Permanent virtual circuit
Permanent virtual circuits (PVC) are full-period virtual circuits (VC) that are manually provisioned at each switch in a network. The provisioning process identifies the end-points of the circuit and the traffic parameters associated with the circuit. For example, in frame relay it would include the specification of the CIR and EIR. In ATM it would include the specification of the virtual circuit type (e.g., CBR, VBR-rt, VBR-nrt, or UBR) and the specific traffic parameters associated with the circuit type (e.g., PCR, SCR, BT, etc.). Typically the switch software checks attributes to ensure the service guarantee can be met for the VC.
The specific path along which the PVC will flow must also be specified. This can be done in a number of fashions. It can be done manually, although this is increasingly rare. If it is done manually, however, either a back-up path must also be defined or the PVC will need to be manually restored in the event of a failure. Today, path specification is typically done dynamically once the network knows the end-points. The network service defines an internal signaling mechanism to establish such paths, and to restore the PVC on a new path in the event of failure.
Applications perfectly suited for PVCs include the following:
- Corporations having stable, closed user groups, known end points, and fixed communications patterns
- Situations having a known class of service (e.g., when ATM provides tie-line service between two ATM-capable PBXs)
- Situations supporting single destinations for traffic (e.g., a corporate access link to the Internet)
|| PVC vs. switched virtual circuit (SVC)|