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

6.4 Digital Multiplexing 289
Figure 6.25
North American
digital hierarchy
(AT&T system).
Channels
1
64 kbit/s 2
DSO signal
24
Channel
bank or
digital
switch
I
2
DM 1/lC
3.152 Mbi tis
DSlC sign al
1.544 Mbit/s
DS I signal
II
2
3
4
6.312 Mbit/s
DS2 signal
DM 1/2
1 1
2
7
DM 2/3
44.736 Mbit/s
DS3 signal
11
2
DM 3/4NA
1
139.264 Mbit/s
There are four orders, or levels, of multiplexing. The first level is the Tl multiplexer
or channel bank, consisting of 24 channels of 64 kbit/s each. The output of this multiplexer
is a DSl (digital level 1) signal at a rate of 1.544 Mbit/s. Four DS I signals are multiplexed
by a DMl/2 multiplexer to yield a DS2 signal at a rate 6.312 Mbit/s. Seven DS2 signals are
multiplexed by a DM2/3 multiplexer to yield a DS3 signal at a rate of 44.736 Mbit/s. Finally,
three DS3 signals are multiplexed by a DM3/4NA multiplexer to yield a DS4NA signal at a
rate 139.264 Mbit/s. There is also a lower rate multiplexing hierarchy, known as the digital
data system (DDS), which provides standards for multiplexing digital signals with rates as
low as 2.4 kbit/s into a DS0 signal for transmission through the network.
The inputs to a Tl multiplexer need not be restricted only to digitized voice channels
alone. Any digital signal of 64 kbit/s of appropriate format can be transmitted. The case of
the higher levels is similar. For example, all the incoming channels of the DMl/2 multiplexer
need not be DS 1 signals obtained by multiplexing 24 channels of 64 kbit/s each. Some of them
may be 1.544 Mbit/s digital signals of appropriate format, and so on.
In Europe and many other parts of the world, another hierarchy, recommended by the
ITU as an standard, has been adopted. This hierarchy, based on multiplexing 30 telephone
channels of 64 kbit/s (E-0 channels) into an E-1 carrier at 2.048 Mbit/s (30 channels) is shown
in Fig. 6.26. Starting from the base level of E-1, four lower level lines form one higher level line
progressively, generating an E-2 line with data throughput of 8.448 Mbit/s, an E-3 line with
data throughput of 34.368 Mbit/s, an E-4 line line with data throughput of 139.264 Mbit/s, and
an E-5 line with data throughput of 565.148 Mbit/s. Because different networks must be able
to interface with one another across the three different systems (North American, Japanese,
and other) in the world, Fig. 6.26 demonstrates the relative relationship and the points of their
common interface.