20.8
B-ISDN/ATM ROUTING AND SWITCHING
529
of ATM cells between two consecutive ATM entities where the VPI value is translated.
A VP link is originated or terminated by the assignment or the removal of the VPI value.
Routing functions for VPs are performed at a VP switch/cross-connect. This routing
involves translation of the VPI values of the incoming VP links into the VPI values of
the outgoing VP links. VP links are concatenated to form a VPC. A VPC extends two
VPC end-points or, in the case of point-to-multipoint arrangements, more than two VPC
end-points. A VPC end-point is the point where the VCIs are originated, translated, or ter-
minated. At the VP level, VPCs are provided for the purpose of useruser, usernetwork,
and networknetwork information transfer.
When VPCs are switched, the VPC supporting the incoming VC links are terminated
first and a new outgoing VPC is then created. Cell sequence integrity is preserved by the
ATM layer for cells belonging to the same VPC. Thus cell sequence integrity is preserved
for each VC link within a VPC.
Figure 20.14 is a representation of a VP and VC switching hierarchy where the physical
layer is the lowest layer composed of, from bottom up, a regenerator section level, a digital
Figure 20.14
Representation of the VP and VC switching hierarchy. (a) VC and VP; (b) VP switching.
[From ITU-T Rec. I.311, Figure 4/I.311, p. 5 (Ref. 12).]
Summary :
A VPC extends two VPC end-points or, in the case of point-to-multipoint arrangements, more than two VPC end-points. When VPCs are switched, the VPC supporting the incoming VC links are terminated first and a new outgoing VPC is then created.
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