Virtual – circuit network is a category of packet switching network, where a virtual path is established between the source and the destination systems for data communication to occur. This path appears to the user as if it is a dedicated physical path, but actually is a logical circuit allocated from a managed pool of circuit resources as per traffic requirements. The network resources forming parts of this path can be shared by other communications, however, is not visible to this user.
There are three phases of transmission by virtual circuits, set up, data transfer and teardown.
Set up Phase − In this phase, a virtual circuit or a route is established from the source to the destination through number of switches. The source and destination use global addresses using which the switches make routing table entries.
Data Transfer − Once the virtual circuit is set up, all packets follow the route established during the set up phase adhering to the routing tables.
Teardown Phase − When data transfer is complete, the source sends a teardown request. The destination responds using a teardown confirmation. The switches flush their routing table entries, thus relinquishing the circuit.
In the adjoining diagram, we can see that the Internet Service Provider (ISP) has six routers (1 to 6) connected by transmission lines shown in black lines. There are three hosts, host H1 and H3 are connected to router 1, while host H2 is connected to router 6.
Suppose that hosts H1 and H3 both wants to send data packets to host H2. Virtual circuits are established between the hosts to enable data transmission. For H1, a virtual circuit via the routers 1 – 2 – 6 is established, as denoted by green dotted lines. All its packets, 1A, 1B, 1C and 1D are routed through this circuit. In the same way, another virtual circuit via the routers 1 – 2 – 3 – 6 is established, as denoted by red dotted lines. H3’s packets 3A, 3B and 3C are
routed through this circuit. Each router maintains a separate entry in the routing table for each virtual circuit that it is a part of.