Cisco PA-A2, PA-A2-4T1C-OC3SM=, PA-A2-4T1C-T3ATM=, PA-A2-4E1XC-OC3SM=, PA-A2-4E1XC-E3ATM= Installation And Configuration Manual

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PA-A2 ATM CES Port Adapter Installation
and Configuration

Product Numbers: PA-A2-4T1C-OC3SM(=), PA-A2-4T1C-T3ATM(=),
PA-A2-4E1XC-OC3SM(=), PA-A2-4E1XC-E3ATM(=)
Platforms Supported: Cisco 7200 Series, Cisco uBR7246

Text Part Number: OL-3460-01

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PA-A2 ATM CES Port Adapter Installation and Configuration

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CONTENTS

Preface: PA-A2 ATM CES Port Adapter Installation and Configuration vii

Objectives vii
Organization viii
Related Documentation viii
Obtaining Documentation ix

World Wide Web ix
Document ation C D-R OM ix
Ordering Documentation x
Document ation Fe edb ack x

Obtaining Technical Assistance x

Cisco.com x
Technical Assistance Center xi

Cisco TAC Web Site xi
Cisco TAC Escalation Center xii

CHAPTER

1 Overview: PA-A2 ATM CES Port Adapter 1-1

Circuit Emulation Services Overview 1-1

Circuit Emulation Services Internetworking Function 1-2

T1/E1 Unstructured (Clear Channel) CES Services 1-2
T1/E1 Structured (N x 64) CES Services 1-4
Channel-Associated Signaling (for Structured CES Services Only) 1-6

Network Timing Services for CES Operations 1-8

Network Clock Synchronization 1-8
Designating a PRS Source Port in a Cisco 7200 Series Router or Cisco uBR7200 series 1-8
Network Clock Distribution in a Cisco 7200 Series Router or a Cisco uBR7200 series 1-9

Clocking Modes for CES Operations 1-11

Synchronous Clocking Mode 1-11
Synchronous Residual Time Stamp Clocking Mode 1-12
Adaptive Clocking Mode 1-12
Summary of Clocking Modes 1-13

Other Network Factors Relevant to CES Operations 1-13

PA-A2 ATM CES Port Adapter Overview 1-15

Features 1-16

LEDs 1-17

Contents

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PA-A2 ATM CES Cables, Connectors, and Pinouts 1-18

ATM Port Cables 1-18

CBR Port Cables 1-19
ATM Management Information Base Specifications 1-20
Fibre-Optic Transmission Specifications 1-20

Maximum Transmission Distances for SONET 1-20

Power Budget 1-21

Approximating the ATM CES Port Adapter Power Margin 1-21
Using Statistics to Estimate the Power Budget 1-23

For Further Reference 1-23
Setting ATM CES Port Adapter Jumpers 1-23
PA-A2 ATM CES Port Adapter Slot Location s on the Supported Platforms 1-24

Cisco 7200 Series Routers and Cisco uBR7200 Series Routers Slot Numbering 1-25

Identifying Interface Addresses 1-27

Interface Addresses of Cisco 7200 Series Rout ers and Cisco uBR7200 Series Routers 1-27

CHAPTER

2 Preparing to Install the PA-A2 ATM CES PortAdapter 2-1

Required Tools and Equipment 2-1
Minimum Software and Hardware Requirements 2-1
Checking Hardware and Software Compatibility 2-2
Safety Guidelines 2-2

Safety Warnings 2-2
Electrical Equipment Guidelines 2-4
Telephone Wiring Guidelines 2-4

Preventing Electrostatic Discharge Damage 2-5
Laser/LED Safety 2-5
FCC Class A Compliance 2-6
BABT Compliance 2-7
CE Compliance 2-7

CHAPTER

3 Removing and Installing the PA-A2 ATM CES Port Adatper 3-1

Handling Port Adapters 3-1
Online Insertion and Removal 3-2
Warnings and Cautions 3-3
Port Adapter Slot Divider 3-3

Cisco 7200 Series Routers—Removing the Slot Divider 3-4

Cisco uBR7200 Series Routers—Removing the Slot Divider 3-5

Contents

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Port Adapter Removal and Installation 3-6

Cisco7200 Series Routers and Cisco7200 VXR Routers—Removing and Installing a Module 3-7
Cisco uBR7200 Series Routers—Removing a Module 3-8
Cisco uBR7200 Series Routers—Installing a Module 3-9

CHAPTER

4 Configuring the PA-A2 ATM CES Port Adapter 4-1

Using the EXEC Command Interpreter 4-1
PA-A2 ATM CES Configurations 4-2

Shutting Down an Interface 4-3
Performing a Basic Interface Configuration 4-4
Configuring the PA-A2 ATM CES Port Adapter for T3 4-5
Configuring the PA-A2 ATM CES Port Adapter for E3 4-5
Configuring the PA-A2 ATM CES Port Adapter for OC-3 4-6
Configuring VCs 4-6
Configuring PV Cs 4-7

Configuring a PVC on a Point-to-Point Subinterface 4-7
Configuring a PVC on a Multipoint Subinterface 4-8

Configuring SVC s 4-9

Configuring the PVC That Performs SVC Call Setup 4-10

Configuring the Network Service Access Point Address 4-11
Configuring Classical IP and ARP over ATM 4-13
Classes of Service and Transmit Priority on the PA-A2 ATM CES Port Adapter 4-13

Customizing the PA-A2 ATM CES Port Adapter 4-14

Setting the MTU Size 4-15
Configuring an ATM Interface for Local Loopback 4-15
Configuring an ATM Interface for External Loopback 4-15

Checking the Configuration 4-15

Using show Commands to Verify the New Interface Status 4-16

Using the show version or show hardware Commands 4-16

Using the show diag Command 4-17

Using the show interfaces Command 4-17
Using the ping Command to Verify Network Connectivity 4-18
Using loopback Commands 4-19

Traffic Management 4-20
Troubleshooting the PA-A2 ATM CES Port Adapter Installation and Configuration 4-21

PA-A2 ATM CES Port Adapter Statistics 4-21
Using the debug atm Commands 4-22
Displaying ATM Information 4-23

ATM Configuration 4-24

Contents

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Upgrading Your Boot Flash Image 4-25
Port Adapter Error Messages 4-26

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Preface: PA-A2 ATM CES Port Adapter
Installation and Configuration

This prefa ce de s cr ibe s t he o bj ect ives an d organ ization o f th is d o cu ment and ex plains how to fi nd
additional information on related products and services. This preface contains the following sections:

• Objectives, page vii

• Organiz at ion, pa ge v iii

• Related Documentation, pageviii

• Obtaining Documentation, page ix

• Obtaini ng Technical A ssistance, p ag e x

Objectives

This document describes how to install and configure the dual-wide ATM circuit emulation services
(CES) port adapter, hereafter referred to as the PA-A2 ATM CES, which is used in the following
platforms :

• Cisco 7200 series routers, including the 2-slot Cisco 7202, 4-slot Cisco 7204

and Cisco 7204VXR, and the 6-slot Cisco 7206 and Cisco 7206VXR

• 6-slot Cisco uBR7246

For complete descriptions of interface subcommands and the configuration options that support ATM
CES por t adapter f u nc tio n ality, refer to p ub li cat ions lis ted i n th e “Related Documentation” section on

page v iii.

