al means. Our defini- tion is not SO broad. Transmission provides the transport of a signal from an end-user source to the destination such that the signal quality at the destination meets certain performance criteria. Switching selects the route to the desired destination that the transmitted signal trav- els by the closing of switches in either the space domain or the time domain or some combination(s) of the two. Prior to about 1985, transmission and switching were separate disciplines in telecom- munication with a firm dividing line between the two. Switching engineers knew little about transmission, and transmission engineers knew little about switching. As we men- tioned in Chapter 1, that dividing line today is hazy at best. Signaling develops and carries the control information for switches. If a transmission path becomes impaired, signaling becomes ineffectual and the distant-end switch either will not operate or will not function correctly, misrouting the connectivity. Timing, which is SO vital for the digital transmission path, derives from the connected switches. 4.2 TRAFFIC INTENSITY DEFINES THE SIZE OF SWITCHES AND THE CAPACITY OF TRANSMISSION LINKS 4.2.1 Traffic Studies As we have already mentioned, telephone exchanges (switches) are connected by trunks or junctions. 1 The number of trunks connecting exchange X with exchange Y is the number of voice pairs or their equivalent used in the connection. One of the most important steps in telecommunication system design is to determine the number of trunks required on a 1 The term junction means a trunk in the local area. It is a British term. Trunk is used universally in the long-distance plant. Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK route or connection between exchanges. We could say we are dimensioning the route. To dimension the route correctly we must have some idea of its usage-that is, how many people will 