Some Traffic Characteristics of Communications Networks with Automatic Alternate Routing

The recent rapid expansion of long-distance communications facilities to serve increasing civilian and military demands, along with the evolution of cheaper trunking facilities and more sophisticated switching techniques, continues to bring the problem of network design and engineering to the attention of communications engineers. Although methods have been developed for engineering certain types of networks for the most economical distribution of trunking facilities, several critical problems remain. 769 770 THE BELL SYSTEM TECHNICAL JOURNAL, MALICH 1902 One of these is the lack of understanding of the behavior of alternate routing networks under overload conditions, whether the overload is local or system wide. Local disasters, such as storms, earthquakes, etc., have caused severe deterioration of service in certain regions owing to increased loads directed toward the affected area. At other times, such as Christmas Day in the United States, the pattern of traffic shifts radically ^ again causing serious overloads and long delays in completing calls. Finally, some concern is felt for the behavior of the system under the impact of some widespread disaster, where overloads may appear everywhere simultaneously. Such considerations lead in turn to two questions. First, how shall networks be designed to be efficient during normal operation and yet not deteriorate catastrophically under overloads, and second, given a network design, can the switching pattern be altered for the duration of an overload to improve performance, and if so, how? Another problem is our present inability to engineer any but the limited class of alternate routing networks of a "hierarchical" nature which have been widely used in the Bell System and elsewhere.