Bandwidth

I

Introduction

Bandwidth, in computer science, 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 alloted 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.

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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.

II

Modems

In the 1980s, in the early days of home computer use, computers relied on telephone lines and modems to connect to other computers. These modems worked by translating, or modulating, the streams of bits (1s and 0s) from a computer into sound waves; this stream is known as the digital signal. The sound waves were played through a speaker and picked up by the microphone of a telephone handset. The telephone’s microphone translated the sounds into an analog signal, a signal that uses an electrical current to simulate sound waves. The analog signal was sent through telephone wire to a second telephone, which transferred the analog signal to the receiving modem’s microphone. The receiving modem translated, or demodulated, the analog signal back into a digital signal. Modern modems operate in much the same way, but they use electronic circuits to pass the digital signal to the telephone line instead of microphones and speakers.

In 1990 a fast modem operated at 9.6 kbps. By the year 2000 modems were able to receive data at more than 5 times that speed, reaching a maximum of 56 kbps. A variety of factors can slow the transfer of data, however, and many computer users with a 56 kbps modem do not experience information transfer that fast. For instance, some older computers and certain laptops cannot support a connection with a 56 kbps modem. In addition, a 56 kbps modem often has to fall back to lower speeds when static interference on the telephone line distorts its signal. Another bandwidth bottleneck occurs if the modems at each end of the connection support different levels of bandwidths—a 33.6 kbps modem has to fall back to a lower speed when connecting to a modem that only supports 28.8 kbps, for example.

III

New Technologies with Larger Bandwidth

By the late 1990s, a variety of other technologies became available that vastly improved bandwidth to 1.5 mbps or more. Cable modems send data back and forth over a cable television network using broadband technology, in which multiple signals, or channels, are sent over a single coaxial cable. Cable television companies allocate one of their cable television channels for sending data to their cable modem users. A second channel is used to send information back. Cable modem technology can send data from the Internet out to its customers at speeds as high as 30 mbps. However, that bandwidth is shared by all the cable modem users in a particular neighborhood, which may range from 500 to 2,000 subscribers. The actual bandwidth that most cable modem users experience when they retrieve, or download, information from the Internet is generally in the range of 200 to 400 kpbs.

Digital subscriber lines (DSL) send data over the same telephone lines that carry regular telephone traffic. There are many different types of DSL. The most popular types can share a pair of telephone wires with a regular voice telephone service. This type of DSL uses broadband technology to send a radio-like signal over a telephone line. This signal cannot be heard, so the telephone line can be used simultaneously for both voice calls and computer connections. DSL connections range in speed from 128 kbps to 6 mbps, with the most common speeds ranging from 384 kbps to 1.5 mbps. Experts hope that newer DSL technologies will push download speeds as high as 50 mbps.

Not all technologies with high bandwidth rely on telephone wires or cable. Wireless services use radio waves to transmit information. Cellular telephones can transfer data to compatible modems at speeds of about 9.6 kbps. Multipoint Microwave Distribution Service (MMDS) systems use microwaves to provide computer communications to fixed locations. Available primarily in cities and nearby suburbs, MMDS provides data speeds similar to those of cable modems and DSL. Satellite dish television companies were due to begin offering two-way wireless Internet access via satellites that orbit the Earth over the equator in late 2000 and early 2001. These services transfer data from the Internet to a home computer at speeds up to 400 kbps.

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