Alarm indication signal

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 telecommunications 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. Downstream intermediate elements of the transport circuit propagate the AIS onwards to the destination element.