 Nyquist sampling rate) and then convert- timeslots, of which 30 were for voice and two for synchronisation ing each sample into an 8 bit word or byte. The original technique and signalling information, giving a total of 256 bits. At a sweep rate used by the PTTs was to use frequency division multiplexing with of 8000 frames per second this gives a speed of 2.048Mbit/s. 12 or 24 voice channels being frequency multiplied. For example, All digital exchanges and the space/time switches used these in the CCITT 960 channel system, channel 1 would be frequency speeds at the core of their design. Multiplexing and packet switching 40/9 As with all synchronous TDM networks timing is a major con- 40.5 Multiplexing and packet switching sideration in all network design. Simply put, in order to maintain data integrity, the whole network must run off a single clock or The evolution of multiplexing over the past 20 years has seen the timing source, with every node 'slaved' to that clock designated the generalisation of the original design techniques into two distinct 'master'. Failure to do SO results in 'bit slippage' in the frames, with types: circuit switching and packet switching. data loss. On a small network, geographically spread over a small Circuit switching has developed out of the synchronous digital area, this may not present a serious implementation problem. On a techniques and, as its name implies, supports network source to national network it requires careful planning with contingency back destination connections as a single, end-to-end circuit. Bandwidth up clocks available automatically if the master fails. International over the network links is permanently allocated for the duration of networks resolve the problem by clocking parts of the network the call. It is then set aside whether information is being sent or not. independently or plesiochronously, that is off a clock that is SO stable In this it is analogous to a telephone connection. (1 part in 10-12) that the slippages are low enough to be acceptable.