rrect order creates significant flexibility and robustness in a telecommu- nications system design. Special protocols have been designed to cater for delay sensitive service such as voice communications. These allow for prioritisation of real-time services and minimise delay and delay variations. This chapter gives an overview of packet switching technologies and protocols. Circuit Switching and Packet Switching As discussed in the Historical Note at the beginning of this chapter, data communications technologies and protocols trace their origins back to the 1950s. With the rise of computers it became necessary to find ways to facilitate machine-to-machine transfer of data. The nature of such communications was significantly different to that of voice telecommunications and audio and video broadcasting. A voice call has a well-defined statistical probability and con- tinues for an average length of 2-3 min, and therefore lends itself well to a circuit-switched mode of communications where a dedicated end-to-end circuit could be set up for the call. The process of such a call set-up is shown in Figure 8.1. The process involves connection between exchanges, and the called party, ensuring there are circuits available for an end-to-end connec- tion, checking to see whether the called party is not busy, and waiting for the called party to answer the telephone. This process can take a considerable amount of time and resource. At the end of a call, control signalling is necessary to disconnect the call and allow the circuit to be used for another call. Other processes such a billing and verification are also active during a call. These control processes are all overheads on the actual 'information' communi- cation. However, since an average call continues for several minutes, the overall overhead is comparatively small and does not reduce system efficiency significantly. Moreover, a voice call is bidirectionally balanced: the two parties are expected to talk and listen in equal propor- tions. This allows for two equal c