X.25 Permanent Virtual Circuits (PVC)
PVCs may at first glance appear to be simpler to handle than SVCs, as there is no call set-up and clearing with which to deal.
However, working out the state of remote DTE is not so simple - for example, it be may be powered off, or there may not be an application attached to the PVC. This therefore leads to a number of complications, so it is the opinion of the author that SVCs should be used instead of PVCs whenever there is a choice.
Having said that, PVCs are suitable:
- when a higher-layer protocol is used on top of the PVC to determine explicitly whether the remote application is responding;
- for the same sort of applications as used with UDP.
For example, market-feed applications are common on PVCs - indeed, some networks support a PVC broadcast facility, such that data packets transmitted on a PVC are then sent out by the network to a number of different remote DTEs.
The Call set-up and Clearing packets are not used on PVCs - the PVC starts in the data transfer state. Because the PVC will probably have been used previously, when attaching to a PVC an application should cause the PVC to be Reset.