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Broadband in general electronics and telecommunications
is a term which refers to a signal or circuit which includes or handles
a relatively wide range of frequencies. 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).
Introduction
Broadband in data communications may have the same
meaning as above, so that data transmission over a fibre 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.
Multiplexing
Communications may utilise 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 CDMA). 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 utilised
for the same communication could be described as broadband.
Confusing usage
While many lower rate forms of data transmission, such
as analog modems above 600 bit/s, are broadband, broadband has been more
closely associated with higher data rate forms of broadband data
transmission such as T-carrier and Digital Subscriber Lines. Therefore,
the word "broadband" has also come to mean a relatively high rate, while
the term "narrowband" is used to mean a relatively low rate. It is now
quite common to hear a broadband method such as a 9600 bit/s modem
described as "narrowband", while a high rate baseband transmission such
as 10BASE-T is described as "broadband". The International
Telecommunication Union Standardization Sector (ITU-T) recommendation
I.113 has defined broadband as a transmission capacity that is faster
than primary rate ISDN, at 1.5 to 2 Mbit/s. However speeds of 256 kbit/s
and greater are commonly marketed as "broadband" and this convention is
held to by policy makers and ISPs alike. See Broadband Internet access.
Note: The term "narrowband" is also used to mean the opposite of
"wideband" instead of the opposite of "broadband". |
