3Com CoreBuilder 9000, User Manual

Page: 351 / 468

Virtual Channels 351
The figure shows three ATM devices: two ATM edge devices and an ATM
Enterprise Switch between them. The virtual channel begins at the left
edge device, passes through the ATM Enterprise Switch and terminates at
the right edge device. Two external links of the virtual channel, shown as
2
and
4
connect output ports of one device with input ports of the
adjacent device; the internal link within the ATM Enterprise Switch,
shown as
3,
associates the input port of the ATM Switch Fabric Module
with the correct output port.
External Links — ATM Connections
Each external link, referred to as an ATM connection , is uniquely
characterized by a pair of integers VPI, VCI called the Virtual Path
Identifier and Virtual Channel Identifier . The default range is VPI: 0-2
8
;
VCI: 0-2 16 .
The VPI/VCI pairs are allocated by the ports at both ends of the external
link. For example, the values VPI/VCI=0/32 in link number 2, shown in
Figure 16, were allocated by the output port of the edge device and the
input port of the ATM switch. The fact that separate devices can allocate
exactly the same VPI/VCI value is the result of UNI signaling negotiation
between them during establishment of the virtual channel. For more
information about UNI signaling, see “Signaling — External Links” on
page 354.
Internal Link — Switch Matrix
The virtual channel is completed by adding the internal link inside the
ATM devices. The internal link does not run over a fixed cable like the
external link, but is rather a correspondence between the adjacent
external links that is stored in the internal memory matrix of the switch.
The ATM switch stores the correspondence
3
(Figure 16) between link
2
and link
4
in its internal memory matrix. Thus, the VPI/VCI values of link
2
at the specific input port of the switch are associated with the VPI/VCI
values of link
4
at the specific output port of the ATM Enterprise Switch.
Similar to the external links, this correspondence is established during the
signaling phase.