ELECOMMUNICATIONS 6.7 PACKET SWITCHING NETWORKS The pioneering efforts towards sharing the resources of networks among a community of users were based on a technique of channel allocation called packet switching. The first successful packet switched network was the Advance Research Projects Agency Net- work (ARPANET) which became operational in the United States in 1969, followed soon after by others in Europe. Packet switching is a derivative of message switching and uses many message switching features. The key difference is that the mes- sage is divided into maximal length blocks called packets. The chan- nel capacity is allocated to the user only for the duration of each packet. Each packet is individually addressed and transmits the net- work in a store and forward fashion (see Fig. 6-12). When a packet is transmitted, the channel is immediately available for allocation to packets being generated by other users. In this way the expensive facility costs may be shared by many users. DAIDC AD BE BD CE CF AB AB|EC AB|FB DA EC DC FA FB EF BEICE Fig. 6-12. Packet switching addressing scheme. NETWORK STRUCTURES 99 Packet switching networks use the concept of virtual calls and vir- tual circuits. When a request to enter packets into the network is received a 'virtual call' is established. The virtual call is somewhat analogous to a dialed switched network connection (see Fig. 6-13). It exists only for the duration of the call. For that period the network behaves as though a fixed path exists from the source to destination. Those users with a high volume of traffic can establish a permanent virtual circuit. This type of path behaves like a dedicated channel and eliminates the need for set up and termination sequences. The virtual circuit concept is made to work by the use of logical channel numbers. Each packet entering the network is assigned a logical channel number which indicates the session or 'conversation' to which it belongs. The network can then associate the logical chan- nel number of one subscriber to the