Frame organisation

Digital time division multiplex structure

For each digital channel there is assigned a group of b bits, called a word which corresponds to a digital message element (data). The groups belonging to the same channel are transmitted at a frequency fs, equal to the sampling frequency. Thus, the bit rate of channel i can be calculated as in Equation 21.2.

In the case of digital PCM telephony, these words are composed of 8 bits and called octets. Hence the bit rate of each channel equals 64000bit/s.

When n digital channels are assembled into a time division multi­plex, the collection of the n words of b bits (and eventually auxiliary bits which are added to them), within a period given by Equation 21.3, constitutes a frame.

Although the structure of the frame is strictly repetitive, their content obviously is not because the channels contain variable digital information.

Two types of frame organisation can be considered, as shown in Figure 21.3:

1. Word interleaved, where the interleaving is performed on a character by character basis, i.e. a character from each data source is accumulated and combined as a word or frame for transmission. To do so, the frame is subdivided into у ≥ z time slots each containing b grouped bits corresponding to the same digital channel or auxiliary bits.

2. Bit interleaved, where the interleaving is performed on a bit by bit basis, i.e. one bit from each data source is accumulated and combined as a word or frame for transmission. To do so, the frame is subdivided into b groups each containing z bits of the same order belonging to each of the channels.

The word interleaved structure corresponds well to the mode of functioning of a PCM modulator, while the bit interleaved structure has advantages for time division digital switching because it suffers less throughput delay.

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