5.11.1 Session Hold and Resume Procedures

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

5.11.1.0 General

This clause gives information flows for the procedures for placing sessions on hold that were previously established by the mechanisms of clauses 5.4, 5.5, 5.6, and 5.7, and resuming the session afterwards. Two cases are presented: mobile-to-mobile (UE-UE), and a UE-initiated hold of a UE-PSTN session.

For a multi-media session, it shall be possible to place a subset of the media streams on hold while maintaining the others.

These procedures do not show the use of optional I‑CSCFs. If an I‑CSCF was included in the signalling path during the session establishment procedure, it would continue to be used in any subsequent flows such as the ones described in this clause.

5.11.1.1 Mobile-to-Mobile Session Hold and Resume Procedures

An IMS session was previously established between an initiating UE and a terminating UE. Each of these UEs has an associated P‑CSCF, and a S‑CSCF assigned in their home network. The procedures are independent of whether the P‑CSCFs are located in the home or visited networks. Therefore there is no distinction in this clause of home network vs. visited network.

The hold and resume procedures are identical whether the UE that initiated the session also initiates the session-hold, or whether the UE that terminated the session initiates the session-hold.

When a media stream has been placed on hold, it shall not be resumed by any endpoint other than the one that placed it on hold.

The procedures for placing a media stream on hold, and later resuming the media stream, are as shown in the following information flow:

Figure 5.28: Mobile to Mobile session hold and resume

Information flow procedures are as follows:

1. UE#1 detects a request from the user to place a media stream on hold. UE#1 stops sending the media stream to the remote endpoint, but keeps the resources for the session reserved.

2. UE#1 sends a Hold message to its proxy, P‑CSCF#1.

3. P‑CSCF#1 forwards the Hold message to S‑CSCF#1.

4. S‑CSCF#1 forwards the Hold message to S‑CSCF#2.

5. S‑CSCF#2 forwards the Hold message to P‑CSCF#2.

6. P‑CSCF#2 forwards the Hold message to UE#2.

7. UE#2 stops sending the media stream to the remote endpoint, but keeps the resources for the session reserved.

8. UE#2 acknowledges receipt of the Hold message with a 200-OK final response, send to P‑CSCF#2.

9. P‑CSCF#2 forwards the 200 OK final response to S‑CSCF#2.

10. S‑CSCF#2 forwards the 200 OK final response to S‑CSCF#1.

11. S‑CSCF#1 forwards the 200 OK final response to P‑CSCF#1.

12. P‑CSCF#1 forwards the 200 OK final response to UE#1.

13. UE#1 detects a request from the user to resume the media stream previously placed on hold. UE#1 sends a Resume message to its proxy, P‑CSCF#1.

14. P‑CSCF#1 forwards the Resume message to S‑CSCF#1.

15. S‑CSCF#1 forwards the Resume message to S‑CSCF#2.

16. S‑CSCF#2 forwards the Resume message to P‑CSCF#2.

17. P‑CSCF#2 forwards the Resume message to UE#2.

18. UE#2 resumes sending the media stream to the remote endpoint.

19. UE#2 acknowledges receipt of the Resume message with a 200-OK final response, sent to P‑CSCF#2.

20. P‑CSCF#2 forwards the 200 OK final response to S‑CSCF#2.

21. S‑CSCF#2 forwards the 200 OK final response to S‑CSCF#1.

22. S‑CSCF#1 forwards the 200 OK final response to P‑CSCF#1.

23. P‑CSCF#1 forwards the 200 OK final response to UE#1.

24. UE#1 resumes sending the media stream to the remote endpoint.

5.11.1.2 Mobile-initiated Hold and Resume of a Mobile-PSTN Session

An IMS session was previously established between an initiating UE and a MGCF acting as a gateway for a session terminating on the PSTN, or between an initiating MGCF acting as a gateway for a session originating on the PSTN to a terminating UE. The UE has an associated P‑CSCF, an S‑CSCF assigned in its home network, and a BGCF that chooses the MGCF. The procedures are independent of whether the P‑CSCF is located in the subscriber’s home or visited network. Therefore there is no distinction in this clause of home network vs. visited network.

The session hold and resume procedure is similar whether the UE initiated the session to the PSTN, or if the PSTN initiated the session to the UE. The only difference is the optional presence of the BGCF in the case of a session initiated by the UE. Note that the BGCF might or might not be present in the signalling path after the first INVITE is routed.

