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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 [[Digital_Signal_1#Alarms|T-carrier]] system, and became a standard feature of subsequent [[plesiochronous]] and [[Synchronous circuit|synchronous]] circuit-based communication systems, and is also part of the [[Asynchronous Transfer Mode]] 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).
==References==
{{Reflist}}
{{DEFAULTSORT:Alarm Indication Signal}}
[[Category:Synchronous optical networking]]
{{Telecomm-stub}}
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