18 Chapter 1 Getting startedN0060720N0060720When a branch user IP telephone calls a main office IP telephone and vice versa, the call is asimple station-to-station call within the main office call server. Since the branch user IP telephoneis physically remote from the call server, the media path goes through the WAN connectionbetween the main office and the SRG50, and thus uses WAN bandwidth, as demanded by thecodec used in the call.Local modeIn the event of a WAN failure or the call server at the main office becomes unavailable, the SRG50reverts to local mode automatically. In local mode, the IP users connected to the SRG50 are underthe control of the SRG50. When in local mode, main office call features are not available to usersattached to the SRG50. The SRG50 offers a set of basic features for the IP telephones, includingaccess to the local PSTN, dialing emergency service numbers, and calling local extensions. (For acomplete list of local mode features, refer to “Features in local mode” on page 73.) Local mode isillustrated in Figure 2.Figure 2 Local modehe SRG50 handles all call processing. Calls between two IP telephones at the SRG50 are handledlocally as a simple station-to-station call. When an IP telephone initiates a local PSTN call, theSRG50 routes the call to a trunk that is connected to the local PSTN. Incoming DID calls are alsohandled by the SRG50 and terminated on the appropriate IP telephone set.In local mode, the IP telephones do not have access to the main office network over the VoIPtrunks. If alternate routes are configured, then calls can be made to the main office or other branchoffices using the available PSTN trunks.Several situations, described below, can cause the IP phone to be in local mode.Branch office Main officeWAN connectivity lost,IP phonesSRG50IP phonesLocal modeSRG50 pollingSoftwarephonesWLANhandsetsPSTN provides fallback
