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.

Enabling EATF Capability

Use the following procedure to enable EATF operations.

Use the following procedure to move to sip-config configuration mode.

ACMEPACKET# configure terminal
ACMEPACKET(configure)# session-router
ACMEPACKET(session-router)# sip-config
ACMEPACKET(sip-config)#

Use the eatf-stn-sr parameter to enable EATF operation and to provide E.164 telephone number of the EATF service.
Provide the E.164 telephone number as either a tel: or a sip: URI. When confirming the E-STN-SR value, the match succeeds based solely on the E.164 portion regardless of the URI type.
```
ACMEPACKET(sip-config)# eatf-stn-sr tel:+1-237-555-9999
ACMEPACKET(sip-config)#
```
Use done and exit to complete configuration.

Monitoring SRVCC Sessions

The show sipd srvcc command displays SRVCC handover counts including ATCF and EATF sessions.

ORACLE# show sipd srvcc
08:22:13-188
SRVCC Handover Stats                         ---- Lifetime ----
                                   Recent      Total  PerMax
Total Calls                             0          0       0
  Success                               0          0       0
  Failed                                0          0       0
Calls After Answer                      0          0       0
  Success                               0          0       0
  Failed                                0          0       0
Calls During Alerting                   0          0       0
  Success                               0          0       0
  Failed                                0          0       0
ATCF Cancellation                       0          0       0
Emergency Calls                         0          0       0
  Success                               0          0       0
  Failed                                0          0       0
EATF Cancellation                       0          0       0

These counters are defined as follows:

Total Calls - Total calls subjected to SRVCC
Total Success - Total successful SRVCC hand-off
Total Failed - Total failed SRVCC hand-off
Calls After Answer - Total calls subjected to SRVCC in established phase
After Answer Success - Total successful SRVCC hand-off in established phase
After Answer Failed - Total failed SRVCC hand-off in established phase
Calls During Alerting - Total calls subjected to SRVCC in alerting phase
During Alerting Success - Total successful SRVCC hand-off in alerting phase
During Alerting Failed - Total failed SRVCC hand-off in alerting phase
ATCF Cancellation - Total ATCF cancellations
Total Emergency Calls - Total SRVCC hand-off for Emergency calls
Emergency Success - Total successful SRVCC hand-off for Emergency calls
Emergency Failed - Total failed SRVCC hand-off for Emergency calls
EATF Cancellation - Total EATF Cancellations