data communications


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data communications,

application of telecommunications technology to the problem of transmitting data, especially to, from, or between computerscomputer,
device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculator, by being able to store a computer program (so that it can repeat its operations and make logical
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. In popular usage, it is said that data communications make it possible for one computer to "talk" with another. Telephone circuits are often used to transfer data, although their relatively limited bandwidth makes them relatively slow paths for data. Recent techniques, however, have made it possible to send data over phone lines at rates of 28,800 bits per second and higher. A modemmodem
[modulator/demodulator], an external device or internal electronic circuitry used to transmit and receive digital data over a communications line normally used for analog signals.
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 is required for such telephone communications when they occur over standard (analog) telephone circuits. Where cost can be justified, high speed data links are constructed; these are often fiber-optic or coaxial cables designed for wide frequency range, or microwave, radio links. Local-area and wide-area networks link computers together so that they can transfer and share data. Because many computers can be on the network at any given time, techniques such as time-division multiplexing are used; each computer is assigned a short time slot during which it can use the full bandwidth of the network. Packet switching allows a single channel to be used for multiple concurrent transmissions. Data packets contain addresses that indicate the intended destination. To minimize data-communication errors, special codescode,
in communications, set of symbols and rules for their manipulation by which the symbols can be made to carry information. By this extended definition all written and spoken languages are codes.
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 are used.

data communications

[′dad·ə kə‚myü·nə′kā·shənz]
(communications)
The conveying from one location to another by electrical means of information that originates or is recorded in alphabetic, numeric, or pictorial form, or as a signal that represents a measurement; includes telemetering, telegraphy, and facsimile but not voice or television. Also known as data transmission.

Data communications

The conveyance of information from a source to a destination. Data means the symbolic representation of information, generally in a digital (that is, discrete) form. (Analog information refers to information encoded according to a continuous physical parameter, such as the height or amplitude of a waveform, while digital information is encoded into a discrete set of some parameter.) Usually, this digital information is composed of a sequence of binary digits (ones and zeros), called bits. The binary system is used because its simplicity is universally recognizable and because digital data have greater immunity to so-called noise than analog information and allow flexible processing of the information. Groups of eight bits create a data byte or character. These characters make up the so-called alphabets (including alphabetic, numeric, and special symbols) which are used in data communications. The most common data sources and destinations are computers and computer peripherals, and the data represent groups of characters to form text, hypertext (text organized according to topic rather than linear sequence), or multimedia information, including audio, graphics, animation, and video information. See Bit, Computer, Computer graphics, Computer peripheral devices, Digital computer, Multimedia technology

Data communications may be accomplished through two principal functions, data transmission and data switching.

Data transmission always involves at least three elements: a source of the information, a channel for the transmission of the information, and a destination for the information. In addition, sometimes the data are encoded. The codes can be used for error detection and correction, compression of the digital data, and so forth. See Data compression

The communications channel is carried over a transmission medium. Such media can be wired, as in the cases of twisted-pair telephone wires, coaxial cables, or fiber-optic cables, or they can be wireless, where the transmission is not confined to any physical medium, such as in radio, satellite, or infrared optical transmission. Sometimes, even when the source of the information is digital, the transmission medium requires analog signaling, and modems (modulators-demodulators) are required to convert the digital signals to analog, and vice versa. For example, data communication between personal computers transmitted over telephone lines normally uses modems. See Coaxial cable, Communications cable, Modem, Optical communications, Radio, Telephone service

The directionality of the information can be either one-way (simplex communications) or two-way. Two-way communications can be either half-duplex (information goes both ways over the communications link, but not at the same time) or full-duplex (information goes both ways at the same time).

The data channel can be a serial channel, in which the bits are transmitted one after another across a single physical connection; or a parallel channel, in which many bits are transmitted simultaneously (for instance, over parallel wires). Generally, parallel channels are used for short-distance links (less than 300 ft or 100 m), whereas serial links are used for larger distances and high data rates.

At low data rates (less than a few hundred kilobits per second) communications channels are typically dedicated, whereas at higher data rates, because of the cost of high-speed transmitters and receivers, shared channels are used by multiplexing the data streams. For example, two independent data streams with constant data rates of 10 megabits per second (Mb/s) could use a shared channel having a data-rate capability of 20 Mb/s. The multiplexing system would select one bit from each of the two channels to time-multiplex the data together.

In many cases, the source, the destination, and the path taken by the data may vary; thus switching is required.

The two primary types of switching employed in data communications are circuit switching and packet switching. In circuit-switched data communications, an end-to-end connection is established prior to the actual transmission of the data and the communications channel is open (whether or not it is in use) until the connection is removed.

A packet is a group of data bytes which represents a specific information unit with a known beginning and end. The packet can be formed from either a fixed or variable number of bytes. Some of these bytes represent the information payload, while the rest represent the header, which contains address information to be used in routing the packet. In packet switching, unlike circuit switching, the packets are sent only when information transmission is required. The channel is not used when there is no information to be sent. Sharing the channel capacity through multiplexing is natural for packet-switched systems. Furthermore, the packet switches allow for temporary loading of the network beyond the transmission capacity of the channel. This information overload is stored (buffered) in the packet switches and sent on when the channel becomes available.

In order to transfer information from a sender to a receiver, a common physical transmission protocol must be used. Protocols can range from very simple to quite complex. The Open Systems Interconnect (OSI) model, developed by the International Standards Organization, reduces protocol complexity by breaking the protocol into smaller functional units which operate in conjunction with similar functional units at a peer-to-peer level. Each layer performs functions for the next higher layer by building on the functions provided by the layer below. The advantage of performing communications based on this model is that at the application layers (user processes) there is no concern with the communications mechanisms. See Electrical communications, Integrated services digital network (ISDN), Packet switching, Switching systems (communications)

data communications

Transmitting text, voice and video in binary form. See communications.
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