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Bandwidth (the width of a band of electromagnetic frequencies) is used to mean (1) how fast data flows on a given transmission path, and (2), somewhat more technically, the width of the range of frequencies that an electronic signal occupies on a given transmission medium. Any digital or analog signal has a bandwidth. Generally speaking, bandwidth is directly proportional to the amount of data transmitted or received per unit time. In a qualitative sense, bandwidth is proportional to the complexity of the data for a given level of system performance. For example, it takes more bandwidth to download a photograph in one second than it takes to download a page of text in one second. Large sound files, computer programs, and animated videos require still more bandwidth for acceptable system performance. Virtual reality (VR) and full-length three-dimensional audio/visual presentations require the most bandwidth of all.

In digital systems, bandwidth is expressed as data speed in bits per second (bps). Thus, a modem that works at 57,600 bps has twice the bandwidth of a modem that works at 28,800 bps. In analog systems, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component. frequency is measured in the number of cycles of change per second, or hertz. A typical voice signal has a bandwidth of approximately three kilohertz (3 kHz); an analog television (TV) broadcast video signal has a bandwidth of six megahertz (6 MHz) -- some 2,000 times as wide as the voice signal.

Communications engineers once strove to minimize the bandwidths of all signals, while maintaining a minimum acceptable level of system performance.
This was done for at least two reasons :
(1) low-bandwidth signals are less susceptible to noise interference than high-bandwidth signals.
(2) low-bandwidth signals allow for a greater number of communications exchanges to take place within a specified band of frequencies.
However, this simple rule no longer applies in general. For example, in spread spectrum communications, the bandwidths of signals are deliberately expanded. In digital cable and fiber optic systems, the demand for ever-increasing data speeds outweighs the need for bandwidth conservation. In the electromagnetic radiation spectrum, there is only so much available bandwidth to go around, but in hard-wired systems, available bandwidth can literally be constructed without limit by installing more and more cables.

source :
tech target
systems mgmt