Specifications

46 HW-MTP2 Level 2 - Level 3 Interfaces and Message Flows HighWire MTP-2 - 1.2, September 4, 2002
5-3-3. Changeover following local
detection of link failure
Figure 5-6 Message flows involved when L2 detects that a link has failed.
1. When L2 detects that the link has become unusable, it sends a
CBI_M2_OUT_OF_SERVICE signal to inform L3.
2. L3 then issues a CBI_M2_RETRIEVE_BSNT signal to request that L2
process all PDUs received from the remote L2 so far and then send a
CBI_M2_BSNT signal back to L3. The CBI_M2_BSNT signal contains the
sequence number of the last received message. If the sequence
number could not be obtained, L2 sends back a
CBI_M2_BSNT_NOT_RETRIEVABL signal.
3. L3 then sends a CBI_M2_RETRIEVAL_REQ_FSNC signal to request that
L2 return all messages that L3 has passed to it for transmission but
which have not yet been acknowledged by the partner. L2 returns the
messages in CBI_M2_RETRIEVED_MESSAGE signals.
4. When all unacknowledged messages have been returned, L2 sends a
CBI_M2_RETRIEVAL_COMPLETE signal to L3. (Even if L2 does not send
any CBI_M2_RETRIEVED_MESSAGE signals, it must return the
CBI_M2_RETRIEVAL_COMPLETE signal to L3.) If the retrieval of
unacknowledged messages is not possible, L2 sends a
CBI_M2_RETRIEVAL_NOT_POSS signal to indicate this situation to L3.