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Broadband in
telecommunications is a term which refers to a
signaling method which includes or handles a
relatively wide range of frequencies which may
be divided into channels or frequency bins.
Broadband is always a relative term,
understood according to its context. The wider
the bandwidth, the more information can be
carried. In radio, for example, a very
narrowband signal will carry Morse code; a
broader band will carry speech; a yet broader
band is required to carry music without losing
the high audio frequencies required for
realistic sound reproduction. A television
antenna described as "normal" may be capable
of receiving a certain range of channels; one
described as "broadband" will receive more
channels. In data communications a modem will
transmit a bandwidth of 64 kilobits per
seconds (kbit/s) over a telephone line; over
the same telephone line a bandwidth of several
megabits per second can be handled by ADSL,
which is described as broadband (relative to a
modem over a telephone line, although much
less than can be achieved over a fibre optic
circuit, for example). With Cable Broadband
there is a higher chance of maintaining a
constant broadband speed compared to ADSL
services.
Broadband in data communications may have the
same meaning as above, so that data
transmission over a fiber optic cable would be
referred to as broadband as compared to a
telephone modem operating at 600 bits per
second.
However, broadband in data communications is
frequently used in a more technical sense to
refer to data transmission where multiple
pieces of data are sent simultaneously to
increase the effective rate of transmission,
regardless of actual data rate. In network
engineering this term is used for methods
where two or more signals share a medium.
Various forms of Digital Subscriber Line
service are broadband in the sense that
digital information is sent over one channel
and voice over another channel sharing a
single pair of wires. Analog modems operating
at speeds greater than 600 bit/s are
technically broadband. They obtain higher
effective transmission rates by using multiple
channels with the rate on each channel limited
to 600 baud. For example, a 2400 bit/s modem
uses four 600 baud channels (see baud). This
is in contrast to a baseband transmission
where one type of signal uses a medium's full
bandwidth such as 100BASE-T Ethernet.
Communications may utilize a number of
distinct physical channels simultaneously;
this is multiplexing for multiple access. Such
channels may be distinguished by being
separated from each other in time (time
division multiplexing or TDMA), in carrier
frequency (frequency division multiplexing (FDMA)
or wavelength division multiplexing (WDM)), or
in access method (code division multiplexing
or CDM). Each channel that takes part in such
a multiplexing exercise is by definition
narrowband (because it is not utilising the
whole bandwidth of the medium), whereas the
whole set of channels taken together and
utilized for the same communication could be
described as broadband. |
