SRVCC PS-CS Access Transfer

This section describes various scenarios for transferring sessions between Packet-Switched (PS) and Circuit-Switched (CS) networks, providing details about the following cases:

• Mobile Switching Center (MSC) server-assisted mid-call feature supported by the SCC AS
• Call flows for the MSC server-assisted mid-call feature supported by the SCC AS
• Failure cases
• Confirmed dialog
• Early dialog

For PS-CS transfers in SRVCC, the ATCF performs several actions based on the STN-SR when it first receives the SIP INVITE. First, the ATCF determines the set of transferrable sessions. It defines this set as the SRVCC-transferrable sessions that are associated with a C-MSISDN matching the URI in the P-Asserted-Identity header field in the INVITE. The ATCF compares the INVITE to its set of transferrable sessions to see if the INVITE is part of the set; if so, its responds with 2xx response for the initial INVITE.

Active session transfers require the ATCF to act as a back-to-back user agent (B2BUA) if the session undergoing transfer has media anchored at the ATGW. In this case, the ATCF responds with a 200 OK to the INVITE; the OK carries an SDP answer with ATGW IP address and port information. Then the ATCF updates the following information and sends the INVITE to the remote UE:

• Original SDP offer is replaced with an SDP offer containing media information currently in use with the ATGW IP address and port information.
• The Request-URI with the ATU-SI associated with the session being transferred is inserted.
• The Target-Dialog header field with the dialog identifier of the session being transferred is inserted.

When the ATCF receives an SDP answer and if media is anchored at the ATGW for the session being transferred, the ATCF directs the ATGW to start sending and receiving media to/from the remote UE according to the newly received answer.

If it receives a BYE with a 503 Service Unavailable header on the source access leg, the ATCF retains session state information and ATGW resources associated with the session until either:

• The ATCF receives an INVITE request because of STN-SR, or
• An operator-defined time period elapses. The default value for the operator-defined time period is eight (8) seconds.

Thus, the session remains recognizable for SRVCC transfer for a time. Release of the session is also delayed by the ATCF’s forwarding the BYE to the SCC AS. If the transferrable session cannot progress, the ATCF responds with a 404 Not Found. For transfer cases when only hold or alerting sessions exist and the session in question is an early dialog or a confirmed dialog with inactive media, the ATCF replaces the Request-URI received in the original INVITE with the ATU-STI associated with a session in the transferrable set before forwarding the request; this proxy-role behavior complies with the 3GPP standard.