Beta Draft Confidential
2-121/19/05 ATM Services Configuration Guide for CBX 3500, CBX 500, GX 550, and B-STDX 9000
About ATM Logical Ports
VPs and VCs
Figure 2-2. Virtual UNI with VP Multiplexer
VPs and VCs
To establish connections, ATM uses virtual channels (VCs) and virtual paths (VPs). A
VC is a connection between two communicating ATM entities. It may consist of a
group of several ATM links, CPE to central office switch, and
switch-to-switch or switch-to-user equipment. All communications proceed along this
same VC, which preserves call sequence and provides a certain level of QoS.
A VP is a group of VCs carried between two points. VPs provide a way to bundle
traffic headed in the same direction.
VPIs and VCIs are hardware addressing identifiers (similar to Frame Relay’s Data
Link Connection Identifier [DLCI]) that route cell traffic. The ATM cell header
contains both a VPI and a VCI, which gives an ATM cell a unique VCI and associates
it with a particular VP. Every ATM cell uses these VP/VC identifiers.
Switching equipment checks the VPI portion of the header to route traffic over certain
trunks. It uses the VCI portion of the address to deliver the cell to an individual user
within that destination.
CPE
CPE
CPE
VPI 0, 1
VPI 0, 1
VPI 0, 1
VP MUX
VPI 0, 1; 2, 3; ... ;x, x+1
Direct UNI
VPI 0, 1
Virtual UNI
VPI 2, 3
Virtual UNI
VPI x, x+1
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