e an example of time-dependent routing. For each originating and terminating exchange pair, a particular route pattern is planned depending on the time of day and day of the week. This is illustrated in Figure 8.15. A weekday, for example, can be divided into different time periods, with each time period resulting in different route patterns being defined to route traffic streams between the same pair of exchanges. This type of routing takes advantage of idle circuit capacity in other possible routes between originating and terminating exchanges which may exist due to noncoincident busy hours. 11 Crankback may be utilized to identify downstream blocking on the second link of each two-link alternative path. Exchanges Data links Routing processor Figure 8.14 State-dependent routing example with centralized processor. 11 Noncoincident busy hours: in large countries with two or more time zones. LOCAL AND LONG-DISTANCE NETWORKS A to B period Route pattern AB, ACB, ADB AB, AEB, AFB Figure 8.15 Example of time-dependent routing. (From Figure 7/E.170 of Ref. 3.) The following is an example of event-dependent routing. In a fully connected (mesh) network, calls between each originating and terminating exchange pair try the direct route with a two-link alternative path selected dynamically. While calls are successfully routed on a two-link path, that alternative is retained. Otherwise, a new two-link alternative path is selected. This updating, for example, could be random or weighted by the success of previous calls. This type of routing scheme routes traffic away from congested links by retaining routing choices where calls are successful. It is simple, adapts quickly to changing traffic patterns, and requires only local information. Such a scheme is illustrated in Figure 8.16 (Refs. 3, 4). A to B route pattern Choice Current After call failure Figure 8.16 Event-dependent routing in a mesh network. 8.5 TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONY TRANSMISSION FACTORS IN LONG-DISTANCE TELEPHONY 8.5.1 Introduction Long-d