lock for T1/E1 is This allowed a number of asynchronous devices to utilise one circuit 64kbit/s, which is arrived at for historical reasons by the manner at for onward connection. (See Figure 56.5 and Chapter 21.) which analogue voice connections are converted to a digital stream. The statistical multiplexer's role is to handle the inward, and in In the 1930s, Standard Telephones and Cables Ltd. (STC), de- reverse the outward, connections using buffering and retrans- veloped a technique for converting analogue voice to a digital mission techniques. This simple process provided incredible im- stream called Pulse Code Modulation (see Chapter 19). This tech- provement in the efficiency of the time and combined with modem nique was adopted as standard by CCITT as G712. The concept is Digital networks 56/5 as follows. The analogue network had been characterised to operate In Europe, BT was once again the first to provide 64Kbit and at 300Hz to 3400Hz (or 0 to 4kHz), this frequency range essentially below service under the banner of KiloStream. Now throughout enabling acceptable voice communication across a telecommunica- Europe one is able, dependent on the PTT, to receive digital speeds tion infrastructure. The frequency range of 4kHz therefore requires below 64kbit/s, or N X 64kbit/s service (where N = 1 to 30) or a sample rate of twice its value in order to provide accurate repro- 2Mbit/s (E1) or indeed Fractional E1 the equivalent to North duction. America. Given this sampling rate, there was need to code the repre- The arrival of T1 and E1 services, although initially tariffed high sentation of the waveform. Two forms of representation were in the early 1980s provided the means for users to save cost; to adopted, one North American called u-law and one European called improve quality of service; to meet the growing demands of service; A-law. The sampling rate and the subsequent code provide an 8Kbit and lastly to consider their network as a single point of integration code which represented therefore a 64Kbit sig