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Preface: PA-A2 ATM CES Port Adapter Installation and Configuration

Organization

Organization

This docu m en t co nt ain s t he f ollow in g ch ap ter s:

Related Documentat ion

The documentation listed below is ava ilable online, on the Documentation CD-ROM, or as printed
docum ents.

Your router, switch, or gateway and the Cisc o IOS software running on it con tain extensive featur es and
functionality, which are documented in the following resources:

• Cisco IOS software:

–

For configuration information and support, refer to the modular configuration and modular
command reference publications in the Cisco IOS software configuration documentation set
that corresponds to the software release installed on your Cisco hardware.

–

T o ch eck the m inimum so ftware require ment s of Cisco I OS sof tware wi th the hardwa re ins talled
on your router, Cisco maintains the Software Advisor tool on Cisco.com:
http:/ /w w w.cisco.c om/cgi- b in /S u p po r t/C o m p Nav /I nd ex.pl. You must be a r egi s te red use r o n
Cisco.co m to access thi s t oo l.

Note You can access Cisco IOS s o ftwa re co nfi gu ration and h ar d ware insta lla ti on

and maintenance documentation on the World Wide Web at

http://w w w.cisco.c om/publi c/ cou n tr ie s _l an gu ag es.shtml .

• Cisco 7200 series routers:

–

For port ad apte r hardw ar e and memo ry conf i gu rati on gui del ines , refer to the Cisco 7200 Series
Port Adapter Hardware Configuration Guidelines.

–

For hardware installation and maintenance information (including the Cisco 7206 or
Cisco 7 20 6 V XR a s a router s h el f in a C isc o A S 58 0 0 Un iversa l A ccess S erver ), r ef er to the
installation and configuration guide that shipped with your Cisco 7200 series router.

–

For information on network processing engines or network services engines, refer to the

Network Processing Engine and Network Services Engine Installation and Configuration

publication.

Section Title Description

Chapter 1 Overview: <XXXXX Module> Describes the module and its LED displays,

cables, and re cepta cles.

Chapter 2 Preparing to Install the <XXXXX

Module>

Describes safety considerations, tools required,
and procedures you should perform before the
actual in stallati on .

Chapter 3 Removing and Installing the

<XXXXX Module>

Describes the procedures for installing and
removing module.

Chapter 4 Configuring the PA-A2 ATM CES

Port Adapter

Provides instructions for configuring the module.

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Preface: PA-A2 ATM CES Port Adapter Installation and Configuration

Obtaining Documentation

–

For info rmation on the router boot images, refe r to the Cisco 7200 Series Routers Boot Im ages
Information publication.

• Cisco 7200 VX R routers:

• For hard w are inst all at io n and main te na nc e inf orm at ion , re fe r to the Cisco 7200 VXR Installation

and C on figuratio n Guide or t h e Cisco 7200 VXR Quick Start Guide.

• Cisco uBR7200 series routers:

• For insta llation and ma in ten ance in f orm a tion, ref er t o t he Cisco uBR7200 Series Hardware

Instal la tion Gu ide.

• For inte rnat ional age ncy co m p li an ce, safety, and statutory inform ati on f o r WAN interfaces:

–

Regulatory Compliance and Safety Information for the Cisco 7200 Series Routers

–

“Regulatory Compliance and Safety Information” appendix in the Cisco uBR7200 Series
Universal Broadband Router Hardware Installation Guide

–

Site Prep ar at ion and Saf ety Guid e

• To view Cisco documentation or obtain general information about the documentation, refer to the

followi ng sec t ions:

–

Obtaining Documentation, page ix

–

Obtaini ng Technical A ssistance, p ag e x

–

Customer service at 800 553-6387 or 408 526-7208. Customer service hours are 5:00 a.m. to
6:00 p.m. Pacific time, Monday through Friday (excluding Cisco-observed holidays). You can
also send e-mail to c s -rep @ cisco.c om.

Obtaining Document ation

The following sections explain how to obtain documentation from Cisco Systems.

World Wide Web

You can acces s th e most cur re nt C isco d o cu m en ta tio n o n t h e World Wide Web at the f o ll owin g URL:

http:/ /w w w.cisco.com

Translated do cu mentation i s availa ble at the fol lowin g U R L:

http://w w w.cisco.c om/publi c/ cou n tr ie s _l an gu ag es.shtml

Documentat i on CD-ROM

Cisco docum en tatio n and addi tional li terat ure ar e available in a Cisco Do cume ntati on CD- ROM
package , which is shipp ed wit h your pr oduct. The Doc umentat ion CD -ROM i s updat ed mont hly and may
be more cu r r en t t ha n p r in te d do cu m en ta ti on . T he CD - ROM p ack ag e is available as a single u n it or
through an annual subscription.

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Preface: PA-A2 ATM CES Port Adapter Installation and Configuration

Obtaining Technical Assistance

Ordering Docum entation

Cisco documentation is available in the following ways:

• Registered Cisco Direct Customers can order Cisco product documentation from the Networking

Products M a rk etP l ace :

http:/ /w w w.cisco.com/cg i- bin/or de r/order_r oot.pl

• Regis tere d Cis co. com use rs c an o rde r the Doc ument at ion CD-ROM t hro ugh t he onli ne Su bsc ri ption

Store:

http://www.cisco.com/go/subscription

• Nonregist er ed C isco.com us er s can orde r do cu m e nt ati on t hr o ugh a local ac co unt r epresent ative by

calling Cisco corporate headquarters (California, USA) at 408 526-7208 or, elsewhere in North
America, by calling 800 553-NETS (6387).

Documentati on Feedback

If you are reading Cisco product documentation on Cisco.com, you can submit technical comments
electronically. Click Leave Feedback at the bottom of the Cisco Documentation home page. After you
complete the form, print it out and fax it to Cisco at 408 527-0730.

You can e-mail your c o m ments to bug-d oc@ci s co . co m .
To submit your comments by mail, use the response card behind the front cover of your document, or

write to th e fo ll owi ng a dd r es s:
Cisco Systems

Attn: D oc ument R es o ur c e Connect io n
170 West Tasman Drive
San Jose, CA 95134-9883

We appreciate your comments.

Obtaining Technical Assistance

Cisco p rovi des Cisco.com a s a star t in g po in t fo r al l t ech n ica l assistan ce. Customers and p ar t ne rs can
obtain documentation, troubleshooting tips, and sample configurations from online tools by using the
Cisco T e c hnical Assistance Center (TAC) Web Site. Cisco.com registered users have c omplete ac cess to
the technical support resources on the Cisco TAC Web Site.

