6.3.2 LCLS established, Basic Call Example with BICC based CS core network, forward bearer establishment

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

Figures 6.3.2.1, 6.3.2.2 and 6.3.2.3 show the message sequence example for the basic call establishment for LCLS. In this example the oUE and the tUE belong to the same BSS (marked as oBSS and tBSS) and the CN permits LCLS. The example is based on examples from 3GPP TS 23.205 [2] for the basic mobile originating call, forward bearer establishment (case when access bearer assignment is requested on the originating side after reception of Bearer Information message) and the basic mobile terminating call, forward bearer establishment.

Figure 6.3.2.1: Basic Call Establishment Flow when call is locally switched, forward bearer establishment

1. Service Request handling.

2. Originating Call SETUP.

2a. The oMSC server replies with the CALL PROCEEDING message to indicate that the call is being processed.

3. If the oMSC server supports LCLS it retrieves the oBSS ID and generates the Global Call Reference for the call.

4. The oMSC server sends the IAM message including supported codecs list, GCR with encapsulated oBSS ID, the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE.

5. If the iMSC server supports LCLS it may modify the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE due to CAMEL, supplementary service requirements etc. before sending the IAM message containing the GCR with the encapsulated oBSS ID, the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE.

6. The tMSC server pages the tUE.

7. The tMSC server performs call Setup.

8. The tUE confirms the call.

9. The tMSC server requests the tMGW to prepare for the network side bearer establishment.

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

11a. When the bearer information is received the iMSC server requests the seizure of the outgoing network side bearer termination.

11b. After the outgoing side bearer termination is seized the iMSC server requests the seizure of the incoming network side bearer termination.
During the seizure of the outgoing side and the incoming side bearer termination the iMSC server will also request the iMGW to through-connect the bearer terminations so that the bearer will be both way through-connected.

12. The iMSC server transfers the APM message with the selected codec, available codec list, the LCLS-Negotiation Response IE and the LCLS-Configuration-Preference IE.

Figure 6.3.2.2: Basic Call Establishment when call is locally switched, forward bearer establishment (continuation of figure 6.3.2.1)

13a. When the bearer information is received the oMSC server requests the seizure of the network side bearer termination.

13b. After the network side bearer information is seized the oMSC server requests the seizure of the access side bearer termination.
During the seizure of the network side or the access side bearer termination the oMSC server will also request the oMGW to through-connect the bearer terminations so that the bearer will be backward through-connected.

14. The oMSC server determines whether LCLS is allowed in the core network based on the returned LCLS-Negotiation IE and if so the oMSC server includes the LCLS-Configuration IE in the ASSIGNMENT REQUEST message along with the GCR IE.

15. The oBSS returns the ASSIGNMENT COMPLETE message with the LCLS-BSS-Status IE indicating "call not possible to be locally switched".

16. When the access assignment is completed the oMSC server sends the Continuity (COT) message to the iMSC server.

17. The iMSC server transfers the COT message to the tMSC server.

18. The tMSC server requests the seizure of the access side bearer termination. If not requested during the seizure of network side bearer termination (in step 9) the tMSC server will also request the tMGW to through-connect the bearer terminations so that the bearer will be backward through-connected.

19a. If the tMSC server supports the optional "intra-Network call detection" procedure it compares its own Network ID with the Network ID received within the Global Call Reference IE.
If the tMSC server supports the optional "intra-BSS call detection" procedure it compares the BSS ID of the selected terminating BSS with the oBSS ID received within the Global Call Reference IE at this step. Since the oUE and the tUE belong to the same BSS the call continues the same way as for the basic LCLS establishment without this pre-check.

19b. The tMSC server performs the access bearer assignment and sends the ASSIGNMENT REQUEST message containing the GCR IE and the LCLS-Configuration IE if LCLS is permitted in the core network.

20. The oBSS/tBSS performs the GCR correlation. Since the GCR correlation has identified the call as an intra BSS call and LCLS is allowed in the BSS, the tBSS returns the ASSIGMENT COMPLETE message with the LCLS-BSS-Status IE indicating "Call not yet locally switched".

20a. Since the GCR correlation has identified the call as an intra BSS call and LCLS is allowed in the BSS, the oBSS signals the LCLS status change by sending the LCLS_NOTIFICATION message with the LCLS-BSS-Status IE set to "Call not yet locally switched".

21. The tUE reports alerting.

22a, b. The tMSC server returns the ACM message and requests the tMGW to provide a ring-back tone.

23. The iMSC server transfers the ACM message to the oMSC server.

24. The oMSC server reports alerting.

Figure 6.3.2.3: Basic Call Establishment when call is locally switched, forward bearer establishment (continuation of figure 6.3.2.2)

25. The tUE answers the call.

25a. The tMSC server returns the CONNECT ACKNOWLEDGE message to the tUE.

26. The tMSC server indicates to the tBSS that this call leg is ready to be locally switched by sending the LCLS_CONNECT_CONTROL message (note the BSS cannot through-connect LCLS until it receives the same indication from the oMSC server).

27. The tBSS returns the LCLS_CONNECT_CONTROL_ACK message with the LCLS-BSS-Status IE set to "Call not yet locally switched" since the BSS has not received the same order from the oMSC server.

28. When the tMSC server receives the Connect message it requests the tMGW to stop providing ring-back tone to the calling party and requests to bothway through-connect the bearer.

29. The tMSC server returns the ANM message with the LCLS-Status IE indicating "LCLS is feasible but not yet connected".

30. The oMSC server receives the ANM message with the LCLS-Status IE indicating "LCLS is feasible but not yet connected".

31. The oMSC server request the oMGW to bothway through-connect the bearer.

32. The oMSC server reports Answer/Connect to the oUE.

32a. The oUE completes the call establishment with the CONNECT ACKNOWLEDGE message.

33. The oMSC server requests the oBSS to connect LCLS since the received ANM message indicated "LCLS is feasible but not yet connected".

34a. 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 "call is locally switched with requested LCLS configuration".

NOTE 1: If the BSS cannot locally through-connect the call at this time then it is indicated by setting the LCLS-BSS-Status IE set to "the call is not yet locally switched". If at a later time the BSS can locally switch the call, this is indicated by sending the LCLS_NOTIFICATION message with the LCLS-BSS-Status IE set to "the call is locally switched with requested LCLS configuration".

34b. 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 "call is locally switched with requested LCLS configuration".

34c. If the oMSC server supports the option to configure its Access MGW to isolate the access side termination from the network side termination and LCLS negotiation indicated that no succeeding node requires the UL data from the oUE then the oMSC server requests the oMGW to isolate the access side termination T1 from the network side termination T2.

34d. If the tMSC server supports the option to configure its Access MGW to isolate the access side termination from the network side termination and LCLS negotiation indicated that no preceding node requires the UL data from the tUE then the tMSC server requests the tMGW to isolate the access side termination T4 from the network side termination T3.

NOTE 2: The MSC server can also use the Change Through-Connection procedure and requests the MGW to change the through-connection of the bearer to inactive instead of using of the Isolate Bearer termination procedure, see 3GPP TS 23.205 [2].

35. 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".

36. The iMSC server transfers the change of the LCLS status to the tMSC server.