frame is strictly repetitive, their System) CCITT System) CCITT content obviously is not because the channels contain variable Recommendation Recommendation digital information. G.732 G.733 Two types of frame organisation can be considered, as shown in Figure 21.3: Sampling f ==8kHz f 8kHz Frequency Word interleaved, where the interleaving is performed on a Bit rate per D 64kbit/s Di 64kbit/s character by character basis, i.e. a character from each data channel source is accumulated and combined as a word or frame for Number of transmission. To do so, the frame is subdivided into y N Z time time slots slots each containing b grouped bits corresponding to the same digital channel or auxiliary bits. Number of Bit interleaved, where the interleaving is performed on a bit by channels bit basis, i.e. one bit from each data source is accumulated and Number of 32 X 8 = 256 + 1 = 193 combined as a word or frame for transmission. To do so, the bits/frame frame is subdivided into b groups each containing Z bits of the same order belonging to each of the channels. Total bit rate 256 X 8kHz = 193 X 8kHz = 2.048kbit/s grouped 1.544kbit/s word of 7 bits in the () The word interleaved structure corresponds well to the mode of distributed sequence channel of odd frames 101010 consisting functioning of a PCM modulator, while the bit interleaved structure of the 193rd bit of has advantages for time division digital switching because it suffers odd frames less throughput delay. Signalling Out-of-octet, 21.2.2 Frame alignment grouped in channel 16, consisting of 4 bits per channel, The frame alignment or framing is typical of time division multi- distributed over 16 plexing. It consists of synchronising the receiving equipment both frames (=1 in frequency and in phase to the stream of symbols it receives. This multiframe), operation is obviously necessary each time the receiver is switched on, but also during normal operation. Indeed, once aligned, the receiver needs a periodic time reference in order to check its iso- chronism and 