imeslot bits. Every 6th frame, bit 8 of the caller timeslot (DSPs). These are special purpose microprocessors with an instruc- is replaced by a signalling bit. This is termed robbed bit signalling. tion set specially for these purposes. In early voice processing A multi-frame structure is identified by the use of a spare bit at the systems these functions were carried out in the analogue domain end of each T1 frame bit 193). By this means up to 4 (but usually using op-amps etc. These, by comparison, are difficult to manufac- 2, the A and B bits bits per channel are transmitted. The bits are ture and modify in service. Analogue systems using tightly toler- used in a similar way to the E1 case. There are many variants of the anced components also have a tendency to degrade with time due to use of the CAS bits. component ageing and are inflexible, difficult to modify and per- form field upgrades. On the other hand DSP software can be altered at will to modify and enhance functionality and no degradation over 37.2.2.2 Common Channel Signalling (CCS) time is usually experienced. DSPs use the digital representations of analogue waveforms ob- In common channel signalling systems a more complex and flexible tained from analogue to digital conversion and manipulate them protocol is used. These systems are not at all common in the T1 using software. All the usual analogue filtering techniques can be system because robbed bit signalling is unsuitable for this applica- realised and many more, such as adaptive filtering, correlation and tion. In E1 systems timeslot 16 in the E1 frame is used to carry convolution, delay lines, matched filtering, Fourier analysis and echo cancellation. HDLC formatted data. Any caller timeslot of the frame can have signalling data inserted in timeslot 16 at any time. There is no pre-allocation of bits within the superframe to timeslots. Signalling 37.2.5 Speech compression information is only sent when necessary. When primary rate ISDN protocols become available they will In order to maximise th