One of the real advantages of H.323 and SIP phones is their ability to plug in anywhere on the enterprise voice network and get voice services. This has cost benefits in terms of labor overhead to move and administer phones. However, unless you have a strategy in place to keep the location of each phone up to date, this mobility can create serious problems in terms of Enhanced 911 protection – not to mention asset management.

There are two common methods to track the location of H.323 and SIP endpoints using the network: network regions and layer 2/port-level discovery. This month, I’ll explain network regions, the most widely used method.

With network regions, the voice network is broken up into a series of geographically logical subnets (network regions), each with its own dedicated IP address range in the DHCP server. An example of this might be a 10-story building where each floor is its own network region. When an IP phone plugs into floor 10, it gets an IP address from the DHCP service for the network region on floor 10. The PBX (News - Alert)/call server or the E911 software can determine which network region the phone is located in by looking at its IP address upon registration. The PBX/call server/E911 software can then assign the appropriate emergency line information number so that if that phone dials 911, the correct ELIN is sent out and will display the caller’s correct location to the public safety answering point dispatcher. Once the network regions are established, phones can move from region to region, and their location is automatically updated within seconds.  

The primary benefit of using the network regions approach for your enterprise is that it’s generally faster to implement as you’re associating each phone with a logical geographic area. This approach is often more than sufficient for smaller organizations. For larger organizations, the added detail provided by adopting a layer 2/port-level discovery approach may not only be better, but necessary.