The digital network is based on pulse code modulation (PCM). The general design of
a PCM system was invented by Reeve in 1937, an ITT engineer from Standard Telephone
Laboratories (STL), while visiting a French ITT subsidiary. It did not become a reality
until Shockley's (Bell Telephone Laboratories) invention of the transistor. Field trials of
PCM systems were evident as early as 1952 in North America.
PCM is a form of time division multiplex. It has revolutionized telecommunications.
Even our super-high-fidelity compact disc (CD) is based on PCM.
Two Different PCM Standards
As we progress through this chapter, we must keep in mind that there are two quite
different PCM standards. On one hand there is a North American standard which some call
T1, and we prefer the term DS1 PCM hierarchy. The other standard is the E1 hierarchy,
which we sometimes refer to as the "European" system. Prior to about 1988, E1 was called
+2, where CEPT stood for Conference European Post and Telegraph (from the
French). Japan has sort of a hybrid system. We do not have to travel far from the United
States to encounter the E1 hierarchy, just south of the Rio Grande (Mexico).
BASIS OF PULSE CODE MODULATION
Let's see how we can develop an equivalent PCM signal from an analog signal, typified
by human speech. Our analog system model will be a simple tone, say 1200 Hz, which
we represent by a sine wave.
There are three steps in the development of a PCM signal from that analog model:
The cornerstone of an explanation of how PCM works is the Nyquist sampling theorem
(Ref. 1), which states:
If a band-limited signal is sampled at regular intervals of time and at a rate equal to or
higher than twice the highest significant signal frequency, then the sample contains all the
information of the original signal. The original signal may then be reconstructed by use of a
Consider some examples of the Nyquist sampling theorem from which we derive the
1. The nominal 4-kHz voice channel: sampling rate is 8000 times per second (i.e.,
2. A 15-kHz program channel:
Sampling rate is 30,000 times per second (i.e.,
A program channel is a communication channel that carries radio broadcast material such as music and
commentary. It is a facility offered by the PSTN to radio and television broadcasters.