Don't know how Intrado is doing its thing, but all the wireline 911 systems i've seen use civil location exclusively. lat/long is a recent development for wireless. the databases which translate civil address in MSAG (Master Street Address Guide) format into emergency services regions are truly amazing feats of grunt provisioning. However, this is a still a problem you can push onto a 911 Selective Router in the PSTN (or more accurately it is the *only* way you solve the problem now and still get all the PSAP routing features that an analog residential customer gets.) The only problems the SIP network needs to solve to hand the problem off is to find an appropriate ESRD (usually just your phone number, but if you moved from one SR serving region to another you may need a unique ESRD), and then to route the 911 call to a gateway connected to the correct SR. The mapping between ESRD and SR is a small table lookup, suitable for static configuration in a proxy. INVITE sip:+1911;context=+18314598727@vonage.com;user=phone SIP/2.0 arrives at a vonage proxy, which looks up +18314598727 to see if there is a special ESRD required for me. Let say I moved to Colorado and wanted to keep my Santa Cruz number, so it assigns me a new ESRD and retargets to the emergency ITSP. (It would still retarget, but not change the ESRD if my location was consistent with the SR associated with by DID.) INVITE sip:+1911;context=+13035512921@emergency-itsp.net SIP/2.0 emergency-itsp.net retargets to a specific gateway: INVITE sip:+1911;context=+13035512921@qwest-cama17.boulder.emergency-itsp.net or possibly just: INVITE sip:3035512921@qwest-cama17.boulder.emergency-itsp.net the gateway sends MF tones corresponding to 3035512921 (the 911 ANI or ESRD) to the SR, which does the rest. That is probably too low, probably more accurately between 100 and 1000 I heard that the state of Colorado has 2 SRs total, but I didn't personally verify that (seems low). Minnesota has 12 (they have SRs from two competing companies, so the number is high despite the low population). Texas has 24 SRs, which seems reasonable for large state. There are approximately 24 selective routers in Texas serving approximately 600 PSAPs - Ric Atkins at least 410 and less than 450. ---- http://www.ietf-ecrit.org/comments/draft-schulzrinne-sipping-emergency-req-01_hannes.txt