The construction of the transmission facilities portion (or outside plant) of a telephone system. A telephone system or network consists of one or more transmission paths, called channels or circuits, which have been specifically designed and constructed to carry a particular type of electrical information signal between two or more points. Station equipment, switching systems, and transmission facilities are the components of these transmission paths. Voice, data, video, and program signals, in either analog or digital form, are some of the types of information signals (known as traffic) carried over these paths, and their signal characteristics and requirements dictate the physical makeup of the system.
The telephone network has evolved to provide for the transmission requirements of human speech, and must also provide for the transmission of more complicated information signals. The primary objective in the design and construction of any telephone system is to meet these varying transmission requirements in the most economical way possible. See also Telephone; Telephone service.
The telephone network is made up of a variety of transmission systems, each consisting of the transmission medium and its supporting structure. The ideal transmission facility should provide for the safe and satisfactory transmission of information signals under all types of conditions, and should be flexible enough to meet changing traffic requirements with a minimum amount of expense. Certain types of signals are better suited to certain types of transmission media because of bandwidth requirements and loss limits. Bandwidth is simply a measure of the information-carrying capacity of the medium; generally, the greater the bandwidth of a system, the more expensive it is to construct. Some common structures and systems are listed below. See also Communications satellite; Mobile radio; Radio-wave propagation.
The most common structures used to support telephone systems are pole lines, underground conduit systems, and buried systems. The most common type of transmission facility found in telephone systems is metallic paired cable. Paired cable is composed of copper or aluminum conductors coated with wood-pulp or plastic insulation. These insulated conductors are then twisted together into color-coded cables, ranging in size from 6 to 4200 pairs. These cables are coated with a protective sheath made of lead, aluminum, or polyethylene. These sheaths provide for structural and moisture protection.
Paired cables can carry voice, program, video, and most data signals up to 3.2 megabits/s, although additional cable pairs may be required to meet the required bandwidth. Because of the high attenuation of paired cables, the maximum range of voice-frequency signals is approximately 3 mi (4.8 km). To reduce attenuation, lumped inductances, called load coils, are placed along the transmission line. For digital signals, repeaters are used to eliminate attenuation. See also Communications cable.
Fiber-optic transmission systems offer several advantages over other transmission systems. Ideally suited for digital transmission, the information-carrying capacity of glass silica fibers is unlimited, but due to the limitations of electronic terminating equipment, transmission bit rates do not currently exceed 274 megabits/s. Two optical fibers can carry 4032 voice signals simultaneously with very low attenuation. These systems utilize lasers or light-emitting diodes in converting the digital bit stream into an optical signal. See also Optical communications; Optical fibers.
Coaxial cables offer wide bandwidths and can operate at very high frequencies. They are used in long-haul telephone transmission systems and as the transmission medium for most cable television systems. The cables contain 4 to 22 coaxial tubes. Each tube consists of a copper inner conductor that is kept centered within a cylindrical copper outer conductor by polyethylene-insulated disks. Coaxial tubes are spliced together with connectors that have been press-fitted onto the ends of the tube and then screwed together. The splice is then sealed with a moisture-protecting closure. See also Coaxial cable.
