It was
engineered earlier than the OSI model, and that is why differs greatly.
Fig.1 shows the TCP/IP structure.
TCP/IP protocols are composed of 4 layers:
Layer IV
The lowest layer (Layer IV) corresponds to the physical and data link layers of the OSI reference
model. This layer in TCP/IP is not regulated, but it supports all the popular physical and data link
layer standards: for LANs, this is Ethernet, for WANs, these are Point-to-Point Protocols, SLIP,
Frame Relay. However, when a new LAN and WAN technology appears, it is usually included
into TCP/IP stack because of a specially engineered request for comments (RFC), which
determines the encapsulation method of IP packets into its frames. Thus, for the encapsulation of
IP protocols into ATM cells, there was engineered a special RFC 1483 method. This method is
used in Black Box PAM modems as well.
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Fig.1 Correspondence of TCP/IP layers with the OSI model layers
Layer III
The next layer (Layer III) is a layer of internetwork interconnection, which enables packet
transmission using different transmission media, LANs, WANs, xDSL, etc.
The Internet Protocol is used as the primary protocol of this layer (session layer in terms of the
OSI model).
All protocols, connected with data collecting and updating of routing tables, such as Routing
Internet Protocol (RIP) refer to this layer. This protocol is used in Black Box PAM modems.
Layer II
Layer II is sometimes called basic. The Transmission Control Protocol (TCP) and User Datagram
Protocol (UDP) function at this level. TCP provides reliable packet transmission using virtual
links. UDP, as well as IP, enables datagram application packet transmission. It functions as a
connecting link between network protocols and numerous application processes.
Layer I
Layer I is called the application layer. It contains a great number of application layer protocols
and services. Such widely used protocols as File Transfer Protocol (FTP), Telnet terminal
emulation protocol, Simple Network-Management Protocol (SNMP) (used in the e-mail), WWW
protocols and many others belong to this layer.
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4.2.2 Address assignment in IP networks
Any IP-network device is characterized by the addresses of three groups:
Physical address. It is a hexadecimal MAC address of the network adapter or port. The MAC
address is unique and is 6-byte long: the first 3 bytes are the manufacturer’s identifier and the
other 3 bytes are uniquely assigned by the manufacturer itself. For example, 18-B7-34-39-AA-FC.
Network address (IP address). It is assigned during the configuring of network devices by the
administrator and does not depend on the physical address. The address has a decimal
representation and its length is 4 bytes. It consists of two parts: network number and node
number. Depending on the class of the network, different quantity of bytes is assigned to the
network number..
IP address classes
The network address consists of two logical parts: network and node number. The values of the
first address bits mean what part of the address refers to the network number and what to the
node number:
• Class A networks. The network number takes one byte, the other three show the node number
in the network. Class A networks can only have numbers in the 1.0.0.0–126.0.0.0 range.
Networks with number are not used, and number 127 is reserved. The node count must be more
than 126 but less than 224. The first bit of the network address of Class A must start with 0.
• Class B networks. The network and node numbers take two bytes each. Class B networks can
have numbers in the 128.0.0.0–191.255.0.0 range. The node count must be more than 28 but
less than 216. The network address of Class B must have the first two bits equal to 10.
• Class C networks. The network number takes three bytes. Class C networks can have numbers
in the 192.0.1.0–223.255.225.0 range. The node count must no be more than 28. The network
address of Class C must have the first three bits equal to 110.
• Class D networks. The networks of this class have a special multicast address. Class D
networks can have numbers in the 224.0.0.0–239.255.225.225 range. All nodes that have this
address will receive a packet with an address that belongs to Class D network. The network
address of Class D must begin with a sequence of 1110.
• Class E networks. The networks of this class are not used and they are reserved for future
(experimental) usage. Class E networks can have numbers in the 240.0.1.0–247.255.225.225
range. The network address of Class E must begin with a sequence of 11110.
