Wide-Area Networks - CCIE Routing and Switching Exam Certification Guide - page 229

ATM Connections
ATM is a connection-oriented technology. A connection is established between two or more
ESs to permit communication. A connection between two end points is called point-to-point . A
connection between a single originating end point and multiple destination end-points is called
point-to-multipoint .

ATM uses the concept of virtual connections between ESs rather than physical links. ATM
networks support a connection type known as a PVC, which is established manually from a
source end-point to a destination end-point. Each switch in the path of a PVC has an entry in
the switching tables that identify the VPI/VCI and interface for the ingress and egress of the
connection.

A PVC is always connected and is not dynamically rerouted around a link or switch failure; the
ESs cannot create or release the connection.

Many switches support a special type of PVC known as a soft PVC (SPVC) . A SPVC is estab-
lished manually by using connection management software and can dynamically reroute around
a link or switch failure.

ATM networks also support a connection type known as an SVC . An SVC is established by the
ES by using signaling. In a SVC, the ES dynamically establishes and releases the connection
as needed. SVCs can dynamically reroute around a link or switch failure.

SVCs are established by using UNI signaling. The ATM Forum standard for signaling is UNI
4.0. Most networks support the earlier version, UNI 3.1. UNI signaling is based on the ITU-T
Q.2931 specification.

When an end system wants to establish a connection with another end system, it sends a request
to the switch that specifies the following:
•

Destination ATM ES address

•

QoS parameters, such as cell loss ratio, cell transfer delay, and cell delay variance

The switch examines the connection request parameters and determines if it has a routing table
entry for the destination ES address and if it can accommodate the requested QoS. If the switch
can accept the connection request, it forwards the request to the next switch specified in the
connection routing table.

SSCOP

ATM interfaces support a signaling protocol stack and a special signaling ATM adaptation layer
(SAAL). The SAAL is composed of the SSCOP and the service specific convergence function
(SSCF).