B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

284 SAMPLING AND ANALOG-TO-DIGITAL CONVERSION
to transmit this information. This means that every sixth sample of each voice signal will have
a possible error corresponding to the least significant digit. Every sixth frame, therefore, has
7 x 24 = 168 information bits, 24 signaling bits, and I framing bit. In all the remaining frames,
there are 192 information bits and 1 framing bit. This technique is called 7 i bit encoding, and
the signaling channel so derived is called robbed-bit signaling. The slight SNR degradation
suffered by impairing one out of six frames is considered to be an acceptable penalty. The signaling
bits for each signal occur at a rate of 8000/6 = 1333 bit/s. The frame format is shown
in Fig. 6.21.
The older seven-bit framing format required only that frame boundaries be identified
so that the channels could be located in the bit stream. When signaling is superimposed on
the channels in every sixth frame, it is necessary to identify, at the receiver, which frames
are the signaling frames. A new framing structure, called the superframe, was developed
to take care of this. The framing bits are transmitted at 8 kbit/s as before and occupy the
first bit of each frame. The framing bits form a special pattern, which repeats in 12 frames:
100011011100. The pattern thus allows the identification of frame boundaries as before, but
also allows the determination of the locations of the sixth and twelfth frames within the superframe.
Note that the superframe described here is 12 frames in length. Since two bits per
superframe are available for signaling for each channel, it is possible to provide four-state
signaling for a channel by using the four possible patterns of the two signaling bits: 00, 01,
10, and 11. Although most switch-to-switch applications in the telephone network require
only two-state signaling, three- and four-state signaling techniques are used in certain special
applications.
Advances in digital electronics and in coding theory have made it unnecessary to use
the full 8 kbit/s of the framing channel in a DSl signal to perform the framing task. A new
superframe structure, called the extended superframe (ESF) format, was introduced during
the 1970s to take advantage of the reduced framing bandwidth requirement. An ESF is 24
frames in length and carries signaling bits in the eighth bit of each channel in frames 6, 12, 18,
and 24. Sixteen-state signaling is thus possible and is sometimes used although, as with the
superframe format, most applications require only two-state signaling.
The 8 kbit/s overhead (framing) capacity of the ESF signal is divided into three channels: 2
kbit/s for framing, 2 kbit/s for a cyclic redundancy check (CRC-6) error detection channel, and
4 kbit/s for a data channel. The highly reliable error checking provided by the CRC-6 pattern
and the use of the data channel to transport information on signal performance as received
by the distant terminal make ESF much more attractive to service providers than the older
superframe format. More discussions on CRC error detection can be found in Chapter 14.
The 2 kbit/s framing channel of the ESF format carries the repetitive pattern 001011 ... , a
pattern that repeats in 24 frames and is much less vulnerable to counterfeiting than the patterns
associated with the earlier formats.
For vaiious reasons, including the development of intelligent network-switching nodes,
the function of signaling is being transferred out from the channels that carry the messages
or data signals to separate signaling networks called common channel interoffice signaling
(CCIS) systems. The universal deployment of such systems will significantly decrease the
importance of robbed-bit signaling, and all eight bits of each message (or sample) will be
transmitted in most applications.
The Conference on European Postal and Telegraph Administration (CEPT) has standardized
a PCM with 256 time slots per frame. Each frame has 30 x 8 = 240 information bits,
corresponding to 30 speech channels (with eight bits each). The remaining 16 bits per frame
are used for frame synchronization and signaling. Therefore, although the bit rate is 2.048
Mbit/s, corresponding to 32 voice channels, only 30 voice channels are transmitted.