Virtual Circuit Service

X.25 provides for two types of virtual circuits: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs). An SVC is a dynamically established virtual circuit using call setup and call clearing procedures. A PVC is a permanent, network-assigned virtual circuit that requires no call setup or clearing.

A virtual circuit provides a connection-oriented service, similar to that of circuit switching but with the following exceptions:

• It is limited to a connection between two end points.
• It has the advantage of economical statistical multiplexing for establishing a logical path through the network.

This logical path can be provided either on a permanent basis by a PVC, which is equivalent to a leased circuit, or on a request basis by an SVC. Once the logical path is established, the packets are transferred between connected ends, as desired.

The packets are statistically multiplexed with packets of other users, optimizing the transmission media of the network.

Each packet is associated with a logical channel, which is mapped to the appropriate destination of the virtual circuit. This enables a more effective use of the access circuit for the available bandwidth and the traffic density for each logical channel.

The router can manually maintain IPX, IP, and AppleTalk connections using SVCs between multiple sites. You can set up SVCs that can be connected manually or automatically. Links that are set up manually are easy to install and maintain because they use routing table updates to discover end-user stations and hosts for each X.25 destination automatically. The router can automatically establish and disconnect on-demand IP connections using SVCs.

The types of connections supported by NetWare Link/X.25 for various protocols are shown in the following table.

A permanent SVC is established at initialization and is left in a connected state until the user or application brings it down. An on-demand SVC is established only when data is present for the associated virtual circuit and is brought down after the data has been transmitted and the configured idle timer has expired. An on-demand SVC remains down until more data is queued up to be sent, then the connection is reestablished.

The procedure for setting up a virtual call is to establish a logical path, then the data packets are automatically sent to the appropriate destination.

The packet level provides the virtual circuit service of PVCs and SVCs. Logical channels differentiate the virtual circuits supported by the packet level. Multiple connections are provided simultaneously by multiplexing virtual circuits over the access line. Only one PVC or SVC can be established at a time on each logical channel.