'''Alarm indication signal''' ('''AIS''') is a signal transmitted by a system that is part of a [[concatenated telecommunications system]] to let the receiver know that some remote part 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. Other systems attached to a system transmitting AIS then relay the AIS indication onwards to other systems.
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There are a number of types of AIS signals, which signal failure of different logical or physical segments of the system, including:
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* '''Alarm indication signal path''' (AIS-P)
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* '''Alarm indication signal line''' (AIS-L)
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|ATM]] standards.
As the use of [[Ethernet]] for long-distance data links has increased, the need for a similar end-to-end [[operations and management|OAM]] function has led to the development of a similar [[Ethernet alarm indication signal]] ([[EthAIS]]).
