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Chapter 1 Overview
Avaya M770 M-ACCF/SF ATM Access Modules User’s Guide 19
Figure 1.11 Connection Terminology
Many virtual channels can exist on the same physical link. Each virtual channel is
identified by a pair of numbers:
•The Virtual Path Identifier (VPI) and
•The Virtual Channel Identifier (VCI).
Any end-system that wishes to communicate with another end-system must first
use the Signalling protocol to set up the VCC.
The Signalling protocol negotiates with each ATM device between the end-systems
to set up a series of virtual channels. Each of these virtual channels is identified
using the VPI and VCI values.
Figure 1.12 shows how ATM cells are switched through an ATM network with
Legacy Avaya Edge Devices.
Instead of containing the ATM address of the final destination device, each cell
header contains the VPI/VCI values associated with the virtual channel it is going
to take to get to the next ATM Switch in the connection.
Each ATM switch knows that when it receives a cell with a particular VPI/VCI
value on one port that it must transmit the cell on another port with another VPI/
VCI.
Cells are switched through the network based on these VPI/VCI values, and
switching is performed independently for every cell. Each cell can be thought of as
taking a virtual channel connection.
Note: The VPI/VCI values are only meaningful in the context of that user-to-switch,
or switch-to-switch, interface. Identical VPI/VCI values can exist on different
interfaces within the network.
Edge-device
Edge-device
ATM
Switch
A
AT M
Switch
A
ATM
Switch
B
Virtual Channel
Virtual Channel Connection (VCC)
(Also known as Virtual Circuit or Call)
Virtual Channel Virtual Channel