VoIP I2 Assumptions and Building Blocks

The PSTN 911 service is described elsewhere.

Location Delivery

In I2, not all end systems can deliver location information in the call signaling (SIP) messages, either because they don't have access to DHCP servers that are equipped to deliver this data or because they have SIP stacks that are not capable of including location information as multipart MIME bodies. In addition, not all end systems are SIP-capable.

Components

VPC (VoIP Position Center?)

The VPC translates location information into an ESQK. The VPC also updates the ALI database with the location information for the call, using the E2 interface. The VPC is responsible for assigning ESQKs and managing them.

ESGW (Emergency Services Gateway)

Translates SIP calls into PSTN calls onto trunks connected to a selective router. Each ESGW can serve multiple SRs and any number of VSPs and VPCs. The ESGW may consist of a SIP proxy server that performs additional translations on the call, combined with a standard trunk gateway that can initiate SS7 or CAMA calls. This proxy can also perform contingency routing to 10-digit numbers.

Identifiers

ESQK (Emergency Service Query Key)

ESQK is the calling party number. This identifier is used for location lookup and (possibly) routing the call to the right PSAP. Each ESQK will be associated with an ESN at the SR (as a result of the Selective Routing process being performed) and the SR will use existing processing to determine the PSAP based on the ESN information. There may be multiple ESQK numbers for each ESN.

There is no need for global uniqueness; the same number could be used in different SRs.

ESRN (Emergency Service Routing Number)

ESRN is the called party number. Each ESRN identifies one SR outgoing trunk group. Is globally unique within North America.

Both ESQK and ESRN will typically, but not necessarily, use non-dialable numbers with an area code (NPA) or 211 or 511.

Available SIP identifiers

From

Logical call source. Can't be changed by proxies.

To

Logical call destination. Can't be changed by proxies. Will be sip:sos@caller-aor.com.

Contact

Call back identifier (tel or SIP URI).

Request URI

Current destination; can be changed by each proxy.

P-Asserted-Identity (RFC 3325)

Caller identity. Can possibly be changed on route.

Contingency Routing

Contigency routing can occur if the ESGW cannot route to the SR, placing a call using a 10-digit number. It must be configurable to be used only when the ESGW is unable to complete calls to the SR due to network or switch failure (a contingency re-route), or whether the PSAP 10-digit number can also be used for overflow routing when the trunks are traffic busy.

If handled by the ESGW, it needs to able to place calls to 10-digit numbers.

References

• Routing configurations

Last updated by Henning Schulzrinne