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IMS Session Flow
- 1. IMS Session Flow IP Multimedia Subsystem Series
- 2. IMS Operator Originating and Terminating P-CSCF UE S-CSCF AS Invite (SDP) (1) Invite (SDP) (2) Invite (SDP) (2) 200 OK P-CSCF I-CSCF HSS UE S-CSCF AS (6) Invite (SDP) (3) Invite (SDP) (8) Invite (SDP) 5 4 Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network (7) Invite (SDP) (7) 200 OK Invite (SDP)
- 3. IMS Operator Originating and Terminating (Cont’d) P-CSCF UE S-CSCF AS P-CSCF I-CSCF HSS UE S-CSCF AS (9) Offer Resp Offer Resp Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network Offer Resp (10) Offer Resp(11) Offer Resp (12) Offer Resp
- 4. IMS Operator Originating and Terminating (Cont’d) P-CSCF UE S-CSCF AS Resp Conf (SDP) (13) Resp Conf (SDP) P-CSCF I-CSCF HSS UE S-CSCF AS (15) Resp Conf (SDP) (14) Resp Conf (SDP) (16) Resp Conf (SDP) Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network Resp Conf (SDP)
- 5. IMS Operator Originating and Terminating (Cont’d) P-CSCF UE S-CSCF AS P-CSCF I-CSCF HSS UE S-CSCF AS (17) Conf Ack (SDP) Conf Ack (SDP) Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network Conf Ack (SDP) (18) Conf Ack (SDP) (19) Conf Ack (SDP) (20) Conf Ack (SDP)
- 6. IMS Operator Originating and Terminating (Cont’d) P-CSCF UE S-CSCF AS Rsrv Conf (21) Rsrv Conf P-CSCF I-CSCF HSS UE S-CSCF AS (23) Rsrv Conf (22) Rsrv Conf (24) Rsrv Conf Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network Rsrv Conf
- 7. IMS Operator Originating and Terminating (Cont’d) P-CSCF UE S-CSCF AS P-CSCF I-CSCF HSS UE S-CSCF AS (25) Rsrv Conf Rsrv Conf Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network Rsrv Conf (26) Rsrv Conf(27) Rsrv Conf (28) Rsrv Conf
- 8. IMS Operator Originating and Terminating (Cont’d) P-CSCF UE S-CSCF AS P-CSCF I-CSCF HSS UE S-CSCF AS (29) Ringing Ringing Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network Ringing (30) Ringing(31) Ringing (32) Ringing
- 9. IMS Operator Originating and Terminating (Cont’d) P-CSCF UE S-CSCF AS P-CSCF I-CSCF HSS UE S-CSCF AS (33) 200 OK 200 OK Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network 200 OK (34) 200 OK(35) 200 OK (36) 200 OK
- 10. IMS Operator Originating and Terminating (Cont’d) P-CSCF UE S-CSCF AS ACK (37) ACK P-CSCF I-CSCF HSS UE S-CSCF AS (39) ACK(38) ACK (40) ACK Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network ACK Media
- 11. IMS Operator Session Release P-CSCF UE S-CSCF AS (1) Bye (4) Bye (5) Bye (5) 200 OK P-CSCF I-CSCF HSS UE S-CSCF AS (6) Bye (8) Bye Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network (7) Bye (7) 200 OK (10) Bye(2) (3) (9)
- 12. IMS Operator Session Release (Cont’d) P-CSCF UE S-CSCF AS P-CSCF I-CSCF HSS UE S-CSCF AS (13) 200 OK (11) 200OK Originating Visited Network Originating Home Network Terminating Visited Network Terminating Home Network (16) 200 OK (14) 200 OK (15) 200 OK (12)
- 13. Single or Different Operators • Same Operator: The Serving-CSCF handling session origination performs an analysis of the destination address, and determines that it belongs to a subscriber of the same operator. The request is therefore forwarded to a local I-CSCF. The I-CSCF queries the HSS for current location information, and finds the user either located in the home service area, or roaming. The I-CSCF therefore forwards the request to the S-CSCF serving the destination user. • Different Operator: The Serving-CSCF handling session origination performs an analysis of the destination address, and determines that it belongs to a subscriber of a different operator. The request is therefore forwarded to a well- known entry point in the destination operator's network, the I-CSCF. The I-CSCF queries the HSS for current location information, and finds the user either located in the home service area, or roaming. The I-CSCF therefore forwards the request to the S-CSCF serving the destination user.
- 14. thank you
Editor's Notes
- The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt S‑CSCF#1 performs an analysis of the destination address, and determines the network operator to whom the subscriber belongs. Since it is local, the request is passed to a local I‑CSCF. I‑CSCF shall query the HSS for current location information. HSS responds with the address of the current Serving‑CSCF for the terminating user. I‑CSCF forwards the INVITE request to the S‑CSCF (S‑CSCF#2) that will handle the session termination. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt The sequence continues with the message flows determined by the termination procedure.
- 9-12. The terminating end point responds with an answer to the offered SDP and this message is passed along the established session path.
- 13-16. The originator decides on the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation, and forwards this information to S‑CSCF#1 by the origination procedures. This message is forwarded via the established session path to the terminating end point. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 12 or a subset.
- 17-20. Terminating end point responds to the offered SDP and the response if forwarded to the originating end point via the established session path.
- 21-24. Originating end point sends successful resource reservation information towards the terminating end point via the established session path.
- 25-28. Terminating end point sends successful resource reservation acknowledgement towards the originating end point via the established session path
- 29-32. Terminating end point sends ringing message toward the originating end point via the established session path.
- 33-36. The SIP final response, 200-OK, is sent by the terminating endpoint over the signalling path. This is typically generated when the user has accepted the incoming session setup attempt. The message is sent to S‑CSCF#2 per the termination procedure.
- 37-40. The originating endpoint sends the final acknowledgement to S‑CSCF#1 by the origination procedures and it is then sent over the signalling path to the terminating end point.
- One party hangs up, which generates a message (Bye message in SIP) from the UE to the P‑CSCF. Depending on the bearer establishment mode selected for the IP‑CAN session, release resource(s) shall be initiated either by the UE or by the IP‑CAN itself. The P‑CSCF/PCRF removes the authorization for resources that had previously been issued for this endpoint for this session. This step will also result in a release indication to the IP‑CAN to confirm that the IP bearers associated with the session have been deleted. The P‑CSCF sends a Hangup to the S‑CSCF of the releasing party. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session. The S‑CSCF of the releasing party forwards the Hangup to the S‑CSCF of the other party. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session. The S‑CSCF of the other party forwards the Hangup on to the P‑CSCF. The P‑CSCF/PCRF removes the authorization for resources that had previously been issued for this endpoint for this session. This step also results in a release indication to the IP‑CAN to confirm that the IP bearers associated with the UE#2 session have been deleted. The P‑CSCF forwards the Hangup on to the UE.
- The terminal responds with an acknowledgement, the SIP OK message (number 200), that is sent back to the P‑CSCF. Depending on the bearer establishment mode selected for the IP‑CAN session, release resource(s) shall be initiated either by the UE or by the IP‑CAN itself. The UE initiates the release procedures for the resources used for this session as shown in Figure 5.22. Otherwise, the IP‑CAN initiates the release of used resources after step 11. The SIP OK message is sent to the S‑CSCF. The S‑CSCF of the other party forwards the OK to the S‑CSCF of the releasing. The S‑CSCF of the releasing party forwards the OK to the P‑CSCF of the releasing. The P‑CSCF of the releasing party forwards the OK to the UE.