Nortel SRG200 Configuration Manual page 21

Chapter 2 Introducing SRG200/400 Release 1.5 21
Figure 1 Normal mode
Main office
Branch office
SRG
IP phones
IP phones
VoIP connection over WAN
Software
WLAN
phones
handsets
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. 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 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.
SRG200/400 Release 1.5 Configuration Guide