The transmission of words, sounds, images, or data in the form of electronic or
electromagnetic signals or impulses. Transmission media include the telephone
(using wire or optical cable), radio, television, microwave, and satellite. Data
communication, the fastest growing field of telecommunication, is the process of
transmitting data in digital form by wire or radio. Digital data can be
generated directly in a 1/0 binary code by a computer or can be produced from a
voice or visual signal by a process called encoding. A data communications
network is created by interconnecting a large number of information sources so
that data can flow freely among them. The data may consist of a specific item of
information, a group of such items, or computer instructions. Examples include a
news item, a bank transaction, a mailing address, a letter, a book, a mailing
list, a bank statement, or a computer program. The devices used can be computers,
terminals (devices that transmit and receive information), and peripheral
equipment such as printers (see Computer; Office Systems). The transmission line
used can be a normal or a specially purchased telephone line called a leased, or
private, line (see Telephone). It can also take the form of a microwave or a
communications-satellite linkage, or some combination of any of these various

Hardware and Software

Each telecommunications device uses hardware, which connects a device to the
transmission line; and software, which makes it possible for a device to
transmit information through the line.


Hardware usually consists of a transmitter and a cable interface, or, if the
telephone is used as a transmission line, a modulator/demodulator, or modem. A
transmitter prepares information for transmission by converting it from a form
that the device uses (such as a clustered or parallel arrangement of electronic
bits of information) to a form that the transmission line uses (such as, usually,
a serial arrangement of electronic bits). Most transmitters are an integral
element of the sending device. A cable interface, as the name indicates,
connects a device to a cable. It converts the transmitted signals from the form
required by the device to the form required by the cable. Most cable interfaces
are also an integral element of the sending device. A modem converts digital
signals to and from the modulated form required by the telephone line to the
demodulated form that the device itself requires. Modems transmit data through a
telephone line at various speeds, which are measured in bits per second (bps) or
as signals per second (baud). Modems can be either integral or external units.
An external unit must be connected by cable to the sending device. Most modems
can dial a telephone number or answer a telephone automatically.


Among the different kinds of software are file-transfer, host, and network
programs. File-transfer software is used to transmit a data file from one device
to another. Host software identifies a host computer as such and controls the
flow of data among devices connected to it. Network software allows devices in a
computer network to transmit information to one another.


Three major categories of telecommunication applications can be discussed here:
host-terminal, file-transfer, and computer-network communications.


In these types of communications, one computer—the host computer—is connected to
one or more terminals. Each terminal transmits data to or receives data from the
host computer. For example, many airlines have terminals that are located at the
desks of ticket agents and connected to a central, host computer. These
terminals obtain flight information from the host computer, which may be located
hundreds of kilometers away from the agent\'s site. The first terminals to be
designed could transmit data only to or from such host computers. Many terminals,
however, can now perform other functions such as editing and formatting data on
the terminal screen or even running some computer programs. Manufacturers label
terminals as "dumb," "smart," or "intelligent" according to their varying
capabilities. These terms are not strictly defined, however, and the same
terminal might be labeled as dumb, smart, or intelligent depending upon who is
doing the labeling and for what purposes.


In file-transfer communications, two devices are connected: either two computers,
two terminals, or a computer and a terminal. One device then transmits an entire
data or program file to the other device. For example, a person who works at
home might connect a home computer to an office computer and then transmit a
document stored on a diskette to the office computer. An outgrowth of file
transfer is electronic mail. For example, an employee might write a document
such as a letter, memorandum, or report on a computer and then send the document
to another employee\'s computer.


In computer-network communications, a group