Emergency Calls from Unregistered Users
Version S-CZ7.2.0 provides compliance with Section 5.1.6.2 of 3GPP TS24.229. This section mandates that unregistered, or roaming User Endpoints (UE) must perform an emergency registration before sending any SIP request (for example, an INVITE) related to the emergency situation.
Version S-CZ7.2.0 adds a new parameter (options reject-unreg-priority-calls) that enforces compliance with Section 5.1.6.2, Initial Emergency Registration, of 3GPP TS24.229, IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP);Stage 3. Section 5.1.6.2 mandates that unregistered, or roaming User Endpoints (UE) must perform an emergency registration before sending any SIP request (for example, an INVITE) related to the emergency situation.
Prior to Version S-CZ7.2.0, the default SBC behavior (acting as a P-CSCF) was not compliant with Section 5.1.6.2 of 3GPP 24.229. Rather, an emergency call from an unregistered or roaming UE was processed and routed in exactly the same way as a similar call from a currently registered UE.
Processing of emergency calls received from an unregistered or roaming UE as specified by 3GPP TS24.229 can be summarized as follows.
First, the emergency call (for example an INVITE) is rejected by the SBC with a 380, Alternative Service response message, which includes
- a Content-Type header field set to
application/3gpp-ims+xml
- a P-Asserted-Identity header set to the value of the SIP URI of the last entry on the path header field value received during registration -- the SIP URI of the P-CSCF. This is the UE-side Service Route that the SBC sends out when sip-ims-feature is enabled.
- an Alternative Service child element containing a Type element set to "emergency" ;
an Action element set to “emergency-registration”emergency-value = %x65.6D.65.72.67.65.6E.63.79 ; "emergency"
a Reason element set to%x65.6D.65.72.67.65.6E.63.79.2D.72.65.67.69.73.74.72.61.74.69.6F.6E ; "emergency-registration"
"priority calls not allowed from unregistered UEs"
Second, upon receipt of the 380, Alternative Response, the UE responds with an emergency registration.
Finally, upon receipt of the emergency registration, the SBC processes that registration as it would any other registration request.