This Gauge counts the number of Alarm Indication Signal events encountered at the Line layer, a cumulative tally rather than a per-interval bucket. An admin cares because AIS at the Line layer typically indicates an upstream failure being relayed downstream rather than a local fault, so a climbing AIS count points investigation toward equipment further up the SONET path rather than this local card. For example, a rising Line AIS counter on a device whose own Section-layer defects are clean strongly suggests the actual fault lies upstream of this device.