TELECOMMUNICATIONS SYSTEM

any set of devices and techniques employed for the transmission of voice, facsimile, data, teleprinter, or television signals. All telecommunications systems operate according to the same basic principles. They differ from each other chiefly in the specific types of information handled and the media over which they are transmitted. Familiar examples of such systems are radios, televisions, telephones, telegraphs, and computer-based data exchange or retrieval systems. In most cases, telecommunications systems transmit information via wire, optical fibre, terrestrial radio, or space satellite. Wire transmission involves sending electrical signals over various types of wire lines, such as twisted pair, multipair cable, and coaxial cable. These lines can be used to transmit voice frequencies, telegraph messages, computer-processed data, and television programs. In optical transmission, electrical signals converted to light signals by a laser transmitter carry both speech and data over bundles of thin glass filaments. Terrestrial radio communication entails the transmission of electromagnetic waves in specific frequency bands through the air. It includes both amateur and commercial broadcasting (the latter consisting of AM, FM, and TV broadcasting for general public use), radio navigation, and radio telephony. Satellite communication refers to the exchange of signals between widely separated locations by means of microwave radioi.e., very short radio waves. Since satellite systems do not require the construction of intermediate relay or repeater stations, as do ground-based microwave systems, they can be put into service much more rapidly. Either of two transmission techniquesanalog or digitalare utilized in telecommunications. In analog transmission, voice and visual information are sent over communications media through the use of signals that correspond to the sound or image being relayed. In digital transmission, signals are processed by a special coding component of the transmitter, which translates them into digital form (e.g., the two digits 0 and 1 of binary arithmetic). The coding process is omitted in the transmission of computer-generated data, however, because the signals are already in digital form. Whether in analog or digital form, the signal is subsequently routed to a modulator. This device impresses the intelligence signal onto a carrier wave of the appropriate frequency for the particular transmission medium in use. In most cases, high-frequency signals are employed for radio transmission and low-frequency signals in coaxial cables and other wire lines. Noise and interference are often encountered by a communications signal as it travels along the transmission path. Some of these disturbances originate from malfunctioning circuits or poorly designed antennas, but most are produced by solar flares, electrical storms in the Earth's atmosphere, nearby electrical circuits, and signals from other transmitters in the same frequency range. Various coding schemes and modulation techniques have been devised to help minimize noise and signal distortion problems. In fact, one of the primary reasons for employing binary signal transmission is its relative immunity to noise. Communication between a large number of information sources has been accomplished through the organization of the sources into networks. Networking has made it possible to concentrate communications traffic in localized areas, with interconnections provided between the various points of concentration to effect efficient and economical distribution of transmission signals. Telephone and telegraph networks (e.g., Telex) have long been in operation. More recently, cable television and telemetry networks (such as those used in the remote control of automobile traffic and widely distributed industrial operations) have become prominent. Rapid advances in computer technology also have led to a dramatic growth of data-retrieval and exchange networks. These networks are composed of individual computers and intelligent peripheral equipment (e.g., automatic teller machines and point-of-sale terminals) that are interconnected by telephone lines, microwave relays, and other high-speed communications links. Computer networking has developed on all levels, from local to international. Computer-to-computer communication has also become commonplace for owners of home computers. Subscribers to special information services can interact with a host computer to access educational and entertainment materials as well as news and stock-market reports.