communication in one direction and an output module for independent communication in the
opposite direction. Later chapters consider modems in much greater depth, including how noise
affects the channel waveform and how that affects the reliability of the recovered binary sequence
at the output. For now, however, it is enough to simply view the modulator as converting a
binary sequence to a waveform, with the peer demodulator converting the waveform back to the
binary sequence.
As another example, the source coding/decoding layer for a waveform source can be split into 3
layers as shown in Figure 1.3. One of the advantages of this layering is that discrete sources are
an important topic in their own right (treated in Chapter 2) and correspond to the inner layer
of Figure 1.3. Quantization is also an important topic in its own right, (treated in Chapter 3).
After both of these are understood, waveform sources become quite simple to understand.
The channel coding/decoding layer can also be split into several layers, but there are a number
of ways to do this which will be discussed later. For example, binary error-correction coding/decoding
can be used as an outer layer with modulation and demodulation as an inner
layer, but it will be seen later that there are a number of advantages in combining these layers
into what is called coded modulation.3 Even here, however, layering is important, but the layers
are defined differently for different purposes.
It should be emphasized that layering is much more than simply breaking a system into components.
The input and peer output in each layer encapsulate all the lower layers, and all these
lower layers can be viewed in aggregate as a communication channel. Similarly, the higher layers
can be viewed in aggregate as a simple source and destination.
The above discussion of layering implicitly assumed a point-to-point communication system
with one source, one channel, and one destination. Network situations can be considerably
more complex. With broadcasting, an input module at one layer may have multiple peer output
modules. Similarly, in multiaccess communication a multiplicity of input modules have a single