DS channel (Bell's ment are described in CCITT Recommendation G.732. Each voice Dataphone Digital Service, see Section 21.8) is thus 57kbit/s, with circuit is sampled at 8 kHz using an 8-bit A-law companding analog 7 bits per frame used for customer data and the eighth bit reserved to digital converter as specified in CCITT Recommendation G.711, for network control. This causes complications, particularly for and multiplexed with 29 other sampled channels plus one alignment users interfacing with an international facility, e.g. a 64kbit/s satel- and one signalling channel, resulting in 32 multiplexed channels. lite channel. The standard CEPT frame contains 32 channels of 8 bits each, or In many early repeaters that were installed along the transmission 256 bits. With 8000 samples per seconds, the CEPT data rate cables linking the T1 devices, the synchronisation circuitry was very becomes 8000 X 256, or 2.048Mbit/s. simple. An oscillator was run at a frequency close to the line pulse stream. The pulses were then used to pull the oscillator on fre- 21.5.1 Frame composition quency. This method relied upon the availability of pulses in the data stream. Because there could be many repeaters over a T1 span, As with the DS1 D4 framing format, the CEPT PCM-30 frame the cumulative effect of a drift in frequency could cause an unac- structure is subject of CCITT Recommendation G.704. Each CEPT ceptable amount of error in the transmission. Therefore the number PCM-30 frame consists of 32 time slots to include 30 voice chan- of pulses, or the ones density, on the line is important. For voice nels, one alignment signal and one signalling channel. Time slots 1 transmission this is not a problem. However, for data it is a different to 15 and 17 to 31 are assigned to the 30 voice or data channels. This story. To be able to maintain the ones density, zero code suppression frame composition is depicted in Figure 21.7. schemes are used. These schemes consist of replacing long se- The frame alignment signal FAS (0011011) is transmit