Telephone Traffic

an arbitrary quantity that is a function of the number of calls coming in to a telephone central office from the subscribers of a telephone network per unit time and of the time required for each call, that is, the time required for connection, conversation, and disconnection. The telephone traffic is measured in terms of a unit equal to the traffic created by calls that occupy a period of one hour. The most important characteristic of the telephone traffic is the intensity, equal to the product of the mathematical expectation of the number of calls coming in per unit time and the average time required for a call. Intensity is measured in terms of the erlang, which is equal to the traffic whose calls, if placed end to end, would keep one channel continuously occupied.

The telephone traffic varies over a wide range with the month of the year, day of the week, and hour of the day. The 60-minute interval during the day when the traffic is heaviest (on the average, after many days of measurements) is called the busiest hour. The traffic during this hour exceeds the average hourly traffic by a factor of 2–5, and in large cities the traffic of the busiest hour constitutes one-tenth of the total daily traffic. Statistical studies of traffic characteristics, carried out among identical groups of subscribers, make possible an analysis of the traffic and the busiest hour with regard to such considerations as volume, time of day, and communication channels. From these data, the values of traffic intensity can be calculated that are used in place of average values in designing telephone networks. These values help to determine the amount of switching equipment and the number and distribution of channels needed to ensure a particular level of service.