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PA-A2 ATM CES Port Adapter Installation and Configuration
OL-3460-01
Chapter 1 Overview: PA-A2 ATM CES Port Adapter
Circuit Emulation Services Overview
See
Chapter 4, “Configuring the PA-A2 ATM CES Port Adapter,”
for more information on the
procedures for enabling the CAS feature and configuring structured CES services.
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 networking environment, CBR refers to a particular class of traffic that is generated by edge
(source) devices and propagated into ATM networks for transmission to other edge (destination) devices
in the network.
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 three clocking modes covered in the following sections:
•
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 network must incorporate the following facilities:
•
A primary reference source (PRS)—Throughout this document, the term PRS refers to a precision
reference timing signal that must be made available, wherever required, to synchronize the flow of
CBR data from its source to its destination.
•
Network clock synchronization services—This refers to a network clock synchronization 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.
Network Clock Synchronization
Any constant bit rate (CBR) edge device that communicates with another CBR edge device across an
ATM network must be driven by a clocking signal of identical frequency. This “synchronized” signal
controls the rate of CBR data insertion into the network, and also the rate of extraction of CBR data from
the network. If the clock frequency is not synchronized at both the ingress and egress nodes of the circuit,
the data queues and buffers in the network will either overflow or underflow, resulting in line errors.
Distributing a clocking signal within the network ensures that each CBR device has access to a common
reference clocking signal, called Primary Reference Source (PRS), for synchronizing CBR data
transport.
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
capabilities.
By issuing the network-clock-select command with appropriate parameters, you can define a particular
port in a Cisco 7200 series router or a Cisco uBR7200 series to serve as the source of a PRS for the entire
chassis or for other devices in the networking environment. This command enables you to designate a
particular port in a Cisco 7200 series router or a Cisco uBR7200 series to serve as a “master clock”