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4 bytes
1 2 3 4
Class А
0 Network № Node №
Class B
1 0 Network № Node №
Class C
1 1 0 Network № Node №
Class D
1 1 1 0 Multicast address
Class E
1 1 1 1 0 Reserved
Masks
Network mask is a number, consisting of four bytes. It is a decimal number divided by dots, and it
is used together with the IP address. A mask usually contains decimal numbers – 255. The use of
masks allows providing users with narrow address ranges compared to networks of different
classes. The least dedicated range without masks is Class C network, i.e. 256 addresses. Using
masks, the entry 192.168.1.253 mask 255.255.255.252 defines the address 192.168.1.253 in the
subnet of four-address range: from 192.168.1.252 to 192.168.1.255.
4.2.2.1 Automatic assignment of IP addresses
The administrator can assign IP addresses to network devices either manually or automatically. If
there are many devices in the network, the address assignment is a long and painstaking
process. Dynamic Host Configuration Protocol (DHCP) was developed to facilitate this process.
The primary task of DHCP is dynamic IP address assignment. However, besides dynamic, DHCP
can support simpler means of manual and automatic statistic address assignment.
The administrator takes active part during the manual procedure of address assignment. He
presents information about correspondence of IP addresses to MAC addresses or other
customer’s identifiers to DHCP server.
During the automatic-static address assignment, the DHCP server assigns a free IP address from
the IP address range without reference to the administrator. The administrator gives the
boundaries of the address range during the DHCP-server configuration. In this case, the IP
address remains the same all the time.
During the dynamic address assignment, the DHCP server assigns an address to the customer
for a limited period of time. It means that later other computers can reuse the IP address.
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The dynamic address assignment allows one to create IP networks in which the number of nodes
exceeds the number of the IP addresses administrator has.
4.2.3 Bridging of local networks
Bridges are the simplest devices for logical network structuring. They divide the transmission
network medium into segments (logical segments), forwarding data from one segment to another,
if such a transmission is necessary, i.e. if the destination address belongs to another subnet.
Bridges are data communication devices that operate at the data link layer of the OSI reference
model. They use addresses of computers and other devices. Bridges control data flow, handles
transmission errors, provides physical (as opposed to logical) addressing and manage access to
the physical medium. Bridges provide these functions by using various link-layer protocols that
dictate specific flow control, error handling, and addressing and medium-access algorithms.
The primary advantage of bridging is the upper-layer protocol transparency. Because bridges
operate at the data link layer, they are not required to examine upper layer information. It means
that that they can rapidly forward traffic representing any network layer protocol.
By dividing large networks into self-contained units, bridges provide a range of additional
advantages. First, because only a certain percent of traffic is forwarded, bridges diminish traffic
passing through devices of all connected segments. Second, bridges act as a firewall for some
potentially damaging network errors. Third, bridges allow communication between a larger
number of devices than any single LAN connected to the bridge would support. Fourth, bridges
extend the effective LAN length, permitting the attachment of distant stations.
Types of bridges
Bridges can be either local or remote. Local bridges provide a direct connection of subnet
segments in the same area. Remote bridges connect subnet segments in different areas, usually
over telecommunication lines. The MDS92xxx-10BT, device belongs to remote bridges.
Remote bridging represents several unique internetworking challenges. One of them is the
difference between LAN and WAN speeds. Vastly different LAN and WAN speeds sometimes
prevent users from running delay-sensitive network applications over the WAN.
Remote bridges cannot increase WAN speeds, but they can compensate for the speed
discrepancies by using buffering capacities. If a LAN device capable of a 10-Mbit/s transmission
rate intends to communicate with another remote LAN device, the local bridge must regulate the
10-Mbit/s information flows in order not to overwhelm the 2-Mbit/s serial link. It is done by storing
the incoming data in buffers and transmitting it over a serial link. This can be achieved only for
short bursts of data that do not overwhelm the bridge’s buffering capacity.
The MDS92xxx-10BT device implements “transparent bridge” and “spanning tree” algorithms.
The “transparent bridge” is called so because its presence and operation is transparent to all
network hosts.
A bridge builds its own address table while passively monitoring the traffic. At this stage it
extracts the information about source addresses of data frames. The source address shows that
it belongs to a certain node of this or that network segment. Fig. 2 shows the creation of an
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address table of a simple network consisting of two segments.
