H3C Technologies H3C SR8800 - Manual (Page 9)

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Configuring ATM
Overview
Introduction to ATM
Asynchronous Transfer Mode (ATM) is a technology based on packet transmission mode while
incorporating the high speed of circuit transmission mode. It can satisfy the needs of various
communication services. ATM was specified as a broadband ISDN transmission and switching mode by
the ITU-T in June 1992. Due to its flexibility and support for multimedia services, it is regarded as the core
technology for broadband communications.
As defined by the ITU-T, ATM encapsulates data in cells. Each ATM cell is 53 bytes long, among which
5 bytes are the cell header and the remaining 48 bytes are the payload. The major function of the cell
header is to identify virtual connection, with limited functions regarding flow control, congestion control,
and error control.
ATM connection
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 , you 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 identifier (VPI) and virtual channel identifier (VCI).
Figure 1 Physical link, VP, and VC
NOTE:
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.
ATM interfaces of the router support permanent virtual circuits (PVCs) only.