R3303-HP HSR6800 Routers Layer 2 - WAN Configuration Guide

Configuring ATM

Overview

Asynchronous Transfer Mode (ATM) is a technology based on packet transmission mode while

incorporating the high-speed of circuit transmission mode. ATM was adopted as the transmission and

switching mode for broadband ISDN by the ITU-T in June 1992. Due to its flexibility and support for

multimedia services, ATM is regarded as core broadband technology.

As defined by the ITU-T, data is encapsulated in cells in ATM. Each ATM cell is 53 bytes in length, of

which the first 5 bytes contain cell header information and the last 48 bytes contain payload. The major

function of the cell header is to identify virtual connection. In addition, it can be used to carry limited flow

control, congestion control, and error control information.

ATM connections and ATM switching

ATM is connection-oriented, and ATM connections are logical connections, or virtual circuits. In an ATM

network, you can create logical connections called virtual paths (VPs) and virtual circuits (VCs) on

physical links. As shown in Figure 1, y

ou can create multiple VPs on a physical link, and each VP can be

demultiplexed into multiple VCs. Cells from different users are transmitted over different VPs and VCs,

which are identified by virtual path identifiers (VPIs) and virtual channel identifiers (VCIs).

Figure 1 Physical link, VP, and VC

ATM uses VPI/VCI pairs to identify a logical connection. When a connection is released, all the involved

VPI/VCI pairs are reclaimed for new connections.

As shown in Figure 2, an A

TM switch forwards ATM cells by looking up the switching entries and

changing the VPIs/VCIs. The ATM switching by changing VPIs only is VP switching, and the connection

established through VP switching is Virtual Path Connection (VPC). The ATM switching by changing VCIs

only or changing both VCIs and VPIs is VC switching, and the connection established through VC

switching is Virtual Circuit Connection (VCC).