Fig.2 Simple network
MAC address Port
1
2 3 4
Port 1
Port 2
Segment 2Segment 1
Bridge
1 1
2 1
3 2
4 2
Each port work as an end node of the network segment. Originally, the bridge does not know
what nodes with what MAC addresses are connected to each of its port. That is why it sends any
received frame to all ports excluding the port from which the frame was received. Simultaneously
the bridge studies the source address of the frame and fills its table: what port (of a MAC
address) belongs to this or that segment.
Later, the bridge uses its table to forward the traffic. When one of the bridge interfaces receives
the information unit, the bridge seeks for the destination address in its internal table. If the table
contains an association between the destination address and any of the ports of this bridge,
excluding the one the information unit was received on, then the unit is forwarded from the
indicated port. If such an association is not established, the information is flooded to all ports,
except the inbound port. Broadcast and multicast-address messages are also flooded as in the
previous case.
Transparent bridges isolate in-segment traffic, thus reducing the traffic clearly seen in each
individual segment. This improves the response time of the network, seen by the user. The extent
of the traffic shortening and response-time improvement depends on the volume of intersegment
traffic relative to total traffic as well as the volume of the broadcast and multicast traffic.
One of the drawbacks that interfere with the “transparent bridge” algorithm is the presence of
network “loops”. It is shown in Fig.3:
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Fig.3 Network with loops
Suppose that host A sends an information unit to host B. Both bridges receive this information
unit and conclude that host A belongs to network 2. Unfortunately, after host B receives two
copies of the information unit from host A, both bridges again receive the same information unit
onto their interfaces with network 1, because all hosts receive all messages of broadcast LANs.
In some cases bridges change their internal tables to indicate that host A is on network 1. When
host B rep[lies to the information unit of host A, both bridges will receive and then ignore these
replies, since their tables will indicate that this destination address (host A) is on the same
network segment as the information unit source.
Another disadvantage is cloning (proliferation) of broadcast messages in networks with loops.
Assume that the initial information unit of host A is a broadcast. Both bridges will forward this
information unit endlessly, using the available network bandwidth and blocking the transmission
of other packets on both segments.
To solve the above-described problems there was engineered the spanning tree algorithm (STA).
It preserves the benefits of loops, eliminating their drawbacks. The algorithm was published in the
IEEE 802.1d specification.
The STA designates a loop-free subset of the network's topology by placing those bridge ports
that, if active, would create loops into a standby (blocking) mode. Blocking bridge ports can be
activated in the event of primary link failure, providing a new path through the internetwork. Figs 4
and 5 illustrate how the STA eliminates loops.
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Fig. 4 Network before running STA
The STA calls for each bridge to be assigned a unique identifier. Typically, this identifier is one of
the bridge's Media Access Control (MAC) addresses plus a priority. Each port in every bridge is
also assigned a unique (within that bridge) identifier (typically, its own MAC address). Finally,
each bridge port is associated with a path cost. The path cost represents the cost of transmitting
a unit onto a LAN through that port. In Fig. 4, path costs are noted on the lines emanating from
each bridge. Path costs are usually defaulted, but can be assigned manually by network
administrators.
The first step in spanning-tree calculation is the selection of the root bridge, which is the bridge
with the lowest value bridge identifier. In Fig. 4, the root bridge is Bridge 1. Next, the root port on
all other bridges is determined. A bridge root port is the port through which the root bridge can be
reached with the least aggregate path cost. This value (i.e. the least aggregate path cost to the
root) is called the root path cost.
Finally, designated bridges and their designated ports are determined. A designated bridge is the
bridge on each LAN that provides the minimum root path cost. A LAN's designated bridge is the
only bridge allowed to forward information units to and from the LAN for which it is the designated
bridge. A LAN's designated port is the port that connects it to the designated bridge.
In some cases, two or more bridges can have the same root path cost. For example, in Fig. 4,
both Bridges 4 and 5 can reach Bridge 1 (the root bridge) with a path cost of 10. In this case, the
bridge identifiers are used again, this time to determine the designated bridges. The priority is
given to LAN V of Bridge 4 over LAN V port of Bridge 5.
