Alarm indication signal (AIS) (also called all ones because of the data and framing pattern) is a signal transmitted by an intermediate element of a multi-node transport circuit that is part of a concatenated communications system to alert the receiving end of the circuit that a segment of the end-to-end link has failed at a logical or physical level, even if the system it is directly connected to is still working.
The AIS replaces the failed data, allowing the higher order system in the concatenation to maintain its transmission framing integrity.
When an element of T1 or (DS1) circuit loses signal (LOS) or framing (OOF), the device replaces the erroneous data bits with a series of ones.
Middle 20th century analog carrier systems had Carrier Group Alarms by which the failure of a pilot signal was alerted to telephone exchange equipment, imposing an automated make-busy condition so the trunks carried by the failed system would not be used.
The improved AIS originated with the T-carrier system, and became a standard feature of subsequent plesiochronous and synchronous circuit-based communication systems, and is also part of the Asynchronous Transfer Mode standards.