Cisco.com

Cisco.com is the foundation of a suite of interactive, networked services that provides immediate, open
access to Ci s co in fo rm at io n, networking solutions, services, programs, and resources at any time, from
anywhere in the world.

Cisco.com is a highly integrated Internet application and a powerful, easy-to-use tool that provides a
broad range of features and services to help you to

• Strea m li ne business p ro cesses and i m p rove pr oduct ivit y

• Resolve technical issues with online support

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Preface: PA-A2 ATM CES Port Adapter Installation and Configuration

Obtaining Technical Assistance

• Download and test software packages

• Order Cisco learning materials and merchandise

• Register for online skill assessment, training, and certification programs

You can self-register on Cisco.com to obtain customized information and service. To access Cisco.com,
go to the follow in g U RL:

http:/ /w w w.cisco.com

Technical Assi stance Cen t er

The Cisc o TAC is availab le to a ll customer s wh o need tech ni cal ass i stance wit h a C isco produ ct ,
technology, or solution. Two types of support are available through the Cisco TAC: the Cisco TAC
Web Sit e a nd th e Cisco TAC Escalatio n Cen t er.

Inquiries to Cisco TAC are categorized according to the urgency of the issue:

• Priori ty level 4 (P4) —You need information or assistance concerning Cisco product capabilities,

product installation, or basic product configuration.

• Priori ty level 3 (P3) —Your network performance is degraded. Network functionality is noticeably

impaired, but most business operations continue.

• Priori ty level 2 (P2) —Your production network is severely degraded, affecting significant aspects

of business operations. No workaround is available.

• Priority level 1 (P1)—Your production n et w ork is d o wn, a nd a c rit ica l impa ct to bus i nes s op erat io ns

will occur if service is not restored quickly. No workaround is available.

Which Cisco TAC resource you choose is based on the priority of the problem and the conditions of
service co nt r act s , wh en ap p licable.

Cisco TAC Web Site

The Cisc o TAC Web Site allow s you to re sol v e P3 and P 4 is sue s y our sel f, saving both cost and time. The
site provides around-the-clock access to online tools, knowledge bases, and software. To access the
Cisco TAC Web Site , g o to the foll owi ng U R L:

http:/ /w w w.cisco.com/tac

All custom e rs, par tn er s, an d resellers wh o have a valid Ci s co serv i ces co nt ract have complete access to
the technical support resources on the Cisco TAC Web Site. The Cisco TAC Web Site requires a
Cisco.com login ID and password. If you have a valid service contract but do not have a login ID or
passwo r d, go to the following URL to r e gi s te r:

http:/ /w w w.cisco.com/r egis t e r /

If you cannot resolve your technical issues by using the Cisco TAC Web Site, and you are a Cisco.com
registered user, you can open a case online by using the TAC Case Open tool at the following URL:

http:/ /w w w.cisco.com/tac/caseo pen

If you have Internet access, it is recommended that you open P3 and P4 cases through the Cisco TAC
Web Si te.

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Preface: PA-A2 ATM CES Port Adapter Installation and Configuration

Obtaining Technical Assistance

Cisco TAC Escalation Center

The Cisc o TAC Escala tion Cen ter addresses iss u es t h at are class i fied as priorit y level 1 o r pri or it y
level 2; thes e cl as sifi cat io ns ar e a s sig ne d wh en sever e n et wo r k degr ad at io n s ignificantly i m p act s
busin ess ope ra tion s. Whe n you co ntac t t he TAC Escal at ion Ce nter wi th a P1 or P2 p rob le m, a Ci sco TAC
engineer w il l a ut om ati cally op en a case.

To obtain a directory of toll-free Cisco TAC telephone numbers for your country, go to the following
URL:

http:/ /w w w.cisco .com/wa rp /public/687/D irect or y /D irTAC.shtml

Before c all ing , pl eas e ch eck wit h your ne tw ork oper at ions center to dete rmi ne th e l e v el of Ci sco supp ort
services to which your company is entitled; for example, SMARTnet, SMARTnet Onsite, or Network
Supported Accounts (NSA). In addition, please have available your service agreement number and your
product serial number.

CHAPTER

1-1

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OL-3460-01

1

Overview: PA-A2 ATM CES Port Adapter

This chapter describes the PA-A2 ATM CES port adapter and contains the following sections:

• Circui t Emul at ion Service s O ver view, page 1 -1

• PA-A2 ATM CES Port Adapter Overview, page 1-15

• LEDs, page 1 -17

• PA-A2 ATM CES Cables, Connectors, and Pinouts, page 1-18

• ATM Ma na gemen t I n fo rmation Ba se S p ecificat i on s , page 1- 20

• Fibre-Optic Transmission Specifications, page 1-20

• Setting ATM CES Port Adapter Jumpers, page 1-23

• PA-A2 ATM CES Port Adapter Slot Locations on the Supported Platforms, page 1-24

• Identifying Interface Addresses, page 1-27

Circuit Emulation Services Overview

Voice and video services—circuit emulation services (CES)—allow yo u to interco n nect existin g T1/E1
interfaces and other types of constant bit rate (CBR) equipment. CBR services include such features as
PBX interconnect, consolidated voice and data traffic, and video conferencing.

Wit h circu it emula tion , data recei ved from an e xtern al de vice at the edge of an A T M netwo rk is co n verte d
to ATM cells, sent through the network, reassembled into a bit stream, and passed out of the ATM
network to its destination. T1/E1 circuit emulation does not interpret the contents of the data stream. All
the bit s fl o wi ng int o t he i np ut edge por t of the ATM networ k ar e re pr oduce d at on e cor re spon di ng out put
edge port.

An emulated circuit is carried across the ATM network on a PVC, which is configured through the
network management system.

The ATM CES port ad ap ter o ffe rs tw o t y pe s of services, th at are covered in the follow in g sect io ns :

• Circuit Emulation Services Internetworking Function, page 1-2

• Network Timing Services for CES Operations, page 1-8

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Chapter1 Overview: PA-A2 ATM CES Port Adapter

Circuit Emulation Services Overview

Circuit Emulation Services Internetworking Function

CES-IWF i s a service ba s ed o n ATM Forum sta nd ard s th at al low s co m mu n ications b et w een C BR and
ATM UNI interfa ces, that is, bet ween non-ATM telephony de vices (such as classic PBXs or TDMs) and
ATM devices (such as Cisco 7200 series routers). Thus, a Cisco 7200 series router equipped with an
ATM CES port adapter offers a migration path from classic T1/E1 CBR data communications services
to emulated CES T1/E1 unstructured (clear channel) services or structured (N x 64) services in an A TM
network. A Cisco uBR7200 series equipped with an ATM CES port adapter offers an ATM WAN
connection between the broadband cable network and the Internet.

Figure 1-1 is a simplified representation of CES-IWF functions in an ATM network.

