{{redirect|AIS-P|the data packet protocol|Automatic Independent Surveillance-Privacy}}
'''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.
{{Rcat shell|
There are a number of types of AIS signals, which signal failure of different logical or physical segments of the system, including:
{{R to related topic}}
* '''Alarm indication signal path''' (AIS-P)
}}
* '''Alarm indication signal line''' (AIS-L)
Middle 20th century analog [[carrier system]]s 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|ATM]] standards.
As the use of [[Ethernet]] for long-distance data links has increased, the need for a similar end-to-end [[OA&M]] function has led to the development of a similar Ethernet alarm indication signal ([[EthAIS]]).
A little known fact about AIS is that it once stood for "All ones Signal", this has been verified with network engineer Guru Mike Smith of Verizon Business
