alarm indication signal (SDH) (AIS)

The Alarm Indication Signal(AIS) is a monitoring signal in the Synchronous Digital Hierarchy( SDH), on T1 lines or in B-ISDN and also in OSI network management.

As soon as a transmission error - a loss of signal or packet - is detected in the transmission channel or an error message is received from another unit in the transmission path, the unit sends the AIS signal downstream or upstream, indicating the error message. The alarm indication signal ensures that the operation of the downstream regenerator is maintained, thus eliminating unnecessary alarms.