13.2.6 Example

23.2843GPPLocal Call Local Switch (LCLS)Release 17Stage 2TS

13.2.6.1 Connection Model

Figure 13.2.6.1.1 shows the network model for Call Deflection (CD).

The oMSC server seizes one context with two bearer terminations in the oMGW. The bearer termination T1 is used for the bearer towards the oBSS (calling subscriber) and the bearer termination T2 is used for the bearer towards the GMSC selected iMGW. The GMSC server seizes one context with two bearer terminations in the iMGW. The bearer termination T4 is used for the bearer towards the sMSC server selected sMGW and the bearer termination T3 is used for the bearer towards the preceding oMGW. The sMSC server seizes one context with two bearer terminations in the sMGW. The bearer termination T5 is used for the bearer towards the GMSC selected iMGW and bearer termination Ts is used for the bearer towards the sBSS (served subscriber).

After a call deflection request is accepted the sMSC server replaces the bearer termination for the served mobile subscriber Ts with the bearer termination for the forwarded-to subscriber T6 in an existing context in the sMGW.

The tMSC server seizes one context with two bearer terminations in the tMGW. The bearer termination T7 is used for the bearer towards the sMSC selected sMGW and bearer termination T8 is used for the bearer towards the tBSS (forwarded-to subscriber).

Connection Model 1: Before Call Deflection (CD) Request from Served UE

Connection Model 2: After CD is accepted, Announcement towards calling party

Connection Model 3: CD, After Answer, Call locally switched

Figure 13.2.6.1.1: Connection Model for Call Deflection

13.2.6.2 Basic Sequence

Figures 13.2.6.2.1 and 13.2.6.2.2 show the message sequence example for the call deflection with a possible notification to the calling party using an announcement. In the example the sMSC server optionally requests the sMGW to play an announcement and to notify the announcement completion. The sMSC server requests the establishment of the call and the bearer towards the forward-to subscriber after the possible announcement has completed. In this example the calling subscriber (oUE) and the forwarded-to subscriber (tUE) belong to the same BSS (marked as oBSS and tBSS) and the CN permits LCLS. This example is based on examples from clause 6.

Figure 13.2.6.2.1: CD, Call Establishment Flow

1. The incoming call is offered to the served subscriber as a basic mobile terminated call as described in the first part of clause 6.3.2. The Call Deflection (CD) supplementary service is active and a Call Deflection is requested from the served subscriber sUE.

2. The Call Deflection is accepted.

3. The sMSC server initiates call clearing towards the sUE by sending a DISCONNECT message.

4. Upon receiving the DISCONNECT message the sUE sends a RELEASE message to the core network.

5. The sMSC server sends the RELEASE COMPLETE message to the sUE.

6. The sMSC server request the sBSS to release the associated dedicated resource(s) by sending CLEAR COMMAND message.

7. The sBSS informs the sMSC server that the associated dedicated resource(s) has been successfully cleared with the CLEAR COMPLETE message.

8. The sMSC server orders the sMGW to remove the bearer termination (Ts) towards the served mobile subscriber (in case when the radio resources had already been allocated in the sMGW).

9. The sMSC server sends the CPG message with the Generic Notification Indicator parameter set to "Call is diverting".

10. The sMSC server provides the sMGW with the announcement/tone identification and requests the sMGW to notify the announcement/tone completion using the Play Announcement or Send Tone procedure.

11. The GMSC server forwards the CPG message with the Generic Notification Indicator parameter set to "Call is diverting" to the preceding node.

12. The oMSC server notifies the calling user (oUE) about call forwarding.

13. The sMGW notifies the sMSC server when the announcement/tone is completed using the Announcement Completed or Tone Completed procedure.

Figure 13.2.6.2.2: CD, Call Establishment Flow (continuation of figure 13.2.6.2.1)

14. If the sMSC server supports LCLS it may modify the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE before sending the IAM message containing the GCR with the encapsulated oBSS ID, the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE.

15. The tMSC server pages the forwarded-to subscriber (tUE).

16. The tMSC server performs call Setup.

17. The tUE confirms the call.

18. The tMSC server requests the tMGW to prepare for the network side bearer establishment (T7).

19. After the tMGW has replied with the bearer address and the binding reference the tMSC server returns the APM message with the selected codec and if LCLS is supported, the LCLS-Negotiation Response IE and the LCLS-Configuration-Preference IE.

20. The sMSC server transfers the APM message with the LCLS-Negotiation Response IE and the LCLS-Configuration-Preference IE. If codec modification is required then the sMSC server includes the codec related information within the same APM message.

21. The GMSC server transfers the APM message.

22. Based on the returned LCLS-Negotiation Response IE IE and the LCLS-Configuration-Preference IE the oMSC server determines whether LCLS is allowed in the core network and if LCLS-Configuration update is needed.
If codec modification is required then the oMSC server performs codec negotiation according to 3GPP TS 23.153 [4].

23. When performing further call establishment the procedure between the calling subscriber (oUE) and the forwarded-to subscriber (tUE) is the same as specified in steps 18 – 32 of clause 6.3.2.1.

24. Since the received ANM message indicated "LCLS is feasible but not yet connected" the oMSC server checks if LCLS-Configuration updated is needed and if so the oMSC server calculates the new LCLS-Configuration value based on the latest received LCLS-Negotiation IE.

25. The oMSC server requests the oBSS to connect LCLS and if configuration updated is needed, it includes the LCLS-Configuration IE in the LCLS_CONNECT_CONTROL message.

26a. Since the BSS has received the through connect request for both call legs the oBSS returns the LCLS_CONNECT_CONTROL_ACK message with the LCLS-BSS-Status IE set to "the call is locally switched with requested LCLS configuration".

26b. Since the BSS has received the through connect request for both call legs the tBSS signals the LCLS status change by sending the LCLS_NOTIFICATION message with the LCLS-BSS-Status IE set to "the call is locally switched with requested LCLS configuration".

27. The oMSC server signals the change of the LCLS status through the Core Network by sending the APM message with the LCLS-Status IE set to "LCLS connected".

28. The iMSC server transfers the change of the LCLS status to the sMSC server.

29. The sMSC server transfers the change of the LCLS status to the tMSC server.