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

282 SAMPLING AND ANALOG-TO-DIGITAL CONVERSION
Figure 6.20
Tl carrier
system.
Channel
Coder
Digital
processor
Transmission
medium
Channel
Digital
processor
0
0 o o 0
PF
0 0 0
)
I
Decoder ...__.--. 0 -
____ _. o LPF 24
0 0
0 0
0 o o LPF
0
0
2
. 23
(a)
Ch. l
I
I
I
Ch. 2
Ch. 5
Ch. 7
Ch. 4
I Ch. 6
l__ Ch. 3
' ''', '
I
a LIU
a u
a an
' ,
Coder output
(b)
Ch. 2
Ch. l
Ch. 3
Ch. 24
The commutators in Fig. 6.20 are not mechanical but are high-speed electronic switching
circuits. Several schemes are available for this purpose. 1 1 Sampling is done by electronic gates
(such as a bridge diode circuit, as shown in Fig. 4.5a) opened periodically by narrow pulses of
2 µ,s duration. The 1.544 Mbit/s signal of the Tl system, called digital signal level 1 (DSl),
is used further to multiplex into progressively higher level signals DS2, DS3, and DS4, as
described next, in Sec. 6.4
After the Bell System introduced the Tl carrier system in the United States, dozens of
variations were proposed or adopted elsewhere before the ITU-T standardized its 30-channel
PCM system with a rate of 2.048 Mbit/s (in contrast to Tl, with 24 channels and 1.544 Mbit/s ).
The 30-channel system is used all over the world, except in North America and Japan. Because
of the widespread adoption of the Tl carrier system in the United States and Japan before the
ITU-T standardization, the two standards continue to be used in different parts of the world,
with appropriate interfaces in international connections.