Whenever the network layer entity is notified by its data link layer entity via the DL-RELEASE indication primitive that there is a data link layer malfunction, the following procedure shall apply:
a) Any calls not in the Active state shall be cleared internally.
b) For any call in the Active state a timer T309 shall be started (if implemented).
If timer T309 is already running, it shall not be restarted.
The Q.931 entity shall request layer 2 reestablishment by sending a DL-ESTABLISH request primitive.
When informed of layer 2re-establishment by means of the DL-ESTABLISH confirmation primitive, the following procedure shall apply:
the Q.931 entity shall stop timer T309, and either:
- the Q.931 entity shall send a STATUS message with cause No. 31, normal, unspecified to report the current state to the peer entity; or
- the Q.931 entity shall perform the status enquiry procedure according to 5.8.10 to verify the call state of the peer entity.
If timer T309 expires prior to data link reestablishment, the network shall clear the network connection and call to the remote user with cause No. 27, destination out of order;disconnect and release the B-channel; release the call reference, and enter the Null state.
If timer T309 expires prior to data link reestablishment, the user shall clear the attached connection (if any)with cause No. 27, destination out of order; disconnect and release the B-channel; release the call reference, and enter the Null state.
When a back-up D-channel is available, the procedures in Annex F may be used.
The implementation of timer T309 in the user side is optional and in the network side is mandatory.
When a Q.931 entity internally clears the call as a result of data link failure, as an option, it may request ther establishment of the data link in order to attempt to send a DISCONNECT message accross the interface.