The Existing Wireline and Wireless 911 System

Terminology

CAMA (Centralized Automated Message Accounting)
CAMA trunks are used both between the originating end office (EO) and the selective router (SR) as well as between the SR and the PSAP.
ESN (emergency service number)
Identifies a particular combination of fire, police and rescue service; guaranteed to be unique only within one SR. A single PSAP may have hundreds of ESNs, due to the various combinations of these emergency services. ESNs determine the call transfer numbers to be displayed for wireline calls.
NCAS (non-call associated signaling)
Typically, SS7, but also ISDN PRI and BRI.
CAS (call associated signaling)
Signalling is carried with the call on the same trunk, as tones. For 911, CAMA is a typical example.
PANI (Pseudo-Automatic Number Identification)
10-digit number used to identify the location of a wireless caller by cell site/sector or a specific wireless E911 call
ESRK (Emergency Services Routing Key)
A pANI when the pANI identifies a wireless call. ESRKs are used with Non-Call Associated Signaling (NCAS), such as SS7. Each wireless carrier has their own batch of ESRKs. Each ESRK refers to one PSAP, but each PSAP can have any number of ESRKs, typically small (say, 5-15).

Description

An incoming emergency call has two pieces of information:

Called Party Number (CdP)
Number of party being called; typically, 911 or 11 for emergency calls; between originating switch and SR, it can be 
KP 1 NXXXXXX ST
KP 11 NXXXXXX ST
KP 911 NXXXXXX ST
where NXXXX is ?
Calling Party Number (CnP)
Charge number for wireline, also known as pANI or ESRK. This number is mapped to location information, either permanent, for wireline subscribers, or per-call, for wireless subscribers.

Calling party number on a CAMA trunk can be one of four different types (obviously KP-2-ST is lack of ANI), where NXX-XXXX is the ANI:

KP-0-NXX-XXXX-ST
Normal ANI in CAMA (7 digits).
KP-0-NPA-NXX-XXXX-ST
May not be CAMA.
KP-2-ST
EO doesn't know the ANI of the caller.
KP-NPD-NXX-XXXX-ST
This is E9-1-1 Selective Router to PSAP, where NPD is the Numbering Plan Digit, a single MF digit that is expanded into the NPA (area code). Only values 0 through 3 are available.

A CAMA trunk can deliver 8 digits of information to the PSAP in most cases (some CAMA trunks can deliver 10 digits).

There are logically three databases; these can be views into the same database, but provide the same functionality:

MSAG (Master Street Address Guide)
Maps street addresses to ESNs. The MSAG records do not have TNs (ESQKs) associated with them. Each MSAG record has an ESN associated with it. Since MSAG records do not reference TNs, the MSAGs should not require any updates to work with ESQKs.
Selective Router (SR) database:
maps pANI to outgoing (PSAP-bound) trunk and places the call on that trunk, based on the ESNs in the MSAG
ALI (Automatic Location Information)
Consulted by PSAP to obtain location information, keyed by a telephone number. This telephone number is the normal landline number for wireline, and a key (possibly non-dialable) number for wireless. In the I2 discussion, this is the ESQK (Emergency Services Query Key).

The ALI DB can only have one entry per TN (ESQK).

In wireless Phase II, the record for each number is updated by the MPC when the PSAP issues a query for that record. If no location information is available, the "shell record", a constant record, is returned. The shell record contains for wireless:
Street WIRELESS 9-1-1 CALL
Community primary area of cell coverage
ESN one unique to wireless service, with an ALI screen display (ELT) interpretation showing the target PSAP's name, but with VERIFY indicators for the Fire and EMS display.
Customer Name Wireless Company
House Number, Suffix, and lead Directional blank

ALI records cannot be created at call time; they must be created using another, non-call-associated, interface.

The MPC cannot update the ESN, ELT or service provider name during the call via the E2 interface. Thus, all wireless calls within a particular coverage area will have the same ESN, the "wireless ESN".

The MPC updates the location information during the call.

When the VPC is determining what ESQK to assign for a call it would have to do an MSAG lookup to determine the ESN, then it would have to look in its database to find an ESQK for that ESN. When the call gets to the SR, the SR will look up the ESN for that ESQK and route based on it. The ALI DB will ignore the address in the ESQK record and send the address obtained from the VPC to the PSAP (as it does for wireless).

Contingency Routing

Contingency routing occurs when the SR fails or the trunks to the SR fails. The Class 5 switch, which the ESGW is imitating, routes calls to a 10 digit number. The PSTN solution has a 10 digit number per trunk group.

Contingency routing takes two forms:

  1. Route to a specific dedicated PSAP trunk that exists between the selective router and the PSAP. In other words, if the dedicated trunk to the selective router fails, route the call to the selective router via the PSTN. By routing to a specific trunk number of an existing trunk between the router and the PSAP, the call can still arrive at the PSAP via designated trunks. At one time, this was a preferred way to implement "Phase 1/2" in wireless, but it fell out of favor because PSAPs did not want their trunk group numbers being given out.
  2. Route calls via the PSTN to a 10-digit "admin" line at the PSAP, and skip the selective router altogether. This is the most common contingency routing today.

In wireless, ESRKs are routinely mapped to a 10-digit contingency number in the event of trunk failure to the selective router. Since most MSCs cannot distinguish between trunk failure and overflow, the same 10-digit number is used for overflow in most cases today.

References

Last updated by Henning Schulzrinne