case of DS1) that 24 eight-bit slots follow. It synchronizes the receiver. Such identification is carried out by a framing bit, and one full sequence or cycle of samples is called a frame in PCM terminology. Consider the framing structure of the two widely implemented PCM systems: the North American DS1 and the European E1. The North American DS1 system is a 24-channel PCM system using 8-level coding (e.g., 28 = 256 quantizing steps or distinct PCM code words). Supervisory signaling is "in-band" where bit 8 of every sixth frame is "robbed" for supervisory signaling.3-5 The DS1 format shown in Figure 6.8 has one bit added as a framing bit. (This is that indication to tell the distant end receiver where the frame starts.) It is called the "S" bit. The DS1 frame then consists of x 24) + 1 = 193 bits, making up a full sequence or frame. By definition, 8000 frames are transmitted per second (i.e., 4000 X 2, the Nyquist sampling rate), SO the bit rate of DS1 is 193 x 8000 = 1,544,000 bps, or 1.544 Mbps. 3"In-band," an unfortunate expression harking back to the analog world. 4In the DS1 system it should be noted that in each frame that has bit 8 "robbed," 7-bit coding is used versus 8-bit coding employed on the other five frames. 5 Supervisory signaling is discussed in Chapter 7. All supervisory signaling does is tell us if the channel is busy or idle. 6.2 BASIS OF PULSE CODE MODULATION 1 Frame 125 us 67812345678 Sampling frequency 8000 Hz Output bit rate 1.544 Mbps + 50 bps Bits/Frame Time slots/Frame 24 (Sequential assignment) Signaling Eight bit of every sixth frame The S-bit is time-shared between terminal framing (Fi) and signal framing (Fs). Figure 6.8 DS1 signal format. The E1 European PCM system is a 32-channel system. Of the 32 channels, 30 transmit speech (or data) derived from incoming telephone trunks and the remaining 2 channels transmit synchronization-alignment and signaling information. Each channel is allotted an 8-bit time slot (TS), and we tabulate TS 0 through 31 as follows: Type of Information Synch