In circuit switching, a connection is established between two network nodes before they begin transmitting data. Bandwidth is dedicated to this connection and remains available until the users terminate communication between the two nodes. While the nodes remain connected, all data follows the same path initially selected by the switch. This works much like a normal telephone line works for voice communication. Integrated Services Digital Network (ISDN) is a good example of circuit switching. When a router has data for a remote site, the switched circuit is initiated with the circuit number of the remote network. In the case of ISDN circuits, the device actually places a call to the telephone number of the remote ISDN circuit. When the two network node s are connected and authenticated, they can transfer data. When the data transmission is complete, the call can be terminated. There are three phases involved in circuit switching as below:
Circuit establishment – In this phase, the circuit must be established, when signals are transmitted from one network node to other node. This can send information through signals before that the receiving end and sending end must establish their circuit.
Data transfer – In this data transfer phase, information can be transmitted from one network node to other network node through network. The data may be analog signals or digital signals that depend on the nature of the network protocol.
Circuit disconnect – After some time of data transfer, the connection is terminated, generally by the action of one of the two network nodes. Signals must be propagated to given nodes to deallocate the dedicated resources.
The above figure shows that device A is connected to device E through the switches I, III & IV. Other devices can connect to each others by moving the levers of the switches.