Using this process, all but one of the bridges directly connected to each LAN are eliminated,
thereby removing all loops between two LANs. The STA also eliminates loops involving more
than two LANs, while still preserving connectivity. Fig. 5 “Network after running STA” shows the
results of implementing the STA to the network shown in Fig. 4. Comparison of these two figures
illustrates that the STA placed Bridge 3 and Bridge 5 ports to LAN V into the standby mode.
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Fig. 5: Network after running STA
The spanning-tree calculation occurs when the bridge is powered up and whenever a topology
change is detected. The calculation requires communication between the spanning-tree bridges,
which is implemented through configuration messages. Configuration messages contain
information identifying the bridge that is assumed to be the root (root identifier) and the distance
from the sending bridge to the root bridge (root path cost) and also the bridge and port identifier
of the sending bridge and the age of information contained in the configuration message.
Bridges exchange configuration messages at regular intervals (typically 1–4 s). If a bridge fails
(causing a topology change), neighboring bridges will soon detect the lack of configuration
messages and initiate a spanning-tree recalculation.
The MDS92xxx-10BT device implements both transparent bridge and spanning tree algorithms.
4.2.4 Routing of networks
The word “routing” means forwarding information through an internetwork from source to
destination. At least one node must be passed when transmitting data. Routing is often
contrasted with bridging. The main difference between bridging and routing consists in the fact
that bridging occurs at the data link layer of the OSI reference model, while routing occurs at the
network layer. It means that routing and bridging use different information while moving it from
source to destination. It results in different way of implementing their tasks.
4.2.4.1 Routing components
Routing consists of two basic activities: determination of optimal routing paths between source
and destination and data transmission through network. The latter is called switching.
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Optimal path determination
The determination of the optimal path is based on different standards of measurement, for
example, path length, and metric. Routing algorithms calculate path indexes to determine the
optimal path to destination.
To facilitate the process of path determination, routing algorithms initialize and maintain routing
tables, which contain the routing information. This information changes depending on the routing
algorithm used.
Routing algorithms fill in routing tables with different information. “Destination/next hop”
combinations tell a router that a destination can be reached through the shortest path by sending
a packet to a particular router representing the “next hop” on the way to the final destination.
When the router receives an incoming packet, it checks the destination address and makes an
attempt to associate this address with a next hop. An example of a routing table is shown below.
Destination address Next hop
27 Router A
57 Router B
17 Router C
24 Router A
52 Router A
16 Router B
26 Router A
Routing table also contain other information. “Metrics” represent information about the desirability
of a path or a route. Routers compare metrics to determine the optimal routes. Metrics differ
depending on the routing algorithms being used. A variety of common metrics will be described
below in this chapter.
Routers communicate with each other (and maintain their routing tables) by transmitting various
messages. One of these messages is the “routing update”. The routing update usually includes
all or a part of a routing table. By analyzing routing update information from all routers, any router
can build a detailed picture of network topology. Another example of a message exchange
between routers is a “link-state advertisement”. Link state advertisements inform other routers
about sender’s link-states. Link information also can be used to build a full picture of network
topology. After the network topology is determined, routers can determine optimal paths to
destinations.
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Switching
Switching algorithms are relatively simple and are basically the same for most routing protocols.
In most cases, a host determines the necessity of sending a packet to another host. Having
received a router's address, the source host sends a packet addressed specially to a router's
physical (MAC layer) address, however, the packet contains (network-layer) protocol address of
the destination host.
After checking the packet's destination protocol address, the router determines whether the
destination address is in the routing table. If the router did not find the address in the routing
table, it typically drops the packet. If the router knows where to forward the packet, it changes the
destination physical address to that of the next hop and transmits the packet.
During the packet transmission through an internetwork, its physical address changes, however,
the address of the network-layer protocol remains unchanged. Fig. 6 illustrates this process.
