3.19. Signalling procedures for handling of abnormal cases
In this paragraph the procedures for handling of abnormal cases are specified. This includes actions when:
The actions to be taken at receipt of information from the Network Service Part at congestion or prohibited destination, are specified in the specification for SCCP for MUP.
In general, messages received abnormally or received including inconsistent information shall be discarded.
3.19.2 Non call related procedures
The general procedures for handling of abnormal cases for non call related procedures are outlined in figure 3.16.2.a. The MUP applies normally the principle that an MTX initiating a transaction will receive a RESPONSE from the addressed MTX and AR including an acknowledgement (positive or negative) and possible other updating information.
Figure 3.19.2a: Handling of abnormal cases, time-out T1.
An MTX sending a non call related MESSAGE starts two timers, T1 and T2.
If no RESPONSE is received before timer T1 expires, the MTX shall repeat the MESSAGE and restart the timer.
If no RESPONSE is received before timer T2 expires, the MTX shall stop sending the MESSAGE, stop timer T1 if running, and give information to the operation and maintenance system in the exchange.
Figure 3.19.2b: Handling of abnormal cases, time-out T2.
An MTX or an AR receiving a MESSAGE more than once because the RESPONSE sent for the first MESSAGE is lost or discarded, shall update its memory according to each MESSAGE received,
No consistency check shall be made between the messages. Both messages shall be acknowledged in the normal way.
3.19.3 Signalling procedures used when receiving abnormal or unreasonable signalling information
The Network Service Part of the signalling system will avoid missequencing, or double delivery, of messages with high reliability.
However, undetected errors at the signalling link level and exchange malfunctions may produce signalling information in messages that are either ambiguous or inappropriate.
An abnormal message is a message received in a state where it should normally not be received. In general, as stated earlier, messages received abnormally shall be discarded.
This is the case if messages other than those indicated in the normal procedures are received.
If an SRM is received without first having received a PSR (and sent a PSA in response), the SRM shall be regarded as abnormal and discarded.
If a message containing unreasonable information is received, it shall be discarded. Unreasonable information in a message can be: