Analysis of Trunk Groups Containing Short-Holding-Time Trunks

A message trunk, the basic connecting link in the switched telephone network, provides the supervising, signaling, and ringing capabilities essential to call set-up, as well as the communication path. When a condition that prevents proper functioning of a message trunk occurs, and causes a call failure, the trunk is normally released by a switching system on customer abandonment of the failed attempt and is then available to fail another call. As a result, a single undetected faulty trunk can fail a disproportionate fraction of the offered attempts to a group. Figure 9 shows an illustrative case where one trunk, subsequently verified to have been faulty, carried 35 out of 77 attempts offered to a trunk group during one hour. Because of their potential service impact, such trunks are of major concern throughout the Bell System and the object of many preventive maintenance and trouble detection programs. This paper presents models and analyses for the behavior of trunk groups containing short-holding-time trunks. This terminology is used to emphasize that the faulty trunks of interest are accessible by the customer, as opposed to faulty trunks that inhibit seizure by a switching system (false busys) or that result in an automatic retrial on 1127