apacity links to be set up by the network which simplifies the overall design. In contrast, a data communications 'call' is generally characterised by a short transfer of data followed by a long period of inactivity. A transmission may take a fraction of a minute, followed by an inactive period of several minutes before the next portion of data is generated by the source and transmitted. Under these conditions a circuit-switched approach can be very inefficient as the call set-up process may take much longer than the time required for the short burst of data transfer. Furthermore, the process for disconnecting the circuit is difficult to Packetised Data Communications Establish Party A Switch Party B Figure 8.1 A circuit-switched voice call set-up manage since an early termination may increase inefficiency if more short-burst communica- tions appear. A late disconnection on the other hand may occupy a transmission line need- lessly. Moreover, bidirectional data transfer is likely to be unequal due to the client-server traffic nature of most data communications. Therefore, while circuit switching is an effective technology for continuous connectivity, it is not effective for communications for intermittent information transfer. Switching a communication channel for a short duration as needed for a message is more efficient but a mechanism needs to be designed that removes the need for end-to-end channel establishment. Packet-switched communication facilitates independent transmission of the data packets towards the destination. As a result it is not necessary to establish an end-to-end circuit: transmitter only needs to know where a packet is to be sent next, as each packet can carry information that enables an intermediate node to determine where to forward the packet. In practice a packet goes through many intermediate nodes, each of which ideally brings the packet closer to its final destination. The intermediate points are known as routers. Routers switch incoming packets individu- ally based on the destinatio