Alarm indication signal: Difference between revisions

Revision as of 19:00, 15 August 2009

Revision as of 23:01, 1 October 2009

Line 1:

{{redirect|AIS-P|the data packet protocol|Automatic Independent Surveillance-Privacy}}

'''Alarm indication signal''' ('''AIS''') is a signal transmitted by an intermediate element of a systemmulti node transport circuit that is part of a concatenated [[telecommunications system]] to letalert the receiverreceiving knowend thatof somethe remotecircuit partthat a segment 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. OtherThe systemsAIS attachedreplaces tothe afailed data, allowing the higher order system transmittingin AISthe thenconcatination relayto maintain its transmission framing integrity. Down stream intermediate elements of the AIStransport indicationcircuit propagate the AIS onwards to otherthe systemsdestination element.

There are various AIS formats based on the signaling level of the errored circuit. When an element of T-1 or ([[Digital Signal 1|DS-1]]) circuit looses signal ([[Loss Of Signal|LOS]]) or framing ([[Out Of Frame|OOF]]), the device replaces the erroneous data bits with a series of ones. This is where the term All Ones originates. At the [[DS3]] signal level, the intermediate element receiving an errored signal replaces the errored channel data with a signal consisting of valid DS-3 frame with with the overhead bits (the M-subframe alignments bits, M-frame alignment bits, and P bits) with the payload set to a 1010... sequence, the C bits all set to zero, and the X bits set to one. This way, the integrity of the DS-3 frame is maintained even though the underlying data was compromised.

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)

These are [[SONET]] OC-xx level indications that indicate if the errored element is in a section, segment, line segment, or path segment of the SONET circuit.

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-based communication systems, and is also part of the [[asynchronous transfer mode|ATM]] standards.