Figure 1-1 Typical CES-IWF Operations in an ATM Network

The circuit emulation services offered by the ATM CES port adapter are discussed in the following
sections :

• T1/E1 Unstructured (Clear Channel) CES Services, page 1-2

• T1/E1 Structured (N x 64) CES Services, page 1-4

• Channel-Associated Signaling (for Structured CES Services Only), page 1-6

T1/E1 Unstructured (Clear Channel) CES Services

Unstructured CES services in an ATM network emulate point-to-point connections over T1/E1 leased
lines. This s ervi ce maps t he en ti re b an dw i dt h necessary for a T1 /E1 leas ed li ne co nn ec ti on ac r oss t h e
ATM network, allowing users to interconnect PBXs, TDMs, and video conferencing equipment.
Unstructured CES operations do not decode or alter the CBR data in any way.

The ATM CES port adapter supports DSX-1 physical interfaces, providing T1/E1 unstructured (clear
channel) CBR data transmission services to Cisco 7200 series and Cisco uBR7200 series users at a rate
of 1.544 Mbps for T1 and 2.048 Mbps for E1.

The use of an ATM CES port ad ap ter f o r un str u ct ured C E S s e rvices simu lat es a p oi nt -to -po in t T1/E1
leased line across your ATM network.

ATM PBX

PBX

PBX

OC-3/T3/.E3

OC-3/T3/E3

T1/E1

T1/E1

ATM UNI

PVC

ATM UNI
PVC

ATM DSU

S5971

CES-IWF

(CBR to ATM UNI)

CES-IWF

(CBR to ATM UNI)

Router

Router

ATM

network

PBX

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Circuit Emulati on Ser v ices Overview

Figure 1-2 is a generalized representation of how T1/E1 unstructured CES services are set up in

conjunction with a Cisco 7200 series router equipped with an ATM CES port adapter.

Figure 1-2 T1/E1 Unstructured CES Services Across Leased Lines

Figure 1-3 is a general example of unstructured CES applications in an ATM network. In unstructured

CES services, user CBR data received from an edge device at one side of the network is segmented into
ATM cells and propagated through the ATM network. After traversing the network, the ATM cells are
reass embled into a CBR bit s tream that ma tches the orig inal us er data. This CBR dat a is th en passed out
of the network to the edge device at the destination endpoint.

PBX

PBX

T1/E1 OC3/T3/E3 OC3/T3/E3

ATM-CES PA

T1/E1

ATM trunk

Unstructured

CES connection

Unstructured

CES connection

Bandwidth reserved

across trunk connection

for unstructured

CBR traffic

S5886

Router

ATM-CES PA

Router

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Circuit Emulation Services Overview

Figure 1-3 T1/E1 CES Application s in ATM Network

T1/E1 Structu red (N x 6 4) CES Services

The T1/E1 structured (N x 64) CES services enable a CES module to function like a classic digital access
and crossc on n ect s y stem (DACS) switch.

The ATM CES port adapter supports DSX-1 physical interfaces, providing T1/E1 channelized data
transmission services at a rate of 1.544 Mbps for T1 and 2.048 Mbps for E1.

Using an ATM CES por t a da pt er, you can map a si ng l e D S 0 ch an ne l o r mu ltiple D S 0 ch an ne ls across
an ATM network. Each T1 port has up to 24 time slots, and each E1 port has up to 31 time slots for
allocation to structured CES circuits. Each time slot can transmit CBR data at a rate of 64 kbps, or at 56
kbps, if you choose to use optional channel associated signaling (CAS). The “Channel-A ss o cia ted

Signaling (for Structured CES Services Only)” section on page 1-6, describes the CAS mechanism.
Figure 1-4 il lu str at es t he d ig it al cr oss c on n ect an d ch an ne li zed m a pp in g f un ct io ns offer ed by a

Cisco 7200 series ro uter or a Cisco uBR7200 ser ies equippe d with an ATM CES port adapte r. Single or
multiple DS0 time slots can be mapped across the ATM network. Each time slot (or DS0 channel)
represent s a single N x 64 circuit that can t ra ns m it CBR data at a r at e o f 64 k b ps.

Multiple N x 64 circuits can be connected to a single port, if separate time slots are used.
With T1/E1 structured CES services, network designers can simplify networks by eliminating TDM

devices, usin g ATM CES p o rt a d a p te rs inste a d as a mea ns of a llo cating T1/E1 bandwidth t o P BX s and
teleconferencing equipment.

T1/E1

OC3/T3/E3

ATM-CES PA

ATM-CES PA

PBX

OC3/T3/E3

T1/E1

Video conferencing

ATM

network

MUX

T1/E1

T1/E1

MUX

OC3/T3/E3T1/E1

OC3/T3/E3

OC3/T3/E3

OC3/T3/E3

ATM-CES PA

ATM-CES PA

ATM-CES
PA

ATM-CES PA

T1/E1

= CBR bit stream

CODEC

= ATM cells

PBX

PBX

Router

Router

Router

Router

Router

Router

Video conferencing

S5887

CODEC

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Circuit Emulati on Ser v ices Overview

Figure 1-4 Digital Acc ess and Crossconnect (DACS) Functions of CES Modules

As Figure 1-4 demonstr at es , st r uc tu re d ser vi ces in an ATM CES por t ad ap ter a llow T1 /E 1- f orm a tt ed
CBR data to be provisioned into individual DS0 channels (PVCs) or groups of DS0 channels.

Data fro m th es e channels ca n be sent to m u lt ip le individu al outpu t po r ts o n a CE S modu le , w h er e t he
data can be combined with CBR data from other DS0 channels or groups of DS0 channels to form an
outgoing T1/E1 bit stream. Thus, you can combine structured CBR data with great flexibility for
transpor t acr o ss an ATM network .

Figure 1-5 g ives on e exa mple o f how 2 4 available N x 64 time sl ots for T 1 ( 31 tim e slots f or E1 no t

shown) in an ATM CES por t ad ap ter c an be combin ed to accomp lish structured CB R d ata transp or t in
an ATM network. The DS0 channels can be grouped as either contiguous or noncontiguous time slots.

The ingress (source) DS0 channels can be mapped into different DS0 channels on egress from the ATM
network at the destinat io n no de.

(1 x 64)

(1 x 64)

(1 x 64)
(1 x 64)

(12 x 64)

(12 x 64)

(12 x 64)

(12 x 64)

T1/E1

T1/E1

T1/E1

T1/E1

T1/E1

OC3/T3/E3

OC3/T3/E3

Router

Router Router

ATM network

S5888

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Figure 1-5 Time Slots for Structured Services in PA-A2 ATM CES Port Adapters

For CES structured services, each DS0 time slot represents a data bandwidth of 64 kbps for CBR data
transpor t. I f o p ti ona l ch an ne l a s so ciated sign al in g (C A S ) is b ei ng used, how ever, a data b an d w id th o f
56 kbps per DS0 time slot is available for CBR data transport.

