Nortel Networks 4134 - Manual (Page 13)

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ensure that incoming calls are routed through the SSM. The same call flow holds good for
the call originated by the SIP endpoint connected to CS 1000. When the call arrives at SSM,
it will look for a mapped contact in its registration “cache” and routes the call to the UA. If
SSM does not find the mapped contact then it forwards the call to the configured default
gateway.
The following diagram shows the call flow between two SIP endpoints in branch in Normal
(Connected) mode.
SIP Gateway Operation
SIP GW interconnects SIP voice over IP networks with the PSTN. It also provides direct
connections for analog phones, faxes and modems. In branch office deployment for CS
1000, SIP GW registers with the currently active NRS/SPS to enable newly active SPS to
route calls to SIP GW. This is different than a user registration. SIP GW will monitor the
reachability of NRS/SPS to know that current active NRS/SPS. SIP GW does this by
sending OPTIONS messages to both primary and secondary NRS/SPS. If current active
SPS is not reachable, then gateway will switch to the other SPS as the currently active SPS.
The following diagram shows the call flow between a branch PSTN interface/device and SIP
endpoint connected to CS 1000. The SIP GW will route PSTN calls to an active SPS server
with Req-Uri having IP address of the active NRS/SPS via SSM. The SSM will then replace
the IP address with CS 1000 domain in ‘From’, ‘To’ and Req-Uri headers. If none of the NRS
SPSs is available, then SIP GW will send calls to SSM with Req-Uri having host as domain
configured for SLG in SSM e.g. nortel.com. This routing happens on the basis of entries
defined in Normal mode NTML dial plan. Normal mode dial plan defines routes for both
primary SPS and secondary SPS. SSM will choose the route from NTML depending upon
destination set in SIP messages by SIP GW.