The Virtual Path Level

The virtual path (VP) is a generic term for a bundle of virtual channel links; all the links in a bundle have the same endpoints. A VPI identifies a group of VC links, at a given reference point, that share the same VPC. A specific value of VPI is assigned each time a VP is switched in the network. A VP link is a unidirectional capability for the transport

Figure 20.13 Relationship between VC and VP, and the transmission path.

11 VC cross-connect : A network element which connects VC links. It terminates VPCs and translates VCI values and is directed by management plane function, not by control plane functions.

Figure 20.13 Relationship between VC and VP, and the transmission path.

11 VC cross-connect : A network element which connects VC links. It terminates VPCs and translates VCI values and is directed by management plane function, not by control plane functions.

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 terminated. At the VP level, VPCs are provided for the purpose of user-user, user-network, and network-network 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).]

section level, and a transmission path level. The ATM layer resides just above the physical layer and is composed of the VP level, and just above that is the VC level.

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