The ESN (emergency services number) and the MSAG (master street address guide) along with ANI (automatic number identification) and ALI (automatic location identification) are all on the endangered species list.
Chris, here is another pdf to take a look at: http://communications.intrado.com/assets/documents/IEN_Glossary.pdf
The ESN is assigned to a phone number (think landline). It is a little packet of information that includes :
PSAP
Law Response Agency
Fire Response Agency
EMS Response Agency
When you dial 9-1-1 and that packet hits the appropriate switch, the switch says "Oh, you need to go to XYZ PSAP", and off it goes. In the PSAP I work for, the ESN doesn't mean all that much to us. We provide it to the people who manage the MSAG/ANI-ALI as more or less a courtesy. (Remember those three are going the way of the dodo) The CAD does the actual response assignment. For example, if you live in Salt Lake CIty,Utah the esn contains SLC FD and SLC PD, but nothing more specific than that. The CAD assigns which FD station and and which PD 'beat' responds.
NextGEN 911 replaces everything top down in a GIS centric approach. PSAP: polygon boundaries of the area of responsibility per psap, and this will have to be state wide. I'm not sure if the other stuff is included at the state routing level or not, but like I said that's the job of the PSAP. In other words, get the caller connected to the correct PSAP based on geographic location (point in polygon) and then the PSAP takes it from there.
That should just about do it....