Piecewise linear approximation of the
µ-law logarithmic curve used with the DS1 format.
sequence. This identification is necessary so that the far-end receiver knows exactly when
the sampling sequence starts. Once the receiver receives the "indication," it knows a pri-
ori (in the 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., 2
= 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.
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
(8 × 24) + 1 = 193 bits,
making up a full sequence or frame. By definition, 8000 frames are transmitted per second
× 2, the Nyquist sampling rate), so the bit rate of DS1 is
× 8000 = 1,544,000 bps,
"In-band," an unfortunate expression harking back to the analog world.
In 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.
Supervisory signaling is discussed in Chapter 7. All supervisory signaling does is tell us if the channel is busy