1.3.7. Bandwidth

Bandwidth, in computer science, refers to the amount of information that can be sent through a connection between two computers in a given amount of time. Computers may be connected by telephone wires, by coaxial cable, or through radio waves or microwaves. A connection that can transmit more data in a shorter period of time is said to have more bandwidth than another, slower connection.

The term bandwidth originated with radio broadcasting and in that context refers to the amount of the electromagnetic spectrum (i.e. the range of frequencies) that is allocated for a specific use. A band consists of a range of frequencies, and its width is determined by the difference between its highest and lowest frequencies. For instance, FM radio stations are allotted 200 kilohertz (200,000 cycles per second) of bandwidth. Since FM broadcasting relies on changes in frequency to transmit information, a station at 102.5 on the FM dial is using frequencies between 102.4 MHz and 102.6 MHz, with a small buffer at either end, to broadcast its information. In contrast, AM radio stations, which use changes in amplitude rather than frequency for information transmission, do not require as much bandwidth and are allotted only 10kHz.

Bandwidth in computers is measured not in cycles per second but by the number of bits per second (bps) that can be sent over a connection. A bit is the smallest unit of information a computer uses and it may have a value of either 0 or 1. Bits are usually combined into groups of eight to form bytes that can represent visual information, such as a letter, number, punctuation mark, or symbol. Computers manipulate bits by turning small switches, called transistors, off and on. When a transistor is on, bits can pass through the computer connection. When a transistor is off, the bits stop traveling through the connection.

Early telecommunication systems had low bandwidths that sent information at a relatively slow speed. The first teletype machines, used by newspapers to send news stories, operated at just 110 bps, or about 11 characters per second. At this speed it took a full second to transmit the word characters and the space that follows it. That is extremely slow by modern standards—today information can be sent at speeds of millions or even billions of bits per second. The text of this article would reach your computer in less than a second using some of today’s high-speed data connections. To simplify the expression of these larger bandwidth speeds, bandwidth is usually expressed in units of kbps (where k stands for kilo, the metric term for thousand), mbps (where m stands for mega, a million), or gbps (in which g stands for giga, a billion). Using this shorthand, you can shorten 56,000 bps to 56 kbps and 1,500,000 bps to 1.5 mbps.

Bandwidth directly affects the quality of transmitted information. For example, when a caller telephones into a radio show, the caller’s voice is not as clear to a listener as the radio host’s voice because the bandwidth of a telephone connection is smaller than the bandwidth of radio signals. The larger bandwidth of radio signals can carry a broader range of sound frequencies, and this improves the sound quality of the human voice. The telecommunications industry, including telephone companies, cable companies, and Internet service providers, continually seeks new technologies that will increase the amount of bandwidth it can provide in order to improve the quality of information flow and attract more customers.