Figure 1-5 shows one end of three PVCs that have been provisioned for structured CBR data transport.

Think of t he s e D S0 ch an n els as t he so ur ce en ds of t he P V Cs . Th e destinat io n en ds of the P V Cs each
require the allocation of an identical number of DS0 time slots.

However, the actual DS0 numbers assigned at the destina tion node can differ from those assigne d at the
source node. In other words, the DS 0 numbers at e ac h end of the PVC need not corre spond one for one.
Only th e a ctu al n umbers of ti m e s lots p r ovis io n ed at each end of t he PVC mu s t a gre e .

For example, the circuit identified as PVC No. 3 in Figure 1-5 requires a similar bundling of three time
slots at the destination end of the PVC (representing a data bandwidth of 192 kbps) in order for the
circuit t o oper at e p r op er ly.

Channe l -Associat ed Signa l i ng (for Structure d CES Serv i ces Only )

Because th e ATM CES por t ad ap ter e m u lat es constant b it rate se rvi ces over ATM ne tw ork s , i t mu s t be
capable of providing support for handling channel-associated signaling (CAS) information introduced
into structured CES circuits by PBXs and time-division multiplexing (TDM) devices. An optional CAS
feature fo r th e ATM CES por t ad ap te r meets this requi re men t.

With respect to t he C AS inform at io n car ried in a C B R b it stream, an ATM CES por t a da pt er can be
configured to o p er ate as foll ows:

DS0 No. 4 DS0 No. 5

DS0 No. 6

. . .

. . .

Circuit No. 1

Circuit No. 2

Circuit No. 3

CBR
PVC No. 3

CBR
PVC No. 2

CBR
PVC No. 1

Total bandwidth of T1 port (1.544 Mbps)
Total bandwidth of E1 port (2.048 Mbps)

S5889

N x 64

Slot 1

N x 64

Slot 2

N x 64

Slot 3

N x 64

Slot 4

N x 64

Slot 5

N x 64

Slot 6

N x 64

Slot 7

N x 64

Slot 8

N x 64

Slot 24

64 kbps

128 kbps

192 kbps

1 time slot

allocated

2 contiguous time

slots allocated

3 contiguous time

slots unallocated

3 non-contiguous time

slots allocated

24 64-kbps time slots available per ATM-CES port adapter

DS0 No. 7 DS0 No. 8 DS0 No. 24DS0 No. 2 DS0 No. 3DS0 No. 1

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• Without the optional CAS feature enabled (the default state).

In this ca se , th e ATM CES port adapter d oes not sense the CAS inf o rm ati on ( ca rr ie d as s o -called
“ABCD” bits in the CBR bit stream) and provides no support for CAS functions.

• With the op ti on al CAS fe at ur e enabled , bu t w i th ou t the op ti on al (Cisco-propr ietary) “on-hook

detection” fe atur e enabled .
In this ca se , in ad di tion t o pa ckag in g inco ming CBR data i nto ATM AAL1 cells i n the us ual mann er

for transport through the network, the ATM CES port adapter in the ingress node s enses the ABCD
bit patterns in the incoming data, incorporates these patterns in the ATM cell stream, and propagates
the cells to the next node in the network. The ATM cells are transported across the network from
link t o l ink until th e egress n od e i s r eached.

At the egress node, the ATM CES port adapter strips off the ABCD bit patterns carried by the ATM
cells, re assembles t he C A S A BC D b it s an d th e u ser ’s CBR da ta into t he origina l form , and p asses
the frames out of the ATM network in the proper DS0 time slot.

All thes e pr o cess e s occur tran s p ar en tl y w i th ou t user inte rve nt io n.

• With both the optional CAS and on-hook detection features enabled.

In this case, the CAS and on-hook detection features work together to enable an ingress node in an
ATM network to monitor on-hook and off-hook conditions for a specified 1 x 64 structured CES
circuit. As implied by the notation “1 x 64,” the on-hook detection (or bandwidth-release) fe ature is
support ed on ly i n a st ru ct ur ed C ES cir cu it th at involve s a s in gl e D S0 ti m e s lo t a t e ach end of the
connecti on .

The DS0 time slot configured for the structured CES circuit at the ingress node (time slot 2) can be
diff erent from the DS0 time slot configured at the egress node (time slot 4). Only one such time s lot
can be co nfigu re d at each en d o f th e cir cu it w h en th e on -ho o k d et ect io n f eat ur e is u s ed .

When you invoke this feature, the ingress ATM CES port adapter monitors the ABCD bits in the
incomi ng CBR bit stream to detect on-hook and off-hook con ditions in the c ircuit. In an “off-hook”
condition, all the band width provisioned for the specif ied CES circuit is used for transpor ting ATM
AAL1 cells across the network from the ingress node to the egress node.

In an on-hook condition, the network periodically sends dummy ATM cells from the ingress node
to the e g res s node to main ta in the c onn ec tio n. Ho we v er, th ese dumm y cel ls con su me on ly a fra cti on
of the c i rcuit’s reserved bandwidth, leaving the remainder of the bandwidth available for use by
other network tr a ffic. This bandwidth-releas e f e a ture en ables th e network to make m or e efficien t
use of its resources.

When th e C A S f ea tu re is en ab led for a C ES ci r cu it, the ban d w id th of th e D S0 chann el is l im i te d to 56
kbps for user data, because CAS functions consume 8 kbps of channel bandwidth for transporting the
ABCD signaling bits. These signaling bits are passed transparently from the ingress node to the egress
node as part of the ATM AAL1 cell stream.

In summ ar y, when th e o p ti on al CA S an d o n -ho ok d et ection fe atu res a re en ab led, the fo ll owing
conditions apply:

• The permanent vi rtu al connec tio n ( P VC) p rovisioned fo r th e C ES cir cu it alw ay s exi sts.

• The bandwidth for the CES circuit is always reserved.

• During an on-hook sta te, most of t he bandwidth re served for the CES circuit is not in use. (Dummy

cells ar e sen t fr o m t he ingre ss n ode to th e eg r ess n od e to m a in tai n th e connect io n. ) Th er ef ore, this
bandw id t h b e c omes ava ila ble fo r us e by oth er network traffic , s u c h a s availabl e bit ra te (ABR )
traffic.

• During an off-hook state, all the bandwidth reserved for the CES circuit is dedicated to that circuit.

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Chapter1 Overview: PA-A2 ATM CES Port Adapter

Circuit Emulation Services Overview

See Chap te r 4, “Configuring the PA-A2 ATM CES Port Adapter,” for more information on the
procedure s for enabl ing the CA S featur e and configu ring str uctu red CES service s.

Network Timing Services for CES Operations

Circuit emulation services internetworking functions (CES-IWF) and constant bit rate (CBR) traffic
relate to a quality of service (QoS) classification defined by the ATM Forum for Class A (AAL1) traffic
in ATM networks. In general, Class A traffic pertains to voice and video transmissions.

