 - Frequency-division multiplexing (FDM) is
a scheme in which numerous signals are combined for transmission
on a single communications line or channel. Each signal is
assigned a different frequency (subchannel) within the main
channel.
A typical analog Internet connection via a twisted pair telephone
line requires approximately three kilohertz (3 kHz) of bandwidth for accurate
and reliable data transfer. Twisted-pair lines are common in
households and small businesses. But major telephone cables,
operating between large businesses, government agencies, and
municipalities, are capable of much larger bandwidths.
Suppose a long-distance cable is available with a
bandwidth allotment of three megahertz (3 MHz). This is 3,000 kHz, so in theory, it is possible to place 1,000 signals, each 3
kHz wide, into the long-distance channel. The circuit that does
this is known as a multiplexer. It accepts the input from each
individual end user, and generates a signal on a different
frequency for each of the inputs. This results in a
high-bandwidth, complex signal containing data from all the end
users. At the other end of the long-distance cable, the
individual signals are separated out by means of a circuit called
a demultiplexer, and routed to the proper end users. A two-way
communications circuit requires a multiplexer/demultiplexer at
each end of the long-distance, high-bandwidth cable.
When FDM is used in a communications network,
each input signal is sent and received at maximum speed at all
times. This is its chief asset. However, if many signals must be
sent along a single long-distance line, the necessary bandwidth
is large, and careful engineering is required to ensure that the
system will perform properly. In some systems, a different
scheme, known as time-division multiplexing, is used instead.
| LAST UPDATED: |
26 Jul 2001
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