Fig. 7.5. (a) Architecture of T1 link, (b) DS-1 frame format.
Incoming analog signals were time division multiplexed and digitized for transmission. Each individual TDM channel are assigned 8 bits per time slot. Each frame is made of 24 x 8 bits = 192 bits plus one additional bit added to each frame to identity the fame boundaries. Thus each frame contains 193 bits. The frame interval is 125 ^ sec. Hence the basic T1 line rate is 1.544 Mbps. This line rate has been established as the fundamental standard for digital transmission.
The earlier version of the 24 channel system consists of 7 speech bits plus 1 signalling bit. The recent version of 24 channel system uses a 12 frames together called as multi frame. In multi frame, in every 6th frame, the eight bit of each channel is used for signalling instead of speech. The 193rd bit of the PCM frame is used on alterate frames in multi frame arrangement. Except the bits mentioned above, remaing all bits are used for speech. This arrangement reduces the quantization distortion. For a sampling rate of 8 kHz, this arrangement provides a signalling channel capacity of 1.33 kbits/sec. This signalling channel is further subdivided into two separate channels of capacity 667 bits/sec.
30 channel PCM system. It is based on 30 speech channels transmitted within a frame of 32 time slots (0 to 31). The total bit rate is 32 x 8 x 8 kbits/sec = 2048 kbps. Channel 0 is used for providing the framing signal and channel 16 is used for transmitting the signalling information relating to speech channels 1 to 15 and 17 to 31. Fig. 7.6 shows the 30 channel PCM system.
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