The system records the holding time from when a trunk is accessed to when the trunk is released. You set short and long holding-time limits for each trunk group. The system then compares the recorded holding times against these limits.

You enable ACA for the entire system, and administer thresholds for individual trunk groups. You can measure all trunks, or only certain trunks.

Communication Manager deals with long-holding and short-holding calls differently. For every call that is shorter than the administered short holding time, the system increases the short-holding counter by one. For calls over the same trunk that are within the normal range, the system decreases the short holding counter by one. Thus, trunks that handle a normal variety of call lengths are not singled out as faulty. If the counter reaches the administered short holding limit, the system places a referral call.

If one long call exceeds the long holding time, the system makes a referral call.

You cannot measure personal central office (CO) lines, out-of-service trunks, or trunks that are undergoing maintenance testing.