Originating Sessions for SRVCC with ATCF

For initial SIP requests, the ATCF distinguishes SIP INVITE requests with the ATCF URI for originating requests in the topmost Route header field. And when receiving such an originating SIP INVITE request, the ATCF will do the following prior to forwarding it:

• Insert the Record-Route header field with its own SIP URI.
• If the latest SRVCC information received for a session contains a C-MSISDN and ATU-STI:
  • The ATCF will associate the session being established with the C-MSISDN and ATU-STI bound to the registration.
  • The ATCF will replace the SDP offer in the originating SIP INVITE with updated SDP the ATGW provides. Replacement occurs if the originating SIP INVITE contains SDP and a determination to anchor media has been made (according to the operator policy as specified in the 3GPP standard).