Nortel Networks SRG50 - Manual (Page 36)

36 Chapter 4 CS 1000 considerations
NN40140-500 NN40140-500
services, such as transfer, last number redial, and single key access through the PSTN to a
centralized voice messaging system are supported. Local PSTN access and local Emergency
Services access is also supported. No local applications or Business Communication Manager
features are supported in local mode operation.
Analog devices continue to be under the control of the SRG50 system. It is the intent of local mode
to provide continued access to the PSTN for critical calls and emergency services.
In local mode, since the SRG50 handles all call processing, calls between two IP phones at the
SRG50 are handled locally as a simple station-to-station call. When an IP Phone initiates a local
PSTN call, the SRG50 routes the call to a trunk that is connected to the local PSTN. Incoming DID
calls are also handled by the SRG50 and terminated on the appropriate IP Phone.
In the event of a WAN failure, in local mode, the IP Phones do not have access to the main office
network over the VoIP trunks. If the appropriate alternate routes are configured, calls will be
routed to the main office or other branch offices using the available PSTN trunks.
While in local mode, the SRG50 system continues to monitor for a main office CS1000 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 redirection occurs almost immediately. This
reinstates the CS1000 normal user interface and feature set for the IP Phone user, on a user by user
basis.
The SRG50 system implements the same interface used by the MG1000B system to interact with
the main office CS1000 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 which
automatically reregisters users once WAN or system failure has recovered.
Survivability
SRG is specifically designed to provide automatic survivability against WAN failure, main office
Call Server failure, main office Signaling Server failure, and NRS (gatekeeper) failure.
SRG supports the Geographic Redundancy feature.
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.
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 network connection
server (NCS).