Alarm indication signal

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The alarm indication signal ( AIS , English Alarm Indication Signal ) is in the telecommunication that indicates a signal that at a port of a network device is received no signal. It signals the occurrence of a serious error ( loss of signal ). As a rule, a network device forwards the received signal to the next network device or to a terminal device (so-called "line termination"). The simplest network device is a repeater , the task of which is to receive, amplify and transmit a weak signal. However, if a signal fails and is no longer received, it can no longer be forwarded and a replacement signal must be forwarded instead. As a rule, a permanent one is used for this. There are variants of this procedure: for example, instead of a permanent one, some devices use an alternating sequence, ..0101 .., as a substitute signal (the direct current component of an alternating sequence is smaller, some ICs are sensitive in this regard).

If a signal fails, the fault can lie on the path between the sender and receiver, but also in the sending device. It is therefore useful to report the failure of the signal to the transmitting device so that the functionality of the transmitter can be checked there. With bidirectional data transmission, there are two independent transmission paths between the sender and receiver, outward and return path. If the way back is still working and only the way there is disturbed, a sending device can be informed on the way back that it is no longer receiving a signal. This error information is called a “feAIS”: a far end alarm indication signal . A device that does not receive a signal at a port signals this error to the sending device in the opposite direction. So it does two things: On the further transmission path, it uses the substitute signal AIS instead of the lost signal, and on the return path it uses the feAIS. The prerequisite for this is that the signal path contains a management channel in which this error information can be transmitted (management channels can usually transmit a whole series of errors; a different code is specified for each error, i.e. a different bit pattern in each case ). Management channels are defined in the technical standards for PDH and SDH .

The time span in which such an error must be recognized according to the standards of the relevant standardization organizations is in the millisecond range. A yellow signal lamp was originally switched on on network devices of North American multiplex technology when this error occurred. This is why this alarm is also referred to there as yellow alarm , even if a yellow lamp is no longer used.

If a multiplexer does not receive a signal on any transmission path at a tributary port, it cannot multiplex it into an aggregate signal either. He therefore usually replaces it with a permanent one. This indicates to the next multiplexer on the signal path that receives this permanent one that an error has already been registered on the device before it. It forwards this permanent one so that all devices on the transmission path receive it.

If a frame-structured useful signal is transmitted by the multiplex technology devices , as is the case with PDH or SDH methods, the duration one is usually only inserted into the payload of the signal, the OAM overhead is transmitted further without being influenced. The management channel is thus retained. If the synchronization of the overall signal (loss of synchronization) or the frame signal (loss of frame) is lost, this is no longer possible. At SONET , these error causes are differentiated using the codes AIS-L ( line AIS ) and AIS-P ( path AIS ).

The cell-oriented data transmission ATM uses specially coded AIS cells that indicate a loss of signal or a loss of cell synchronization ( header error check ).

literature

  • ITU-T Recommendation G.775, TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS, Digital transmission systems - Terminal equipments - Operations, administration and maintenance features of transmission equipment: Loss of Signal (LOS), Alarm Indication Signal (AIS) and Remote Defect Indication (RDI), defect detection and clearance criteria for PDH signals