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Recovery of the Failed Unit
The host application is informed of recovery of the
communication to the SIU with the same method used
for notification of the failure. The RSI_MSG_LINK_
STATUS message in this case contains a status value of
1 (link to SIU recovered).
The host nominated to receive management
indications (normally host 0) will also receive an
API_MSG_SIU_STATUS message with status value 31
(decimal) indicating a host link has recovered to the
specified host ID.
Transferring Control Back
Immediately following re-establishment of communication
with the primary SIU, the application should send
deactivate circuit group messages to this SIU to ensure
that groups are only active on the secondary unit (this
may be the case if the inter-SIU link had also failed). If the
primary unit has recovered from a complete failure, no
circuit groups will be active. This deactivate command
will fail if the groups were not active and the application
should ignore any acknowledgement of this command
with a status value indicating processing failure. A “busy”
response should cause the application to reattempt the
deactivate.
When communication with the primary SIU has been
re-established, the application should allow sufficient time
to ensure that the communication is stable, thus avoiding
repeatedly transferring circuits between units. After this
time has expired, the application should transfer control
of the affected circuit groups back to the original SIU.
This is achieved by deactivating the groups on the
secondary SIU and re-activating the ones on the primary
SIU. However, before the groups are deactivated, the
circuits in that group should be maintenance blocked.
(Using the Circuit Group Supervision Control API
message). This does not affect any calls in progress, but
will prevent these circuits from being selected for any
new incoming calls. The application should also ensure
that none of the affected circuits are selected for new
outgoing calls.
When all existing calls are completed (all the circuits are
therefore idle), the application should deactivate the circuit
group by sending an API_MSG_COMMAND message
with cmd_type of 9 to the secondary unit. When the
acknowledgement that this command has been success-
fully processed has been received, the groups should be
activated on the primary unit by sending an
API_MSG_COMMAND message with cmd_type of 8.
Once the acknowledgement of the activation has been
received by the application, all affected circuits should be
reset. This will force the circuits to a known (idle) state
and remove the blocking status. When the reset is
acknowledged from the terminating switch (by receipt of
a circuit group supervision control confirmation message)
the application may begin exchanging call traffic with
the SIU.
Circuit Group Conflict
Both SIUs in a dual-resilient configuration periodically poll
each other to determine which circuit groups are active
on each unit. If a group is active on both units at the
same time, an API_MSG_USER_EVENT message is
issued by the unit that detects the conflict, indicating the
group ID of the affected circuit group. The controlling
application host should issue a deactivate command to
the SIU that should not be controlling the circuit group
and re-synchronize the circuits in the group (on the
correct SIU) by issuing a reset.
The SIUs prevent this situation from arising by
automatically sending a deactivate circuit group
command to the other unit on receipt of the activate
command. If the nature of the failure is such that
inter-SIU communication is lost, it may be impossible for
the SIU to issue the automatic deactivate command.
Under this circumstance, when the failed SIU recovers,
circuit groups may be active from the time before the
initial failure. This situation is handled by the application
sending a deactivate command for all of the previously
active groups immediately following restoration of the
communication with the SIU.
Building Fault-tolerant SS7 Systems Using the Intel
®
NetStructure™ SIU520 SS7 Signaling Gateway Application Note
23