Virtual circuit
X.25 enables two DTEs to communicate across a telephone network. The following describes the communication flow:
One DTE sends a connection request call to another DTE.
The called DTE accepts or rejects the request.
Once the connection is set up, the DTEs can transmit data in full duplex mode, and either end can disconnect the connection at any time.
The connection established by X.25 between a pair of DTEs is called a virtual circuit. It provides a virtual channel for communication between the two ends.
X.25 virtual circuits are logical links rather than physical links as in circuit switching networks. They include two categories:
PVCs—Which are suitable for frequent, constant data transmission.
SVCs—Which are suitable for bursty data transmission.
Each virtual circuit is uniquely identified by a virtual circuit ID. Each packet sent by a DTE carries a virtual circuit ID, which enables the DCEs in the switched network to forward the packet correctly to its destination.