CALL -BACK TO A UE THAT HAS MADE AN EMERGENCY CALL VIA A VISITED IMS NETWORK

摘要

The 3GPP Technical Specifications stipulate that when an emergency call is made to an emergency centre such as a Public Safety Access Point (PSAP), special procedures are applied to ensure that the call is correctly routed and not interrupted. However, there may be situations where the PSAP might want to call-back the UE after the call has been terminated (intentionally or unintentionally terminated). As specified in the standards, an emergency call that is established over the IMS is anchored in the IMS entities that serve the UE through the access network at which the UE was attached when the emergency call was established (i.e. in the visited network, not in the user's Home Network as would be the case for a normal call). The current procedures for call-back to a particular device require the PSAP to call back to the right device by indicating an equipment identifier or instance ID or Globally Routable User Agent Uniform Resource Identifier (GRUU) provided by the UE during emergency call establishment (see TS 24.229). However, this requires an IP-PSAP with enhanced functionality, something unlikely to be deployed in many countries. This problem is solved in that the S-CSCF is informed that an emergency call takes place in the visited network without having to route the initial emergency call through the home network. The P- CSCF (23) in the visited network (21) informs the S-CSCF (26) in the user's home network (22) that the user has established an emergency call with a particular equipment identifier (e.g., contact address, instance ID or GRUU). The S-CSCF (26) uses this information to route a terminating emergency call-back to the right device.