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22 Chapter 2 SRG50 overview
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Figure 1 Normal mode
IP telephones connected at the SRG are registered with the main office call server and are under
main office control. They operate as branch user sets and have access to all telephony services and
features that the call server offers to IP telephones connected directly to the main office.
When a branch user set initiates a local PSTN call, the main office sets up the call using the VoIP
trunks, which establishes a local media path. Emergency Services Access calls are similarly routed
to the SRG PSTN. The telephone is redirected to local mode and the SRG initiates a local PSTN
call to 911. For main office callers, the SRG acts as a VoIP-PSTN gateway during normal mode.
When call forwarding has been configured, incoming PSTN calls to the branch user set are
forwarded over VoIP trunks (either H.323 or SIP) to the main office, which terminates the call at
the branch user. Similarly, calls from analog telephones connected to the SRG to the branch user
set are forwarded to the main office over VoIP trunks, which then terminates the call at the branch
user. Calls from the branch user set to the analog telephones at the SRG are routed over the VoIP
trunks to terminate at the analog telephone. In all these call scenarios, only signaling messages go
through the VoIP trunk. The media path is set up directly between the branch user set and the voice
gateway at the SRG. This means that these calls do not use any WAN bandwidth between the main
office and the branch office after calls are established.
When a branch user IP telephone calls a main office IP telephone and vice versa, the call is a
simple station-to-station call within the main office call server. Since the branch user IP telephone
is physically remote from the call server, the media path goes through the WAN connection
between the main office and the SRG, and thus uses WAN bandwidth, as demanded by the codec
used in the call.
Branch office
Main office
VoIP connection over WAN
IP phones
IP phones
Normal mode
Software
phones
WLAN
handsets
SRG