This article originally appeared in the July 2011 issue of INTERNET TELEPHONY

Last month, I talked about the shadow side of the mobility advantages of H.323 and SIP phones. While their ability to plug in anywhere on the enterprise voice network can reduce significantly the time and labor required to move and administer phones, this mobility also can create serious problems in terms of Enhanced 911 protection and asset management if you don’t have a strategy in place to keep the location of each phone up to date.

There are two common network discovery methods to track the location of H.323 and SIP endpoints using the network: network regions and layer 2/port level discovery. Since I covered network regions last month, I’ll explain what could be called HD location information.

Layer 2/port level discovery is more precise than the network regions approach as it allows administrators to detect the location of the phone down to the desktop by assigning a location to every port on a layer 2 switch. This method requires that a detailed wire map be created and maintained that lists all layer 2 switches, their ports, the location for all ports, and ELINs for each port. Physical locations can be assigned to each individual port, or a common location can be assigned to all ports on the switch.

Once the wire map is complete, the data file is uploaded to the E911 system software. As phones move from port to port, they get an IP address from the DHCP server and register with the PBX (News - Alert)/call server. The E911 software gets the registration event from the PBX/call server with the IP address and, in some cases, the MAC address of the phone. The E911 software then performs a search of the layer 2 network, querying the MIB tables on the layer 2 switches, searching for the IP address and MAC address of the phone. When the E911 software finds the phone, it logs the port and layer 2 switch, looks up the location associated with the port/switch in its database, and writes the proper ELIN back to the PBX/call server for the phone. If the phone dials 911, the PBX/call server outputs the ELIN, which describes the location of the phone. Using this method, users can move from port to port on a layer 2 switch, and their locations are automatically updated down to the desktop.

Phones on a Wi-Fi network work in much the same way, where locations are assigned to each access point and event notifications are provided to the E911 software when phones move from access point to access point. The E911 software updates the PBX/call server with the correct ELIN each time the phone moves.

The choice is yours. Whether you choose the HD-level of precision that layer 2/port level discovery offers or the faster-to-implement but more general network regions approach, you’ll have the location information required to comply with state laws and provide E911 protection to everyone in your facilities.

Nick Maier is senior vice president of RedSky (News - Alert) Technologies (www.redskyE911.com).