tructure is used to transmit 4 bits of information per channel during 16 E1 frames. For historic 37.2.4 Analogue to digital conversion reasons the 4 bits are termed the A,B,C and D bits. Each E1 frame consists of 32 timeslots of which 30 can be used to carry one voice conversation each. Timeslot 0 and 16 in the E1 link are reserved for In the case of analogue network connections the caller's speech needs to be digitised before it can be used. In the case of digital signalling use. Two channels' information is sent in each E1 frame connections the digitalisation process is usually carried out in the in timeslot 16. Timeslot 0 is used for other purposes such as frame callers local telephone exchange. A-Law (European) and u-Law synchronisation signalling. The sixteen E1 frames are termed a (USA and Japan) are companding rules used to enhance signal superframe. Each channel's signalling bits are pre-allocated into a quality. Companding is the term used to describe the non-linear particular frame in this scheme. Whether it is necessary or not, new allocation of the 256 levels in the 8 bit timeslot sample to analogue signalling bits are sent in each superframe for each channel. The signal levels. Usually the digitalisation process creates 8 bit quan- signalling bit rate for each timeslot is 4kbit/s, which is adequate for tised values. This gives adequate audio quality for telephone net- most telephony applications. The bits are used to mimic line states, work use. such as battery presented etc. and dialled digits. The dialled digits are represented by simply sampling the dialling waveform and Either before or after the digitisation process the caller's speech is transmitting its state in the relevant bit and frame within the super- processed to detect tones, to amplify it to a reference level etc. (e.g. frame. DTMF tone detection, automatic gain control, etc.). In advanced In the T1 case a similar system is used, but the bits are hidden in systems these processes are carried out by Digital Signal Processes the caller t