Fig. 6 Change of packet addresses
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4.2.4.2 Routing algorithms, RIP
The rate of information processing and its trustworthiness depend on the routing algorithm. But
more complicated and high-speed algorithms imply high requirements to the router’s capacity.
Static routing algorithms are the simplest ones. The network administrator establishes routing
tables, and they do not change until the network administrator changes them. Algorithms of static
routers are simple to design and they work well in simple networks with low traffic.
Dynamic routing algorithms are more complicated ones. They adjust in real time to network
changes. They do this by analyzing incoming routing update messages. If the router receives a
message about a network change, it makes updates it’s routing table and sends out this
information to all the nodes.
The Routing Information Protocol (RIP), implemented in MDS92xxx-10BT modems is a dynamic
routing protocol.
RIP routing tables contain information about packet destination, next hop, and hop counts
(metrics). The routing table can also contain other information such as timers.
Destination Next hop Distance Timers Flags
Network A Router 1 3 t1, t2, t3 x,y
Network B Router 2 5 t1, t2, t3 x,y
Network C Router 1 2 t1, t2, t3 x,y
RIP supports only optimal routes to destinations. If new information provides a better route, this
information updates the old one. Changes in the network topology can cause changes in the
routes, resulting, for example, in creation of better routes to a definite destination. If the network
topology changes, these changes are reflected in updating messages. For example, when a
router finds a failure of one of the links or another router, it recalculates its own routes and sends
out routing updates. Each router that receives routing update messages, includes changes to its
tables and sends them out.
4.2.4.3 Internet Access through LANs, NAT
The Network Address Translation technology allows one to solve to main problems the Internet
faces now. This is a restriction of the address space of IP and routing scaling.
If necessary to get an Internet access, when the number of network nodes connected to the
Internet provider is bigger that the number of IP addresses, NAT allows private IP networks,
using unregistered addresses, to get an access to Internet resources. NAT functions are
configured on a border router, dividing Intranet and Internet networks.
If necessary to change internal address system, instead of a complete change of all the
addresses (and this is quite a pain-taking process), NAT allows translating them according to the
new address plan.
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If there is a necessity to divide traffic on the basis of TCP ports, NAT makes it possible to map
local addresses with one external address using TCP load distribution function.
NAT functioning
The NAT technology defines, as it is stated in the RFC 1631 standard, the ways of IP address
translation, used in one network into another network addresses.
There exist three basic principles of address translation: static, dynamic and masquerading.
Static Network Address Translation
With the help of this concept, NAT can organize translation between the same class networks.
(For example, when each of two networks contain one address (mask – 255.255.255.255). This
strategy is the simplest, because the translation can be described by a couple of simple logical
transformations.
Let us cite an example of address translation from two Class C networks – 194.24.90 and
195.60.3. While passing through NAT to the sender’s address field, the packet, addressed from
the host 194.24.90.13 will contain a change in the IP header from 194.24.90.13 to 195.60.3.13.
Dynamic Address Translation
Dynamic translation is necessary when the number of addresses (internal and external) being
translated is different, however, dynamic translation is sometimes used when static translation
does not work. The number of intercommunicating hosts will be limited, in any case, by the
number of free (available) addresses on the NAT interface.
Dynamic NAT is more complicated, because it requires to keep track of intercommunicating hosts
and possibly even of connections, in case when the information (content) must be modified at
Layer 4 (TCP, for example).
For example it is necessary to translate dynamically all IP addresses in Class B network 138.201
into addresses of Class C network 190.200.112. Then, each new connection receives an address
from Class C network if there are available addresses there.
This technology, in contrast with static translation, introduces a new notion – NAT table. It is a
rendition table of internal addresses and NAT-interface addresses (hereinafter, NAT addresses)
Masquerading (NAPT, PAT)
The Port Address Translation is another case of dynamic translation. Here, we have only one
external address behind which, internal addresses “are hidden” – there can be as many internal
addresses as possible. In contrast to the original dynamic translation, PAT does not mean that
there can be only one connection at a time. To multiplex the number of connections, TCP port
information is used by this masquerading. Thus, only the number of ports available limits number
of simultaneous connections.
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