On receipt of a STATUS message reporting an incompatible state, the receiving entity shall:
a) clear the call by sending the appropriate clearing message with cause No. 101, message not compatible with call state; or
b) take other actions which attempt to cover from a mismatch and which are an implementation option.
Except for the following rules, the determination of which states are incompatible is left as an implementation decision:
a) If a STATUS message indicating any call state except the Null state is received in the Null state, then the receiving entity shall either:
1) send a RELEASE message with cause No. 101, message not compatible with call state, and then follow the procedures of 5.3; or
2) send a RELEASE COMPLETE message with cause No. 101, message not compatible with call state; and remain in the Null state.
b) If a STATUS message indicating any call state except the Null state is received in the release request state, no action shall be taken.
c) If a STATUS message, indicating the Null state, is received in any state except the Null state, the receiver shall release all resources and move into the Null state. If appropriate, the
network shall also clear the network connection, using cause No. 41, temporary failure.
When in the Null state, the receiver of a STATUS message indicating the Null state shall take no action other than to discard the message and shall remain in the Null state.
A STATUS message may be received indicating a compatible call state but containing one of the following causes:
i) No. 96 - Mandatory information element is missing;
ii) No. 97 - Message type non-existing or not implemented;
iii) No. 98 – Message not compatible with call state or message type non-existent or not implemented;
iv) No. 99 - Information element non-existent or not implemented; or
v) No. 100 - Invalid information element contents.
In this case the actions to be taken are an implementation option. If other procedures are not defined, the receiver shall clear the call with the appropriate procedure defined in 5.3, using the cause specified in the received STATUS message.
On receipt of a STATUS message specifying the global call reference and reporting an incompatible state in the restart request or restart state, the receiving Q.931 entity shall inform layer management and take no further action on this message.
When in the Null state, then on receipt of a STATUS message with the global call reference no action shall be taken.
Note - Further actions as a result of higher layer activity (e.g. system or layer management) and implementation dependent(including the retransmission of RESTART).
Except for the above case, the error handling procedures when receiving a STATUS message specifying the global call reference are an implementation option.