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Thoughts on enterprise IP telephony deployments and E911 service. Part 2 of 4.

E911 mechanisms are based on the geographical limitations of circuit switching. All telephone numbers are assigned to specific ports on central office switches which terminate at specific physical addresses, which the phone company keeps really, really good records on.

IP Telephony implementations, on the other hand, have difficulty duplicating the geographical limitations of central offices. Fortunately, IP Telephony is not alone in this regard. In the last blog, we considered service designs from the wireless industry, which uses radio attenuation, triangulation and even geo-stationary satellites to come up with a physical location for emergency responders to act on. In contrast with Rich Tehrani's article on Internet Telephony and E911 , I don't believe that we should wait for GPS-enabled clients, IP phones or the like to do our work for us. This is an excuse.

Instead, we should use knowledge in the layer 2 switching infrastructure to determine where PSTN addresses terminate in IP world. For example, every IP phone in a 3Com VCX IP Telephony implementation uses a well-understood IP address. Each of these devices, typically is contained in a virtual LAN segment which isolates IP telephony traffic from all the other traffic in the network.

Each of these vLAN end points has a MAC address that uniquely defines its relatively stationary point in the enterprise network. Enterprise network managers should be able to link MAC addresses to specific distribution switches, specific cable runs, and specific end points in the wall and thereby linking network elements to specific emergency service responder areas, such as a floor of a building, or a section of the floor of a building.

So, software could form the link between MAC addresses and IP addresses and in this way provide the link between an IP phone's IP address, and the physical location.

But, not all users have IP phones. Some actually use their PC and softphones.

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