Electric currents, electromagnetic waves, and optical energy carry messages on transmission facilities. The availability of ubiquitous transport is a prerequisite for the operation of the networks described in earlier chapters. It is tempting for managers to fantasize about owning all the communication facilities needed to support an enterprise. However, it soon becomes apparent that transmission equipment is expensive, sites are difficult to obtain, and maintenance by enterprise employees is virtually impossible. Consequently, most transport outside corporate buildings uses facilities owned and operated by common carriers. In this chapter, I describe some of the systems likely to be provided by the telephone companies and other entities. Because these facilities work together, all companies providing transport services operate compatible equipment. 7.1 Twisted Pairs Twisted pairs are major components of the public telephone network. They are the dominant bearers in the local loop. In addition, twisted pairs are used extensively for on-premises wiring for enterprise installations. A twisted pair is two insulated wires twisted together and contained in a cable of many pairs. Known as tip and ring, neither of the wires is connected directly to the ground. The twist keeps the conductors balanced with respect to themselves, the cable shield, and other pairs. Often, twisted pairs are called cable pairs. A paired cable is a cable whose conductors are twisted pairs