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)
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 OAM function has led to the development of a similar Ethernet alarm indication signal (EthAIS).
Automatic Independent Surveillance - Privacy (AIS-P) is a signal transmitted by an aircraft transponder system (ATCRBS, Mode S, TCAS, ADS-B, TailLight) in accordance with the internationally standardized Mode S data packet protocol. It is an alternative to ADS-B which transmits position and velocity, but does not transmit identity and cannot be used for automatic targeting, and does not suffer from the capacity reduction.
There are only two types of this signal, which differ significantly in deficiencies.
- ADS-B Dependent, identity, low capacity, targeting
- AIS-P Independent, no identity, high capacity, no targeting