Alarm indication signal: Difference between revisions

Revision as of 04:34, 23 December 2010

Revision as of 22:33, 24 December 2010

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'''Alarm indication signal''' ('''AIS''') (also called “all ones” because of the data and framing pattern) is a signal transmitted by an intermediate element of a multi-node transport circuit that is part of a concatenated [[telecommunications system]] to alert the receiving end of the circuit that 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. The AIS replaces the failed data, allowing the higher order system in the concatenation to maintain its transmission framing integrity. Downstream intermediate elements of the transport circuit propagate the AIS onwards to the destination element.

There are various AIS formats based on the signallingsignaling level of the errored circuit. When an element of T-1 or ([[Digital Signal 1|DS-1]]) circuit loses 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 errorerrored signal replaces the erringerrored channel data with a signal consisting of valid DS-3 frame 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 erringerrored element is in a section, segment, line segment, or path segment of the SONET circuit.

{{Further|[[Digital Signal 1#Connectivity and alarms]]}}

{{Further|[[Digital Signal 1#Connectivity and alarms]]}}

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|ATM]] standards.

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