Trunk Cables

communications cables laid between cities or other large populated areas.

Trunk cables are mainly of the high-frequency type and may be balanced or coaxial. A 60-channel system with a frequency band from 12 to 252 kilohertz is most commonly used for multi channel communication with balanced cable; coaxial cable multiplexes 960, 1,800, 1,920, 2,700, or 3,600 telephone channels. In the last case, the frequency band is up to 20 megahertz.

Most trunk cables are of the composite type, consisting of coaxial pairs of standard size, as well as balanced pairs or spiral quads. The latter type is used for service traffic and remote monitoring and control.

The current conductors of balanced cables are made of copper and have a diameter of 1.1, 1.2, or 1.3 mm. They are insulated by foamed or solid polyethylene, corded polystyrene, corded polyethylene, or corded paper. (In corded insulators, a cord of the insulating material is wound in a spiral around the conductor.) The insulated conductors are twisted into four, seven, or 12 spiral quads.

Coaxial cables contain four, six, eight, 12, or as many as 20 coaxial pairs. Two sizes of coaxial pairs have been standardized internationally: medium-gauge (normalized), or 2.6/9.5, and small-gauge, or 1.2/4.4. The national standards and specifications of various countries provide for coaxial pairs of 0.9/3.2, 1.2/4.6, 1.55/5.6, 2.1/9.4, and 6.85/24.75. The pairs have beaded or envelope-type air-polyethylene insulation. The inner conductor is a single copper wire; the outer conductor is a tube made from copper strip wound longitudinally.

Historically, trunk cables are also considered to include uniform or composite low-frequency balanced cables, with a copper conductor diameter of 0.8-1.6 mm and as many as several hundred spiral or multiple-twin pairs. These cables are used mainly as inserts in overhead communications lines where water barriers (rivers or lakes) must be crossed, and also in mountain passes and tunnels; for lead-ins to telephone exchange buildings; and for connections between automatic exchanges, including their connections with a trunk exchange.