AT&T MERLIN LEGEND System Reference Manual page 473

Functional Description
Commanding
Digital Switching
4-4 Control Unit Operation
Quantizing causes distortion due in part to the "rounding off" of PAM samples
into discrete PCM values. This distortion can be minimized by increasing the
number of bits used to encode each PAM sample or by increasing the sampling
rate. Using 8-bit coded samples taken at an 8-kHz rate results in toll-grade
transmission quality.
Distortion also occurs in quantizing because equal consideration is given to all
amplitude levels. Linear quantizing divides the amplitude range into equal
segments, which results in unnecessary quality at levels where voice signals are
unlikely to occur and not enough quality where they do occur. Commanding
eliminates this distortion by using a nonlinear scale that has smaller divisions
and more accuracy in areas of the scale where voice signals are most likely to
occur.
Mu-Law 255
Mu-Law 255 is a commanding scheme that uses a compression ratio compatible
with modem channel bank equipment. The scale is divided into 16 intervals, with
each interval having 16 levels. Therefore, the scale has 256 discrete values. But
since Mu-Law 255 has a negative and a positive zero, it really has only 255
discrete values. This commanding scheme is used in the United States and
Japan.
The TDM bus allows many users to communicate over a common electrical
connection because it is physically distributed across the backplane of the
control unit and connects all line/trunk and station modules.
The TDM bus can specify time slots for various functions. For example, during a
conversation between station A and station B, a time slot is reserved for
station A to transmit on and for station-B to receive on. For example, station A
talks on time slot 150 and listens on time slot 160. Station B talks on time slot
160 and listens on time slot 150. Because the TDM bus cycles 8000 times per
second, the conversation appears to be continuous.
The TDM bus carries touch-tone and call-progress signals to stations via time
slots 0 to 39. Unlike other bus configurations, the stations on the TDM bus
receive all transmissions. If a station is not assigned to any of the time slots, the
station ignores the data.
The DSE handles voice and data. It also performs the operations for
commanding schemes such as Mu-Law and A-Law. Each module has a DSE to
interface codecs or digital transceivers to the TDM bus. The actual digital
switching occurs when the DSE is programmed by the system I/O bus to
transmit data on or receive data from the TDM bus in specific time slots. In
addition, the DSE can sum digital signals from designated TDM slots to provide
for five-party conferencing.