The procedures for placing a media stream on hold, and later resuming the media stream, are as shown in the following information flow:

Figure 5.29: Mobile-initiated Hold and Resume of a Mobile-PSTN Session

Information flow procedures are as follows:

1. UE detects a request from the user to place a media stream on hold. UE#1 stops sending the media stream to the remote endpoint, but keeps the resources for the session reserved.

2. UE sends a Hold message to its proxy, P‑CSCF.

3. P‑CSCF forwards the Hold message to S‑CSCF.

4. S‑CSCF forwards the Hold message to BGCF.

5. BGCF forwards the Hold message to MGCF.

5a MGCF sends a CPG(hold) in order to express that the call has been placed on hold.

6. MGCF initiates a H.248 interaction with MGW instructing it to stop sending the media stream, but to keep the resources for the session reserved.

7. MGCF acknowledges receipt of the Hold message with a 200-OK final response, send to BGCF.

8. BGCF forwards the 200-OK to the S‑CSCF.

9. S‑CSCF forwards the 200 OK final response to P‑CSCF.

10. P‑CSCF forwards the 200 OK final response to UE.

11. UE detects a request from the user to resume the media stream previously placed on hold. UE sends a Resume message to its proxy, P‑CSCF.

12. P‑CSCF forwards the Resume message to S‑CSCF.

13. S‑CSCF forwards the Resume message to BGCF.

14. BGCF forwards the Resume message to MGCF.

14a. MGCF sends a CPG(resume) in order to resume the call.

15. MGCF initiates a H.248 interaction with MGW instructing it to resume sending the media stream.

16. MGCF acknowledges receipt of the Resume message with a 200-OK final response, sent to BGCF.

17. BGCF forwards the 200 OK final response to the S‑CSCF.

18. S‑CSCF forwards the 200 OK final response to P‑CSCF.

19. P‑CSCF forwards the 200 OK final response to UE.

20. UE resumes sending the media stream to the remote endpoint.

5.11.1.3 PSTN-initiated Hold and Resume of a Mobile-PSTN Session

An IMS session was previously established between an initiating UE and a MGCF acting as a gateway for a session terminating on the PSTN, or between an initiating MGCF acting as a gateway for a session originating on the PSTN to a terminating UE. The UE has an associated P‑CSCF, an S‑CSCF assigned in its home network, and a BGCF that chooses the MGCF. The procedures are independent of whether the P‑CSCF is located in the subscriber’s home or visited network. Therefore there is no distinction in this clause of home network vs. visited network.

The session hold and resume procedure is similar whether the UE initiated the session to the PSTN, or if the PSTN initiated the session to the UE. The only difference is the optional presence of the BGCF in the case of a session initiated by the UE. Note that the BGCF might or might not be present in the signalling path after the first INVITE is routed.

The following information flow shows the procedures, where the session is set on hold from the PSTN side:

Figure 5.29a: PSTN-initiated Hold and Resume of a Mobile-PSTN Session

Information flow procedures are as follows:

1. The call is placed on hold in the PSTN.

2. The MGCF receives a CPG (hold) from the PSTN, which indicates that the call has been placed on hold.

3. MGCF sends a Hold message to BGCF.

4. BGCF forwards the Hold message to S‑CSCF.

5. S‑CSCF forwards the Hold message to P‑CSCF.

6. P‑CSCF forwards the Hold message to the UE.

7. UE stops sending the media stream to the remote endpoint, but keeps the resources for the session reserved.

8. The UE acknowledges receipt of the Hold message with a 200-OK final response, send to P‑CSCF.

9. P‑CSCF forwards the 200-OK final response to S‑CSCF.

10. S‑CSCF forwards the 200 OK final response to BGCF.

11. BGCF forwards the 200 OK final response to MGCF.

12. The call is resumed in the PSTN.

13. MGCF receives a CPG (resume) request from the PSTN, which indicates that the call is resumed.

14. MGCF sends a resume message to BGCF.

15. BGCF forwards the Resume message to S‑CSCF.

16. S‑CSCF forwards the Resume message to P‑CSCF.

17. P‑CSCF forwards the Resume message to UE.

18. UE resumes sending the media stream to the remote endpoint.

19. UE acknowledges receipt of the Resume message with a 200-OK final response, sent to P‑CSCF.

20. P‑CSCF forwards the 200 OK final response to the S‑CSCF.

21. S‑CSCF forwards the 200 OK final response to BGCF.

22. BGCF forwards the 200 OK final response to MGCF.