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Unformatted text preview: If each terminal is operating at 300 bits per second over a
communication line (channel) that can operate at 9600 bits per second, then we see
a very inefficient operation. It has been found that the capacity of a channel
exceeds that required for a single signal. A channel is an expensive resource.
Hence for its optimal utilization, the channel can be shared in such a way so as to
simultaneously transmit multiple signals over it. The method of dividing a
physical channel into multiple logical channels, so that a number of independent
signals may be simultaneously transmitted on it, is known as multiplexing. The
electronic device that performs this task is known as a multiplexer.
A multiplexer takes several data communication lines or signals and converts them
into one data communication line or signal at the sending location. For example,
as shown in Figure 17.11, there may be 4 terminals connected to a multiplexer.
The multiplexer takes the signals from the 4 terminals and coverts them into one
large signal, which can be transmitted over one communication line. Then, at the
receiving location, a multiplexer takes the one large signal and breaks it into the
original 4 signals. Without multiplexers, you would have to have 4 separate
Thus, with multiplexing it is possible for a single transmission medium to
concurrently transmit data between several transmitters and receivers. There are
two basic methods of multiplexing channels. They are frequency-division
multiplexing and time-division multiplexing.
1. Frequency-Division Multiplexing (FDM). The bandwidth or range of a medium
exceeds that of any given signal. This fact is utilized for frequency-division
multiplexing. In FDM, the available bandwidth of a physical medium is split up
(divided) into several smaller, disjoint logical bandwidths. Each of the component
bandwidths is used as a separate communication line (channel). Figure 17.12
illustrates the process of FDM.
The best example of FDM is the way we receive various stations in a radio. Each
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This document was uploaded on 04/07/2014.
- Spring '14