does single mode tors form a pair. A typical cable is formed from 0.63mm or 0.9mm provide economic solutions to today's requirements, it also holds cellular polyethylene insulated conductors, four being twisted into the greatest promise for exploitation as the bearer for high capacity a star quad formation. The quads are then stranded into a concentric systems yet to be designed. layered cable, the direction of standing alternating in successive This section gives a description of the various cables used in layers. The complete cable of between 14 and 520 quads is then telecommunication networks and their impairments on signal trans- sheathed as described for the unit twin design. The resultant cable mission. Copper pairs, coaxial cables and various optical fibres are has a smaller diameter then the equivalent unit twin design and a included. more tightly toleranced characteristic as shown in Table 15.2. The capacitance unbalance at voice frequency in a 500m length is generally less than 40pF between pairs in the same quad, less than Symmetric pair cables 25pF between pairs in adjacent quads and less than 150pF to earth. Table 15.2 Typical characteristics of star quad cable Symmetric pair or balanced pair cables provide the most economic solution for the direct provision of audio circuits over short dis- tances, for example from local exchange to street cabinets and Average mutual capacitance Nominal distribution pillars. Notwithstanding new techniques in mobile diameter (nF/km) resistance radio and optical fibre, conventional copper pair cables for new Nomina Minimu Maximum (ohm/km communities, and for the enhancement and replacement of very old @ 10°C) cables in established areas, continue to be in substantial demand. A typical local area cable is of unit twin construction. The conduc- tors are insulated with cellular polyethylene of various colours which provides for pair identification within the cable. Two insu- lated conductors are twisted together to form a pair, the twist length and that of the other pair