563489578934

Sec. 3–9 Time-Division Multiplexing 213
TDM Hierarchy
In practice, TDMs may be grouped into two categories. The first category consists of multiplexers
used in conjunction with digital computer systems to merge digital signals from
several sources for TDM transmission over a high-speed line to a digital computer. The output
rate of these multiplexers has been standardized to 1.2, 2.4, 3.6, 4.8, 7.2, 9.6, 14.4, 19.2, and
28.8 kbitss and to 10 and 100 Mbitss.
The second category of TDMs is used by common carriers, such as the American
Telephone and Telegraph Company (AT&T), to combine different sources into a highspeed
digital TDM signal for transmission over toll networks. Unfortunately, the standards
adopted by North America and Japan are different from those that have been
adopted in other parts of the world. The North America–Japan standards were first
adopted by AT&T, and another set of standards has been adopted by CCITT under the
auspices of ITU. The North American TDM hierarchy is shown in Fig. 3–40 [James and
Muench, 1972]. † The telephone industry has standardized the bit rates to 1.544 Mbitss,
6.312 Mbitss, etc., and designates them as DS-1 for digital signal, type 1; DS-2 for digital
signal, type 2; etc. as listed in Table 3–8. In Fig. 3–40, all input lines are assumed to
be digital (binary) streams, and the number of voice-frequency (VF) analog signals that
can be represented by these digital signals is shown in parentheses. The higher level multiplexing
inputs are not always derived from lower level multiplexers. For example, one
analog television signal can be converted directly to a DS-3 data stream (44.73 Mbitss).
Similarly, the DS streams can carry a mixture of information from a variety of sources
such as video, VF, and computers.
The transmission medium that is used for the multiplex levels depends on the DS level
involved and on the economics of using a particular type of medium at a particular location
(Table 3–8). For example, higher DS levels may be transmitted over coaxial cables or fiberoptic
cable or via microwave radio or satellite. A single DS-1 signal is usually transmitted
over one pair of twisted wires. (One pair is used for each direction.) This type of DS-1 transmission
over a twisted-pair medium is known (from its development in 1962 by AT&T) as the
T1 carrier system [Andrews, 2011]. DS-1 signaling over a T1 system is very popular because
of its relatively low cost and its excellent maintenance record. (T1 will be discussed in more
detail in the next section.) Table 3–9 shows the specifications for the T-carrier digital baseband
systems. Table 8–2 is a similar table for the capacity of common-carrier bandpass
systems. The corresponding CCITT TDM standard that is used throughout the world except
in North America and Japan is shown in Fig. 3–41 [Irmer, 1975].
With the development of high-bit-rate fiber-optic systems, it has become apparent that
the original TDM standards are not adequate. A TDM standard called the (Synchronous
Optical Network (SONET) was proposed by Bellcore (Bell Communications Research)
around 1985 and has evolved into an international standard that was adopted by the CCITT in
† The Japanese TDM hierarchy is the same as that for North America for multiplex levels 1 and 2, but differs
for levels 3, 4, and 5. For level 3, the Japanese standard is 32.064 Mbitss (480 VF), level 4 is 97.728 Mbitss
(1440 VF), and level 5 is 397.200 Mbitss (5760 VF). Dissimilarities between standards are briefly discussed and
summarized by Jacobs [1986].