UE sends the SIP INVITE request, containing an initial SDP, to the P-CSCF address determined with P-CSCF discovery mechanism. The initial SDP may represent one or more media for a multi-media session.

A connection is reserved in the C-BGF with optional NAT binding list retrieval.

P-CSCF remembers (from the registration procedure) the next hop CSCF for this UE. In this case it forwards the INVITE to the S-CSCF in the home network.

S-CSCF validates the service profile, and invokes any origination service logic required for this user. This includes authorization of the requested SDP based on the user's subscription for multi-media services.

S-CSCF forwards the request, as specified by the S-S procedures. In addition, subject to operator policy, the S-CSCF may insert in the request a reference location of the user, when network-provided location information is not already present. The reference location (e.g. line identification) is determined by the operator as part of the user profile and may be received from the HSS at registration.

The media stream capabilities of the destination are returned along the signalling path, per the S-S procedures.

S-CSCF forwards the Offer Response message to the P-CSCF which triggers RACS. RACS performs admission control based on the Offer and Answer parameters. RACS configures the connections in the C-BGF based on the SDP answer and optionally requests a NAT binding list.

UE decides the offered set of media streams for this session, confirms receipt of the Offer Response and sends the Response Confirmation to P-CSCF. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in step 9 or a subset. If new media are defined by this SDP, a new connection configuration shall be performed following step 2. The originating UE is free to continue to offer new media in this request or in subsequent requests using the Update method. Each offer/answer exchange will cause the P-CSCF to repeat the RACS interactions again.

P-CSCF forwards this message to S-CSCF

S-CSCF forwards this message to the terminating endpoint, as per the S-S procedure.

The terminating end point responds to the originating end with an acknowledgement. If Optional SDP is contained in the Response Confirmation, the Confirmation Acknowledge will also contain an SDP response. If the SDP has changed, the admission control and configure connection flows are repeated.

S-CSCF and P-CSCF forward the answered media towards the UE.

The destination UE may optionally perform alerting. If so, it signals this to the originating party by a provisional response indicating Ringing. This message is sent to S-CSCF per the S-S procedure. It is sent from there toward the originating end along the signalling path. UE indicates to the originating user that the destination is ringing.

When the destination party answers, the terminating endpoint sends a SIP 200-OK final response along the signalling path to the originating endpoint, as specified by the termination procedures and the S-S procedures.

P-CSCF performs the approval of QoS Commit procedure which triggers the Open Gates procedures if required.

P-CSCF passes the 200-OK response back to UE.

UE starts the media flow(s) for this session.

UE responds to the 200 OK with an ACK message which is sent to P-CSCF and passed along the signalling path to the terminating endpoint.

UE#1 sends the SIP INVITE request, containing an initial SDP, via one of the origination procedures and the S-S procedures, to the S-CSCF for the terminating UE.

S-CSCF validates the service profile, and invokes any termination service logic required. This includes authorization of the requested SDP based on the user's subscription for multi-media services.

S-CSCF remembers (from the registration procedure) the P-CSCF address for this UE. The S-CSCF forwards the INVITE to the P-CSCF, which in this case is located in the home network.

The P-CSCF triggers RACS which reserves a connection in C-BGF with optional NAT binding retrieval.

P-CSCF remembers (from the registration procedure) the UE address, and forwards the INVITE to the UE.

UE determines the subset of the media flows proposed by the originating endpoint that it is capable and willing to support, and responds with an Offer Response message back to the originator. The SDP may represent one or more media for a multi-media session. This response is sent to the P-CSCF.

P-CSCF triggers RACS to perform admission control based on Offer and Answer parameters. RACS configures the connection in the C-BGF based on SDP answer with optional NAT binding retrieval.

P-CSCF forwards the Offer Response message to S-CSCF.

S-CSCF forwards the Offer Response message to the originator, per the S-S procedure.

The originating endpoint sends a Response Confirmation via the S-S procedure, to the terminating S-CSCF. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response sent in step 19 or a subset. If new media are defined by this SDP, a new interaction with the RACS (as in steps 4-8) will be done by the P-CSCF. The originating UE is free to continue to offer new media in this request or in a subsequent request using the Update method. Each offer/answer exchange will cause the P-CSCF to repeat the RACS interactions (steps 4-8) again.

UE may alert the user and wait for an indication from the user before completing the session. If so, it indicates this to the originating party by a provisional response indicating Ringing. This message is sent to P-CSCF and along the signalling path to the originating endpoint.

When the destination party answers, UE sends a SIP 200 OK final response to P-CSCF.

P-CSCF indicates that the resources reserved for this session should now be committed.

P-CSCF forwards the 200 OK to S-CSCF, following the signalling path.

The session originator responds to the 200 OK by sending the ACK message to S-CSCF via the S-S procedure and it is forwarded to the terminating end along the signalling path.