This Counter32 counts external AIS (Alarm Indication Signal) AAL2 Type3 packets received on a bearer connection, which are sent by upstream equipment to indicate a maintenance-span defect and warn downstream entities before other alarms fire. A rising count here tells an admin that a defect has been detected somewhere upstream in the transmission path feeding this connection, even before the local circuit itself shows failure. An engineer chasing intermittent call drops on a trunk would check this counter to see if externally-signaled line defects are propagating down to the bearer connections.