Nortel 1000 Configuration Manual page 21

While in Local Mode, the SRG 50 system continues to monitor for a main
office CS 1000 heartbeat signal, and once detected, automatically redirects
phones on an individual basis back to Normal Mode of operation. If a
call is active, the SRG waits until the call is completed before redirecting
the phones; calls in progress are not interrupted. This switch-over occurs
almost immediately once the SRG determines that an individual phone
can be redirected. This reinstates the CS 1000 normal user interface and
feature set for the IP Phone user, on a user by user basis.
The SRG 50 system implements the same interface used by the MG 1000B
system to interact with the main office CS 1000 system. This allows the
main office to identify attached clients and the local PSTN as branch office
entities, enabling proper operation of dial plans and E911 access.
In Local Mode, devices that are physically located at the branch office, that
are controlled by the local system and receive a basic telephony feature
set, provide business continuity for the branch office during the WAN or
system failure. The SRG supports a main office heartbeat or reliable UDP
signaling which automatically reregisters users once WAN or system failure
has recovered.
For information about the features supported in Local Mode, see SRG50
Configuration Guide (NN40140-500).
Survivability
SRG is specifically designed to provide automatic survivability against WAN
failure, main office Call Server failure, main office Signaling Server failure,
and Gatekeeper failure.
SRG supports the Geographic Redundancy feature. For further information
about Geographic Redundancy, see System Redundancy Fundamentals
(NN43001-507).
In the event of a WAN failure, the SRG IP Phones lose communication
with the main office. This causes the SRG IP Phones to reset and register
with the SRG. The IP Phones then operate in Local Mode, providing
basic telephony services delivered by the local SRG system. For further
information about services and features supported on the SRG, see SRG50
Configuration Guide (NN40140-500).
If the main office Call Server fails and call processing services are provided
by an Alternate Call Server, the SRG IP Phones reset and reregister with
the Alternate Call Server and receive call processing services from it. If no
Alternate Call Server is available, the SRG IP Phones go to Local Mode
while the SRG attempts to find an Alternate Call Server by way of the NCS.
Copyright © 2005-2007, Nortel Networks
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Nortel Communication Server 1000
Main Office Configuration Guide for SRG 50
NN43001-307 02.02 Standard
Release 5.0 3 December 2007
Normal Mode and Local Mode overview 21