e transitory, since the technology is evolving rapidly. The cost of optical-fiber cables is expected to come down; better connectors will be developed; and more operating expertise will be acquired. In spite of the many advantages of optical- fiber technology, there remains a place for copper media, especially when the data rates are moderate (10 MBs or less). If the decision has been made to use copper in a local area net- work, the question of twisted-pair versus coaxial cable must be ad- dressed. Coax is an unbalanced medium-the shield and the center conductor have different impedances to ground. Twisted pairs, in contrast, are normally balanced; both wires have the same imped- ance to ground. Since coaxial cable is unbalanced, it is subject to spurious signals that may be picked up in both wires of a pair. The source of this noise is generally outside the cable. If the cable consists of a well-balanced twisted pair, this noise affects both wires equally. If the wire pair is terminated in a balanced receiver the noise is gen- erally eliminated. To keep electromagnetic radiation from affecting the transmission medium, a grounded shield must surround the wires. Since the shield (in the case of coaxial cable) is part of the signal circuit, grounding it introduces noise. A second shield, as in multi-shielded coax, al- leviates this problem, but at great expense. However, coaxial cable generally has lower attenuation than twisted pairs, especially at higher frequencies. 6.3 DISTRIBUTED COMMUNICATIONS There are two generic ways that applications can be distributed SO as to minimize communications and allow for the balancing of proc- essing among several nodes in a network. These two techniques can 92 TELECOMMUNICATIONS be used in combination in much the same way that series and parallel connections of electrical components can be used to implement any electrical circuit. Horizontal distribution is typically characterized by each of the nodes performing essentially the same functions. Usually the differ- ences betwee