In an ATM networki ng environment, CBR refers to a partic ular class of traffic t hat is gene rated by edge
(source ) d ev ice s an d pr opaga t ed in to ATM networks f or t rans mi ss ion to ot her e dge (d est i nati on) de vi ces
in the ne tw or k .

The ATM CES port adapter is designed specifically to handle CBR traffic in an ATM networking
environment. To provide requisite timing functions in support of CES operations, you can specify any
one of t hr e e c l ocking m o d e s covered in the f ol low ing sec t io ns :

• Network Clock Synchronization, page 1-8

• Designating a PRS Source Port in a Cisco 7200 Series Router or Cisco uBR7200 series, page 1-8

• Network Clock Distribution in a Cisco 7200 Series Router or a Cisco uBR7200 series, page 1-9

However, to support synchronous clocking or SRTS clocking in your ATM networking environment,
your networ k m u s t i ncorpor a t e the f ol low ing fac ilities:

• A primary reference source (PRS)—Throughout this docum ent, th e term PRS refers to a preci sio n

referen ce ti min g s ig na l th at mu s t b e mad e available, wh erever r eq ui r ed , to syn ch r on iz e th e f low of
CBR data from its source to its destination.

• Network clock synchronization services—This r ef e r s to a n etw or k cloc k synch r onizati on and

distribution service that provides a PRS to those user and network devices that require a precision
reference timing signal for synchronizing the flow of CBR traffic.

Networ k Clock Sy nchroniz ation

Any const an t bit r ate (CBR ) edge device th at co m mu ni cat es w i th an o th er C BR ed g e device acr oss an
ATM network must be driven by a clocking signal of identical frequency. This “synchronized” signal
contro ls th e rat e of CB R d ata in ser ti on into th e netw ork , and a l so t he ra t e of extra ctio n o f C BR data f rom
the netw ork. If the cl ock freq uency is not s ynchro nized a t both th e ing ress and egress nodes of the ci rcuit ,
the data queues and buffers in the network will either overflow or underflow, resulting in line errors.

Distributing a cloc ki ng si gnal wi th in the net wo rk ens ur es th at ea ch CBR devi c e has ac cess to a common
reference clocking signal, called Primary Reference Source (PRS), for synchronizing CBR data
transp or t.

Designating a PRS Source Port in a Cisco 7200 Series Router or Cisco uBR7200 series

Any port adapter in a Cisco 7200 series router or a Cisco uBR7200 series that can receive and distribute
a network timing signal can propagate that signal to any other port adapter in the chassis that has similar
capabilit ies .

By issuing the network-clock-select command with appropriate parameters, you can define a particular
port in a Cisc o 7200 series router or a Cisco uBR7200 series to serve as th e source of a PRS for the enti re
chassis or f o r ot he r devi ces in the ne two r ki n g e nvironm en t. Th is co m man d en ab les you to d es i gn at e a
particul ar p ort in a C isco 720 0 ser ies rou ter o r a Ci s co uBR720 0 s er ies to s erve as a “master clock”

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source in distributing a single clocking signal throughout the chassis or to other network devices. This
clocking si gn a l c an be di s tr ibu ted whe r ever neede d in the network to globally synch r on ize the flow of
CBR d a ta . Th e us e of this com mand i s d e scribe d in Chap ter 4, “Configuring the PA-A2 ATM CES

Port Adapter.”

Network Clock Distribution in a Cisco 7200 Series Router or a Cisco uBR7200 series

When equi pp ed w ith a n ATM CES port ada pt er an d ap pro pr ia te sof twar e, any Ci s co 720 0 series rout er
or a Cisco uBR7200 series can serve as a me ans for receiving and distributing a P RS to other devices i n
the network.

A Cisco 7200 series router or a Cisco uBR7200 series can make use of a PRS that originates from any
one of sever al sou rces in the net wo rki ng enviro n men t.

Figure 1-6 sh ows t h ree p ossible sou rces of a PRS wh en t wo Cisco 72 06 r o ut er s ar e co nn ec ted

back-to-b ack. However, you shou ld not i nt er pr et th at to m ean th at on ly t hree such cloc ki n g si gn al s can
be made available for u se in the ATM network. In fact, more than three clocking signal s may be present
in the Cisco 7200 series router or Cisco uBR7200 series operating environment.

The important concepts that you should take from Figure 1-6 include the following:

• Up to four clocking signals can be defined per Cisco 7200 series router or a Cisco uBR7200 series

in the ne tw or k .

• The PRS that is used in Cisco 7200 series routers or a Cisco uBR7200 seri es must be traceable to a

single clock source. (In Fi gure 1-6, Rou ter N o . 2 ’s PRS is t ra cea bl e t o Ro ut er N o . 1 ’s PRS.)

Figure 1-6 Cisco 7206 Routers Connected Back-to-Back

Each PRS depicted in Figure 1-6 is external ly g en e r at ed —that is, the timing sig nal orig in at es f r om a
source outside the router. Also shown is an OC-3 trunk line that can propagate a PRS between adjacent
router/network devices.

If the Router No. 1 priority 1 PRS fails (CBR 4/0), the network clock synchronization service
automatically recovers network timing by using the Router No. 1 priority 2 PRS (CBR 4/3).

Assume, for e xa mple, th at th e T1/ E 1 line (CB R 4/0) at Route r No. 1 in Figure 1-6 is currentl y sup pl ying
PRS to th e ne twork. I f th is P R S fa il s , t he T 1 /E1 l in e (CBR 4/3 ) at the same router is used as P R S .

Router 2Router 1

S5969

Cisco 7206

T1/E1 CBR 4/3

OC3/T3/E3

ATM 4/0

ATM WAN

uplink

OC3/T3/E3

ATM 2/0

T1/E1 CBR 4/2

T1/E1

CBR 2/2

T1/E1 CBR 4/1

T1/E1

CBR 2/1

T1/E1 CBR 4/0

T1/E1

CBR 2/3

T1/E1

CBR 2/0

PBX

PBX

PBX

PBX

Cisco 7206

PBX

PBX

PBX

PBX

PRS clock sources

1. CBR 4/0

2. CBR 4/3

PRS clock source

1. ATM 2/0

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Circuit Emulation Services Overview

Router No. 2 has ATM 2/0 as its PRS. If the ATM 2/0 port fails, there is no configured alternate clock
source fo r P R S . Th e de fau lt cl oc k s ou rce i s t he lo cal o s ci lla to r. But, in t hi s ca se, this is not a proble m
because th e t ra ffic from the T 1/ E1 C BR p or ts is a lso d is r u pte d . In th e Cisco 720 0 series ro ut er o r th e
Cisco uBR7200 series, all T1/E1 traffic goes only on the ATM WAN uplink. When ATM 3/0 goes up,
the PRS i s au tomatic all y recovered .

