wish to talk at once over the route. The usage of a transmission route or switch brings us into the realm of traffic engineering; and usage may be defined by two parameters: (1) calling rate, or the number of times a route or traffic path is used per unit time period; or more properly defined, "the call intensity per traffic path during the busy hour (BH)"; and (2) holding time, or "the average duration of occupancy of one or more paths by calls." A traffic path is a "channel, time slot, frequency band, line, trunk switch, or circuit over which individual communications pass in sequence." Carried traffic is the volume of traffic actually carried by a switch, and offered traffic is the volume of traffic offered to a switch. Offered traffic minus carried traffic equals lost calls. A lost call is one that does not make it through a switch. A call is "lost" usually because it meets congestion or blockage at that switch. To dimension a traffic path or size a telephone exchange, we must know the traffic intensity representative of the normal busy season. There are weekly and daily variations in traffic within the busy season. Traffic is random in nature. However, there is a certain consistency we can look for. For one thing, there is usually more traffic on Mondays and Fridays, and there is a lower volume on Wednesdays. A certain consistency can also be found in the normal workday variation. Across a typical day the variation is such that a one-hour period shows greater usage than any other one-hour period. From the hour of the day with least traffic intensity to the hour of greatest traffic, the variation can exceed 100:1. Figure 4.1 shows a typical hour-to-hour traffic variation for a serving switch in the United States. It can be seen that the busiest period, the busy hour (BH), is between 10 A.M. and 11 A.M. (The busy hour from the viewpoint of grade of service was introduced in Section 1.3.4). From one workday to the next, originating BH calls can vary as much as 25%. To these fairly "regular" variations, there ar