MULTIPLEXING

Whenever the bandwidth of a medium linking two devices is greater than the bandwidth
needs of the devices, the link can be shared. Multiplexing is the set of techniques
that allows the simultaneous transmission of multiple signals across a single data link.
As data and telecommunications use increases, so does traffic. We can accommodate
this increase by continuing to add individual links each time a new channel is needed;
or we can install higher-bandwidth links and use each to carry multiple signals. As
described in Chapter 7, today's technology includes high-bandwidth media such as
optical fiber and terrestrial and satellite microwaves. Each has a bandwidth far in excess
of that needed for the average transmission signal. If the bandwidth of a link is greater
than the bandwidth needs of the devices connected to it, the bandwidth is wasted. An
efficient system maximizes the utilization of all resources; bandwidth is one of the
most precious resources we have in data communications.
In a multiplexed system, n lines share the bandwidth of one link. Figure 6.1 shows
the basic format of a multiplexed system. The lines on the left direct their transmission
streams to a multiplexer (MUX), which combines them into a single stream (many-toone).
At the receiving end, that stream is fed into a demultiplexer (DEMUX), which
separates the stream back into its component transmissions (one-to-many) and
directs them to their corresponding lines. In the figure, the word link refers to the
physical path. The word channel refers to the portion of a link that carries a transmission
between a given pair of lines. One link can have many (n) channels.