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In this section:

Steps to Handle Emergency Registration

To establish the emergency call;

  1. SBC determines if the UE registration is an Emergency Registration or not. This is indicated when the "sos" parameter is included in the contact header of the received REGISTER message and the equivalent configuration on the emergency call profile.
  2. SBC then treats this Emergency Registration as priority for CAC, in case the resources are exhausted.
  3. When SBC sends on the emergency REGISTER message, the RPH is only included if it is configured to be included.
  4. When SBC receives the 200 OK to REGISTER, the emergency registration is established. In IMS, a binding established through Emergency Registration is only used to make emergency calls.

The registration statistics and show screens differentiate between the normal and emergency registrations.

Scenarios for Session Establishment

When the call is allowedWhen the call is not allowed
When an emergency call (determined by matching URN prefix or URI prefix setting on the emergency call profile) is received on an Emergency Registration bindingWhen a normal call INVITE is received on an Emergency Registration binding
When an emergency call (determined by matching URN prefix or URI prefix setting on the emergency call profile) is received on a non-Emergency Registration binding, if it is configured as suchWhen an emergency call (determined by matching URN prefix or URI prefix setting on the emergency call profile) is received on a non-Emergency Registration binding, if it is configured as such
When the call is considered as emergency and a PCRF lookup is configured, send AAR with SERVICE AVP indicating that the call is considered as emergency to allow the PCRF to prioritize this request if deemed appropriate 

Emergency Call Establishment

To establish the emergency call,

  1. Add the Resource Priority Header to the outgoing INVITE with a configurable namespace, which indicates to the downstream network that this call is a priority.
  2. Perform Alternative Identity procedures to ensure that for emergency calls a PAI is included that shows the tel and sip URI of the contact. This allows guarantee of emergency callback from the PSAP.
  3. Add the E-CSCF address (returned from PSX lookup) as the URI in the topmost route header.
  4. Transits unchanged any geolocation header information, including location information expressed as an xml PIDF object.

Handling Emergency Session Errors

  1. If the PCRF lookup is rejected (and PCRF lookup is configured as required) then the SBC sends a 380 status message with further information back to the UE
  2. If no route to the E-CSCF is possible, then based on configuration the SBC may send a 380 status message with further information back to the UE.
  3. If an error response is received to an emergency session establishment, then based on configuration, SBC may send a 380 status message with further information back to the UE.
  4. Relay of SIP status 424 back to the UE.

 

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