Emergency Access Transfer Function

The Emergency Access Transfer Function (EATF) is a logical, functional service defined in 3GPP TS 23.167, IP Multimedia Subsystem (IMS) Emergency Sessions, and TS 23.237, IP Multimedia Subsystem (IMS) Service Continuity; Stage 2. The EATF, essentially a special-purpose B2BUA, anchors emergency calls to enable access transfer between packet-switched and circuit-switched networks during eSR-VCC procedures when the LTE equipment is moving outside LTE coverage to either a 2G or 3G carrier network. Similar to the Access Transfer Control Function (ATCF) and ATGW (Access Transfer Gateway), the EATF is always located in the visited network when the user equipment is roaming.

Lacking this capability, the LTE equipment would be forced to re-establish the emergency session in the circuit-switched network through the legacy accesses (2G or 3G).

When the LTE equipment initiates a packet-switched emergency session, the INVITE is sent to the EATF thru the P-CSCF/E-CSCF (collocated on the SBC). This original INVITE is identified as an emergency session by the A-SBC/P-CSCF because it contains either an emergency short number (112, 991, and so forth) or an emergency service URN such as urn:service:sos.fire.

In the event that handoff to a circuit-switched network is required, the Mobile Switching Center (MSC) server initiates the transfer with a SIP INVITE containing an E-STN-SR (Emergency Session Transfer Number for Single Radio VCC) in the Request URI of the INVITE. Each network has a single E-STN-SR, essentially the telephone number of the EATF service, that is used exclusively for emergency session transfer access. The MSC directs the INVITE to the I-CSCF, which, in turn, which forwards the request directly to the EATF.

The EATF checks the E-STN-SR to determine that handoff to the circuit-switched network is requested and proceeds with the access transfer of the active session. The EATF associates the received SIP INVITE with an existing SIP session already anchored at the EATF using the instance-id feature tag. The EATF then sends a re-INVITE to the E-CSCF, which terminates the emergency session.

Once the session modification procedures are complete, as indicated by the reception of the SIP ACK request from the target access leg, the source access leg, previously established via IMS, is released.