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, greater is the information carrying capacity. In radio, for example, a very narrow-band signal will carry Morse code; a broader band will carry speech; a still 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 services 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.
In computer networking, the contention ratio refers to the number of low-bandwidth connections that are multiplexed onto a single high-bandwidth uplink connection. In the UK, an RADSL (Rate Adaptive Digital Subscriber Line) connection usually has a contention ratio between 25:1 and 50:1, meaning that 25 to 50 computers with low-bandwidth connections may be sharing the bandwidth of a single uplink connection. The connection speed for each user will therefore differ depending on the number of computers using the high bandwidth connection at the same time because the uplink (where all the low bandwidth connections join) will only handle the speed that has been implemented on that line.
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.
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