peer output module. It is also possible in network situations for a single module at one level
to interface with multiple modules at the next lower layer or the next higher layer. The use of
layering is at least as important for networks as for point-to-point communications systems. The
physical layer for networks is essentially the channel encoding/decoding layer discussed here, but
textbooks on networks rarely discuss these physical layer issues in depth. The network control
issues at other layers are largely separable from the physical layer communication issues stressed
here. The reader is referred to [1], for example, for a treatment of these control issues.
The following three sections give a fuller discussion of the components of Figure 1.1, i.e., of the
fundamental two layers (source coding/decoding and channel coding/decoding) of a point-topoint
digital communication system, and finally of the interface between them.
1.2 Communication sources
The source might be discrete, i.e., it might produce a sequence of discrete symbols, such as letters
from the English or Chinese alphabet, binary symbols from a computer file, etc. Alternatively,
the source might produce an analog waveform, such as a voice signal from a microphone, the
output of a sensor, a video waveform, etc. Or, it might be a sequence of images such as X-rays,
photographs, etc.
Whatever the nature of the source, the output from the source will be modeled as a sample
function of a random process. It is not obvious why the inputs to communication systems
should be modeled as random, and in fact this was not appreciated before Shannon developed
information theory in 1948.
The study of communication before 1948 (and much of it well after 1948) was based on Fourier
analysis; basically one studied the effect of passing sine waves through various kinds of systems