If the Router No. 1 priority 1 PRS is restored to service, the network clock synchronization service
automatically reverts to this PRS for timing purposes, regardless of which lower priority PRS may be
active at the t ime .

Figure 1-7 illustrates Cisco 7206 routers connected to an ATM network. This figure shows how a PRS

can be deri ved from the ATM network and pro vided to a n edge node and prop aga ted throu gh the netw ork
to synchronize the flow of CBR data between the communicating ATM end nodes.

In Figure 1-7, wh en th e PRS cl o ck so urce is taken f ro m t he ATM network, all th e tr an sm i t n od es at th e
edge of the network on T1/E1 trunk lines on the ATM CES port adapter in Routers 1, 2, and 3 are
synchronized.

Figure 1-7 Cisco 7206 Routers Connected to ATM Network

Figure 1-8 illustrates a typical configuration of a network clock source. In this figure, the PBX T1/E1

transmit clocks, ATM CES T1/E1 transmit clocks, ATM CES OC-3/T3/E3 ATM WAN uplinks, and
ATM net wo r k OC-3/T 3 /E 3 ATM port trans m it c l ocks are all derived f ro m and r ef erenc ed to a s in gl e
clock so u rc e, th e P R S . This lim it s th e ji tter and w an d er in t he em u la ted circuit s.

Router 2

Router 3

Router 1

S5970

Cisco 7206

T1/E1

T1/E1

OC3

OC3

ATM WAN

uplink

ATM WAN

uplink

ATM WAN

uplink
T3/E3

T1/E1

T1/E1

T1/E1

T1/E1

PBX

PBX

PBX

PBX

Cisco 7206

PBX

PBX

PRS clock source

1. ATM WAN uplink

PRS clock source

1. ATM WAN uplink

PRS clock source

1. ATM WAN uplink

ATM

network

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Circuit Emulati on Ser v ices Overview

Figure 1-8 Typical Configuration of Network Clock Source

Clocking Modes for CES Opera t i ons

For CES operations, three clocking modes can be used in conjunction with any ATM CES port adapter.
These cl ocking modes ar e descri bed in t he fol lowi ng sec tions i n the re commended order of consid erat ion
and use :

• Synchronous Clocking Mode, page 1-11

• Synchronous Residual Time Stamp Clocking Mode, page 1-12

• Adapt ive Clocki ng Mode , page 1 - 1 2

• Summary of Clocking Modes, page 1-13

Synchronous Clocking Mode

Synchronous clocking mode requires a PRS and network clock synchronization services. When
equipped with an ATM CES port adapter and appropriate software, any Cisco 7200 series router or
Cisco uBR7200 series can serve as a means for synchronizing the flow of user CBR traffic through the
networ k.

Synchro nous cloc king ca n be use d for unst ructure d servi ce (clea r chann el) and i s the on ly cloc king mod e
for struct ur ed (N x 64 k b ps) C ES s er vices . I t i s th e recommen d e d o pt io n fo r th r ee re as o ns:

• First, this clocking mode is the only one that supports full CES functionality. SRTS and adaptive

clocking do not support structured CES services.

• Second, synchronous clocking exhibits superior stability, reliability, and wander/jitter

characteris t ics.

• Third, synchronous clocking is typically used in public telephone systems, making a precision

referen ce si gn al r ead i ly an d w id ely availa b le for th e synchr onizing of CBR dat a t ra nsp o rt .

For the s e reaso ns , s y nchro no us clocking is the defau lt clocking mode f o r all C E S s er vices .

PBX

T1/E1
T1/E1

OC3
ATM

WAN

uplink

OC3
ATM

WAN

uplink

Network clock source

set to ATM WAN uplink

Primary Reference

Source (PRS)

Network clock source

set to ATM WAN uplink

T1/E1 port Tx
clock set to network
derived, synchronized
clocking type used

T1/E1 port Tx
clock set to network
derived, synchronized
clocking type used

PBX Tx clock
source is
derived from
the Rx clock

PBX Tx clock
source is
derived from
the Rx clock

T1/E1

T1/E1

Network clock synchronization
supported in the ATM and ports
Tx clock uses network clock
as reference

S5992

Router Router

PBX

ATM

network

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Circuit Emulation Services Overview

Synchronous Residual Time Stamp Clocking Mode

Synchronous residual time stamp (SRTS) clocking requires a PRS and network clock synchronization
services . SRTS, which can be used only for unst ru ctur ed servi c es, ca rrie s asyn ch ronous DS1 circ ui ts . In
this case, the in pu t s er vi ce clo ck f r eq ue ncy m u st be recovered at the o ut pu t CES-IWF. SRTS is one of
the clockin g modes that can be used fo r r ec overing t his clock fr equen cy.

The SRTS clocking mode, which requires a network-wide reference clock, measures the service clock
input f re qu e n cy again s t a network -w ide sy nchro ni zation signal t h at must b e p r e se n t in the CES - I W F,
and sends different signals, called residual time stamps, in the AAL1 header to the reassembly IWF. At
the outp ut IW F, the differen ce s ca n be co m b in ed wit h th e n e t work - wid e s y n ch ron ization si gn al , t o
re-crea te the input service clock. The network-wi de referenc e clock is d escribed in the “Network Clock

Distribution in a Cisco 7200 Series Router or a Cisco uBR7200 series” section on page 1-9.

Adapti ve Clocking M ode

Adapti v e cl ocki ng mode r equir es ne ithe r a P RS nor ne t wor k cl ock s ync hro niza tion s ervi ces for e f f ect iv e
handling of CBR traffic. However, as is the case with SRTS clocking, adaptive clocking can be used
only for un str u ct ured ( cl ear ch an n el) C ES s er vices .

Note Althoug h th is clock i ng m o d e i s th e s im p le s t a nd easiest to imp le m en t i n an ATM networ k , it exhibits

the poore st w a nder a nd jitt e r perf orm anc e of all th e a va i labl e cloc king mo des. T here fo re , Ci sco does no t
recommend its use, except in instances where a PRS and network clock synchronization services are not
available.

The term adaptive clocking is used because the rate at which CBR data is propagated through an ATM
network is dr iven by the rate at wh ich such d at a is intr o du ce d into th e network by the u s er’s edge
equipment.

For example, adapt ive clocki ng in an ATM CES po r t ad a p t e r derives ti ming fo r data tr anspor t by
calcula tin g th e “average” rate at which data arrives and conveying that data to the output port of the
module at an equivale nt rate. For this r eason, the actual ra te of CBR data flow t hrough the network may
vary when adaptive clocking is used, depending on how rapidly CBR data is being introduced into the
networ k.

CBR data transport through the network occurs in a “pseudo synchronous” manner that ensures the
integrity of the data.

Rather than using a clocking signal to convey CBR traffic through an AT M network, adaptive clocking
obtain s ap p ro p r iat e timing f o r da ta t r ansport by calcul ati ng a n averag e data ra te fo r the CBR traffic.

