Virtual Circuit

Virtual circuit switching is a packet switching methodology whereby a path is established between the source and the final destination through which all the packets will be routed during a call. This path is called a virtual circuit because to the user, the connection appears to be a dedicated physical circuit. However, other communications may also be sharing the parts of the same path.

Before the data transfer begins, the source and destination identify a suitable path for the virtual circuit. All intermediate nodes between the two points put an entry of the routing in their routing table for the call. Additional parameters, such as the maximum packet size, are also exchanged between the source and the destination during call setup. The virtual circuit is cleared after the data transfer is completed.

Virtual circuit packet switching is connection orientated. This is in contrast to datagram switching, which is a connection less packet switching methodology.

Advantages of virtual circuit switching are:

·         Packets are delivered in order, since they all take the same route;

·         The overhead in the packets is smaller, since there is no need for each packet to contain the full address;

• The connection is more reliable, network resources are allocated at call setup so that even during times of congestion, provided that a call has been setup, the subsequent packets should get through;
• Billing is easier, since billing records need only be generated per call and not per packet.

Disadvantages of a virtual circuit switched network are:

• The switching equipment needs to be more powerful, since each switch needs to store details of all the calls that are passing through it and to allocate capacity for any traffic that each call could generate;
• Resilience to the loss of a trunk is more difficult, since if there is a failure all the calls must be dynamically reestablished over a different route.

Examples of virtual circuit switching are X.25 and Frame Relay.