Bandwidth


Bandwidth describes the amount of data that can be sent through a data-transmitting medium, such as a computer network, telephone line, or coaxial cable in a given amount of time.

A bandwidth of transmitted communication signals is a measure of the range of frequencies the signal occupies. The term is also used in reference to the frequency-response characteristics of a communications receiving system. All transmitted signals whether analog or digital have a certain bandwidth, as do receiving systems. Bandwidth is directly proportional to the amount of data transmitted or received per unit of time.

Bandwidth is therefore also proportional to the complexity of 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.


Digital Systems

In digital systems, digital devices, and devices that process data in coded combinations of binary digits, bandwidth is usually expressed either as a transmission rate called baud or in bits per second (bps). A bit is the smallest unit of information a computer uses. It may have a value of either 0 or 1, and physically corresponds to whether a transistor in the computer is on or off. Combining bits into larger units called bytes creates more meaningful information. Thus, a modem that works at 57,600 bps has twice the bandwidth of a modem that works at 28,800 bps.


Analog Systems

In analog systems, and for analog devices, bandwidth is defined in terms of the difference between the highest-frequency signal component, and the lowest-frequency signal component. Frequency is measured in cycles per second (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. Standard telephone lines are analog and have a low bandwidth. They can only transmit data at a maximum speed equivalent to about 14,400 bps.

Communication 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: firstly, low-bandwidth signals are less susceptible to noise interference than high-bandwidth signals; and low-bandwidth signals allow for a greater number of communication exchanges to take place within a specified band of frequencies. But this simple rule no longer applies in general.


Narrowband, Wideband, and Broadband

Narrowband is a transmission medium or channel with a single voice channel (with a carrier wave of a certain modulated frequency). Wideband is a transmission medium or channel that has a wider bandwidth than one voice channel (also with a carrier wave of a certain modulated frequency). Broadband refers to telecommunication that provides multiple channels of data over a single communications medium using frequency division multiplexing.

All of the interconnected data transmission systems operated by various telephone companies are known collectively as the Public Switched Telephone Network (PSTN) or the Plain Old Telephone System (POTS). The PSTN is now updating the old network from analog to digital. In addition, fiber optic cables are steadily replacing older copper telephone wires that once made up the PSTN. Fiber optic cable uses infrared and visible light pulses to transmit data down thin glass or plastic fibers. This medium has several benefits including its high bandwidth, low susceptibility to interference, and relatively low cost.

Another comparatively fast medium with a larger bandwidth than PSTN is the Integrated Services Digital Network. ISDN is a combination of fiber optic technology and special phone service provided by telephone companies that allows for digital audio and video transmission. Other data transmission media, such as the coaxial cable used by commercial cable companies, can transmit data at more than 500,000 bps.

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 be constructed without limit simply by installing more and more cables.