1.1. STANDARDIZED INTERFACES AND LAYERING 3
• A standardized binary interface between source and channel simplifies networking, which
now reduces to sending binary sequences through the network.
• One of the most important of Shannon’s information theoretic results is that if a source
can be transmitted over a channel in any way at all, it can be transmitted using a binary
interface between source and channel. This is known as the source/channel separation
theorem.
In the remainder of this chapter, the problems of source coding and decoding and channel coding
and decoding are briefly introduced. First, however, the notion of layering in a communication
system is introduced. One particularly important example of layering was already introduced in
Figure 1.1, where source coding and decoding are viewed as one layer and channel coding and
decoding are viewed as another layer.
1.1 Standardized interfaces and layering
Large communication systems such as the Public Switched Telephone Network (PSTN) and the
Internet have incredible complexity, made up of an enormous variety of equipment made by
different manufacturers at different times following different design principles. Such complex
networks need to be based on some simple architectural principles in order to be understood,
managed, and maintained.
Two such fundamental architectural principles are standardized interfaces and layering.
A standardized interface allows the user or equipment on one side of the interface to ignore all
details about the other side of the interface except for certain specified interface characteris
tics. For example, the binary interface2 above allows the source coding/decoding to be done
independently of the channel coding/decoding.
The idea of layering in communication systems is to break up communication functions into a
string of separate layers as illustrated in Figure 1.2.
Each layer consists of an input module at the input end of a communcation system and a ‘peer’
output module at the other end. The input module at layer i processes the information received
from layer i+1 and sends the processed information on to layer i−1. The peer output module at
layer i works in the opposite direction, processing the received information from layer i−1 and
sending it on to layer i.
As an example, an input module might receive a voice waveform from the next higher layer and
convert the waveform into a binary data sequence that is passed on to the next lower layer. The
output peer module would receive a binary sequence from the next lower layer at the output
and convert it back to a speech waveform.
As another example, a modem consists of an input module (a modulator) and an output module
(a demodulator). The modulator receives a binary sequence from the next higher input layer
and generates a corresponding modulated waveform for transmission over a channel. The peer
module is the remote demodulator at the other end of the channel. It receives a more-or
less faithful replica of the transmitted waveform and reconstructs a typically faithful replica
of the binary sequence. Similarly, the local demodulator is the peer to a remote modulator
(often collocated with the remote demodulator above). Thus a modem is an input module for