Alarm indication signal

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.

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

Alarm indication signal path (AIS-P)
Alarm indication signal line (AIS-L)

Middle 20th century analog carrier systems 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 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

See also