Application of the Theory of Probability To Telephone Trunking Problems

F telephone plants were provided in such quantities that when a subscriber makes a call there would be immediately available such switching arrangements and such trunks or paths to the desired point as may be necessary to establish the connection instantly, it would require that paths and switching facilities be provided to meet the maximum demand occurring at any time, with the result that there would be a large amount of plant not in use most of the time. Obviously, this would result in high costs, particularly in cases of long circuits or where the switching arrangements are complex. Sound telephone engineering requires, therefore, that we approach this condition only in so far as it is practical and economical to do so, consistent with good telephone service to the subscriber. To take an extreme case--if enough toll lines were provided between New York and San Francisco so that no call would ever be delayed because of busy lines or busy switching arrangements, the rates it would be necessary to charge would be prohibitive, although the speed of service would be very good. Obviously, it is necessary to adopt a compromise between the number of circuits and amount of equipment and the time required to complete a call. Handling traffic on any other than an instantaneous basis is generally spoken of as handling it on a "delayed basis," even though this delay may be, and generally is, inappreciable to the subscriber. While most of the traffic is handled practically on a no-delay basis, there are certain kinds that are handled on a "delayed basis," such as 1.