For exampl e, if CB R d ata is arrivi ng at an ATM CES port ad ap te r at a rate o f so m any b it s p er se c on d,
then that r a t e i s used, in effect, to gov er n the flow o f CB R data th rou gh the netw or k . M ea nw h ile,
however, the ATM CES po rt adapter au to m at ic all y cal cu lates the aver ag e d at a r at e by me an s of
microc ode (firmware) buil t int o th e b oar d. T his ca lcul a ti on oc curs wh ile use r d ata tr av e rse s t he netw or k.

When the A TM CES port adapter senses that its segmentation and reassembly (SAR) buffer is filling up,
it increases the rate of the transmit (TX) clock for its output port, thereby “draining” the buffer.

Similarly, the ATM CES port adapter slows down the transmit clock of its output port if it senses that
the bu ffe r is b eing “drained” fast er th an C B R d ata is bein g r ece ived. In this mann er, ad ap tive cl oc king
attemp t s to m i nimize w i de variat io ns in SAR buff e r lo ad ing while pr ov id in g an effe ct ive m eans of
propagating CBR traffic through the network.

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Chapte r1 Overview: PA- A2 ATM CES Por t Adapter

Circuit Emulati on Ser v ices Overview

Implemen ting ada pti ve clo ckin g is simpl e and st raight forwa rd, bec ause it d oes not require network clock
synchron i zat io n services, a P R S , o r th e a dvan ce pl an ni ng t yp ic ally asso cia te d w i th th e developmen t o f
a logical network timing map. However, adaptive clocking does not support structured CES services,
and it exhibit s rela tively hig h wand er charac teri stic s.

Summary of Clocking Modes

Table 1-1 summarizes the characteristics of the three clocking modes available for handling CBR traffic

in an ATM networking environment. Although the wander and jitter characteristics of these clocking
modes differ, each mode preserv es the integrity of the user’s CBR data, ensuring its error-free transport
from source to destination.

Other Network Factors Relevant to CES Operatio ns

The following factors affect the functioning of CES circuits:

• The inten de d sou r ce an d d est in at io n no d es f o r C ES cir cu it s.

• The cl ocking mode that be st suits your particular netw ork topology and timing requiremen ts for the

handling of CBR data.
Although synchronous clocking is the recommended (default) clocking mode for CES operations,

this fact does not preclude other clocking modes from consideration.

• The cell d e la y variati on ( C DV) ch ar act er ist ics of the netw ork , measured in m i cr ose co nd s .

• Each end- to - en d CES circui t ex hi bi ts d elay char ac ter ist ics, base d o n th e fo ll owi ng f act or s:

–

The delay characteristics of the individual devices participating in the CES circuit. Each
network d evice contribut es s o me increment of de la y, reflecti ng t ha t d evic e’s uniqu e electr ical
characteris t ics.

Table 1-1 Clocking Mode s for CBR Traffic

Clocking Mode Advantages Limitations

Synchronous Supports both unstructured (clear

channel) an d str u ctu r ed C BR t ra ffic.
Exhibits superio r wa nder a nd jitter

characteris t ics.

Requires network clock
synchronization services.

Ties the CES interface to the network
clock sync hroni zatio n serv ices cl ocking
signal (PRS).

SRTS (synchronous
residual ti m e stamp)

Conveys externally generated user
clocking signal through ATM
network, providing independent
clocking signal for each CES circuit.

Requires network clock
synchronization services.

Supports only unstructured (clear
channel) CBR traffic.

Exhibits moderate wander
characteristics.

Adaptive Does not require network clock

synchron izat ion ser vices.

Supports only unstructured (clear
channel) CBR traffic.

Exhibits poorest wander charac teristics.

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–

The number of intermediate hops through which the CBR data must pass in traversing the
netwo rk from s ource t o dest inatio n. Th e netw ork desi gner/a dmini str ator calc ula tes a CD V v alu e
for each hop in the data path in order to establish a maximum allowable CDV value for the
networ k at large .

–

The type and speed of the trunk lines interconnecting the ATM networks.

–

The volume of traffic being handled by the trunk lines at any given time, that is, the degree to
which the network may be experiencing congestion conditions.

Network designers and administrators calculate a maximum allowable CDV value for the network in
order to establish network cell delay tolerance limits. Thus, to some degree, the network’s maximum
allowable CDV value is a measure of the network’s expected perform an ce.

When a CDV threshold for the network is established, appropriate buffer sizing can be derived for the
network devices involved in any given CES circuit. This helps to ensure that the network will operate
as expecte d.

In the ca se o f an ATM CES por t ad apt er, for exa mp le, the m ax imu m allowable CDV valu e fo r th e
network is used to de te rmine an a pp r op riat e s i ze (d ep th ) f or t he s egm en tation an d re asse m bl y (SA R)
buffer bui lt into t he board . This s izing of t he SAR buffer prevents bu ffer overflow or und e r fl ow
conditio n s. A n overf low co n di tio n can cause a loss o f fra m es, and an u nde rf low co ndit io n can cause
frames to be r ep eat ed .

The actual CDV value for a circuit varies accord in g t o th e p ar t icu la r da ta pa th used for th e circuit .
Consequently, the depth of the SAR buffer may increas e or decrease in proportion to t he CDV value for
the CES circuit being set up.

You can issue th e show ces circuit interface command i n a n un str u ct ured ( cl ear ch an n el) c ir cu it to
measure the current CDV value. See Chapter 4, “Configu r in g the PA-A2 ATM CE S P o r t Adap te r,” for
more information on verifying a configured hard PVC.

For an unst ru ct ur ed h ar d PV C, the CDV value f o r t h e ci r cui t (i nc lu di ng a ll ho p s) s hould not exceed a
maximum allowable CDV value. The procedure for setting up a hard PVC is described in

Chapter 4, “Configuring the PA-A2 ATM CES Port Adapter.”

For an unst ru ct ur ed h ar d P VC, the ne two r k au to m at ica ll y det e rm i ne s th e b est data p ath through th e
network and handles the routing of CBR traffic. The network accomplishes this dynamically by means
of the ATM connection admission control (CAC) mechanism. The CAC determines the best path
through the network by executing a routing algorithm that consults local routing tables in network
devices.

If the requested data path is equal to or les s than the maximum allowable CDV value established by the
network adminis tra to r, the connect io n re qu est is gr an te d. I f t h e r eq ue sted CES cir cu it exc eed s t he
maximum allowable CDV value, th e co nn ec ti on re qu est is d en ied . T he s e co nnection ad mis si o n co n tro l
processes occur “on the fly” as network connection requests are initiated.

For example, when a user requests a connection from source node A at one edge of the network to
destin ati on no de B at th e o ppo sit e edg e of the ne t wor k, th e CAC mec hani sm t ake s int o a cco unt th e CD V
value for each hop in the requested connection to determine a suitable path through the network that does
not exceed the ne twork’s maximum al lowable CDV value.