With a single ESQK, it has to provide call routing to the Selective Router and to provide PSAP routing, and steering back to the VPC, and to identify a specific call in progress at the VPC. This requires the allocation of ESQK values that are specific to these purposes and doesn’t allow, for example, them to also be routable in the in the PSTN. It also requires the call server to include a routing function that copes with all possible values of ESQK. Proposal Break the relationship between routing to the Selective Router and the other functions of the ESQK. That is, utilize a separate parameter for routing to the Selective Router called the ESRN (Emergency Services Routing Number). This is supported by current Selective Router software loads through the use of standard translations. That is in the, e.g., ISUP call setup the following is set: CalledPartyNumber = ESRN CallingPartyNumber = ESRQ ESQK overloading of functions - implications

PSAP E9-1-1 Selective Router SRDBALI DB VoIP domain Emergency Services Provider Network VPC 1. Query (Client-ID, [LIS-ID], [LOC], CallBack *, Provider ID) 2. Response with ESQK + ESRN ESGW 3. Call route V2 V4 3. IAM(CdPN=ESRN, CgPN=ESQK) 5. ESPOSREQ(ESQK) 4.Pulse(ESRQ) Using ESRN for S/R determination

Benefits ESRN can be assigned on a per S/R basis and be reused by all VPCs for all calls. ESRN can be assigned from LEC number range and be routable over the PSTN. For situations where no ESGW with direct S/R trunks is available the call server has the option of routing over the PSTN. Provides a fallback and stopgap solution where ESGW presence is not available. The call routing tables in the call servers only need entries for existing Selective Routers. No requirement to support a routing entry for every possible ESQK value. Implications Where SIP redirect is used, the From field in the redirect message needs to be populated with the ESQK instead of the original callback. The actual callback is cached in the VPC and returned in the esposreq. ESQK overloading of functions - implications