ronizing (framing) Speech Signaling 17-31 Speech In TS 0 a synchronizing code or word is transmitted every second frame, occupying digits 2 through 8 as follows: 0011011 In those frames without the synchronizing word, the second bit of TS 0 is frozen at a 1 SO that in these frames the synchronizing word cannot be imitated. The remaining bits of TS 0 can be used for the transmission of supervisory information signals (Ref. 16). Again, E1 in its primary rate format transmits 32 channels of 8-bit time slots. An E1 frame therefore has 8 x 32 = 256 bits. There is no framing bit. Framing alignment is carried out in TS 0. The E1 bit rate to the line is 256 X 8000 = 2,048,000 bps, or 2.048 Mbps Framing and basic timing should be distinguished. "Framing" ensures that the PCM receiver is aligned regarding the beginning (and end) of a bit sequence or frame; "timing" refers to the synchronization of the receiver clock, specifically, that it is in step with its companion far-end transmit clock. Timing at the receiver is corrected via the incoming "1"-to-"0" and "0"-to-"1" transitions. It is mandatory that long periods of no transitions 6A transition in this context is a change of electrical state. We often use the term "mark" for a binary 1 and "space" for a binary 0. The terms mark and space come from old-time automatic telegraphy and have been passed on through the data world to the parlance of digital communications technology. DIGITAL NETWORKS do not occur. This important point is discussed later in reference to line codes and digit inversion. 6.3 PCM SYSTEM OPERATION PCM channel banks operate on a four-wire basis. Voice channel inputs and outputs to and from a PCM multiplex channel bank are four-wire, or must be converted to four- wire in the channel bank equipment. Another term commonly used for channel bank is codec, which is a contraction for coder-decoder even though the equipment carries out more functions than just coding and decoding. A block diagram of a typical codec (PCM channel bank) is shown in Figure 6.9. A 