Alarm indication signal: Difference between revisions

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'''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.

There are various AIS formats based on the signaling level of the errored circuit. When an element of T-1 or ([[Digital Signal 1|DS-1]]) circuit loses signal ([[Loss Of Signal|LOS]]) or framing ([[Out Of Frame|OOF]]), the device replaces the erroneous data bits with a series of ones. This is where the term All Ones originates. At the [[Digital Signal 3|DS3]] signal level, the intermediate element receiving an errored signal replaces the errored channel data with a signal consisting of a valid DS-3 frame with the overhead bits (the M-subframe alignmentsalignment bits, M-frame alignment bits, and P bits) with the payload set to a 1010... sequence, the C bits all set to zero, and the X bits set to one. This way, the integrity of the DS-3 frame is maintained even though the underlying data was compromised.

There are a number of types of AIS signals, which signal failure of different logical or physical segments of the system, including: