7.2.4 LCLS Break Example Call Flows

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

7.2.4.1 LCLS Break Connection Model for LCLS

Figure 7.2.4.1.1 shows the network model for a LCLS break of the mobile call. The "squared" line represents the call control signalling. The "dotted/full" line represents the bearer terminations in the MGW. Bearer termination T1 and T6 are used for the bearer towards BSC and bearer termination T2, T3, T4 and T5 are used for the bearer towards preceding/succeeding MGW.

Connection Model 1: Before LCLS Break

Connection Model 2: After LCLS Break

Figure 7.2.4.1.1: LCLS Break (Network model)

7.2.4.2 MSC server Initiated

Figure 7.2.4.2.1 shows the message sequence example for the MSC server initiated LCLS Break.

Figure 7.2.4.2.1: MSC Server initiated LCLS break

1. The oMSC server determines that local switching should be disconnected.

2. The oMSC server sends to the succeeding node the LCLS Status Change Request message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation".

3. The iMSC server transfers the LCLS Status Change Request message to the succeeding node.

4. The tMSC server sends to the tBSS the LCLS-Connect-Control message with the LCLS-Connection-Status-Control IE set to "Release LCLS".

5. The tBSS confirms the reception of the LCLS release request with the LCLS-Connect-Control Acknowledge message but does not change the LCLS-BSS status since LCLS release request is not yet received for the associated call leg.

6. The tMSC server sends to the preceding node the LCLS Status Change Request Acknowledge message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation" and the Result code IE set to LCLS Status Change Request accepted.

7. The iMSC server transfers the LCLS Status Change Request Acknowledge message to the preceding node.

8. The oMSC server sends to the oBSS the LCLS-Connect-Control message with the LCLS-Connection-Status-Control IE set to "Release LCLS".

9. The BSS reports the LCLS disconnection by sending:
a) The LCLS-Connect-Control Acknowledge message with the LCLS-BSS-Status IE set to "the call is no longer locally switched" to the oMSC server.

b) The LCLS-Notification message with the LCLS-BSS-Status IE set to "the call is no longer locally switched" to the tMSC server.

10. The oMSC server sends the LCLS Status Update Request message with the LCLS-Status IE set to "LCLS Not Connected" to the succeeding node.

11. The iMSC server transfers the LCLS Status Update message to the succeeding node.

12. On the receipt of the LCLS-Notification message with the LCLS-BSS-Status indicating LCLS disconnection the tMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Not Connected" to the preceding node.

NOTE: The iMSC server does not forward the LCLS Status Update message since the same LCLS Status is already received from the oMSC server.

7.2.4.3 BSS Initiated, Immediate LCLS Break

Figure 7.2.4.3.1 shows the message sequence example for the BSS initiated LCLS Break.

Figure 7.2.4.3.1: BSS initiated, immediate LCLS break

1. The BSS determines that local switching should be disconnected.

2a, b. The BSS reports the LCLS disconnection by sending the LCLS-Notification message with the LCLS-BSS-Status IE set to "the call is no longer locally switched" to the tMSC server and oMSC server.

3. a) The tMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Not Connected" to the preceding node.

b) The oMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Not Connected" to the succeeding node.

4a. The iMSC server transfers the LCLS Status Update message to the oMSC server.

NOTE: At reception of the LCLS Status Update message from the oMSC server, the iMSC server does not forward the LCLS Status Update message since the same LCLS Status is already received from the tMSC server.

7.2.4.4 BSS Initiated, LCLS Break requested from Core Network

Figure 7.2.4.4.1 shows the message sequence example for the BSS initiated but when the LCLS Break is requested from the Core Network.

Figure 7.2.4.4.1: BSS initiated, LCLS Release ordered from Core Network

1. The BSS determines that local switching should be disconnected.

2a, b. The BSS requests the LCLS disconnection by sending the LCLS-Notification message with LCLS-Break-Request IE set to "LCLS Break Request" to the oMSC server and tMSC server.

3. a) The oMSC server sends to the succeeding node the LCLS Status Change Request message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation".

b) The tMSC server sends to the preceding node the LCLS Status Change Request message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation".

4a, b. The iMSC server transfers the LCLS Status Change Request message to the succeeding/preceding node.

5. a) The tMSC server sends to the tBSS the LCLS-Connect-Control message with the LCLS-Connection-Status-Control IE set to "Release LCLS".

b) The oMSC server sends to the oBSS the LCLS-Connect-Control message with the LCLS-Connection-Status-Control IE set to "Release LCLS".

6. a) The tBSS confirms the reception of the LCLS release request with the LCLS-Connect-Control Acknowledge message but does not change the LCLS-BSS status since LCLS release request is not yet received for the associated call leg.

b) The oBSS reports the LCLS disconnection by sending the LCLS-Connect-Control Acknowledge message with the LCLS-BSS-Status IE set to "the call is no longer locally switched" to the oMSC server.

c) The tBSS reports the LCLS disconnection by sending the LCLS-Notification message with the LCLS-BSS-Status IE set to "the call is no longer locally switched to the tMSC server.

7. a) On the receipt of the LCLS-Connect-Control Acknowledge message with the LCLS-BSS status still indicating local switching the tMSC server sends to the preceding node the LCLS Status Change Request Acknowledge message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation" and the Result code IE set to LCLS Status Change Request accepted.

b) On the receipt of the LCLS-Connect-Control Acknowledge message the oMSC server sends to the succeeding node the LCLS Status Change Request Acknowledge message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation" and the Result code IE set to LCLS Status Change Request accepted.

8. a) The iMSC server transfers the LCLS Status Change Request Acknowledge message to the preceding node.

b) The iMSC server transfers the LCLS Status Change Request Acknowledge message to the succeeding node.

NOTE1: The oMSC server already requested the LCLS break from the oBSS (step 5b) and due to that it does not perform any action on the receipt of the LCLS Status Change Request Acknowledge message. The tMSC server does not perform any action on the receipt of the LCLS Status Change Request Acknowledge message since it already requested the LCLS break from the tBSS (step 5a) and already received LCLS-Notification message indicating LCLS disconnection (step 6c).

9. At reception of the LCLS-Connect-Control Acknowledge message indicating LCLS disconnection the oMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Not Connected" to the succeeding node.

10. The iMSC server transfers the LCLS Status Update message to the succeeding node.

11. On the receipt of the LCLS-Notification message indicating LCLS disconnection the tMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Not Connected" to the preceding node.

NOTE2: At reception of the LCLS Status Update message from the tMSC server, the iMSC server does not forward the LCLS Status Update message since the same LCLS Status is already received from the oMSC server.

7.2.4.5 Intermediate Node/GMSC Server Initiated

Figure 7.2.4.5.1 shows the message sequence example for the network initiated LCLS Break.

Figure 7.2.4.5.1: Intermediate Node / GMSC Server initiated LCLS break

1. The iMSC server determines that local switching should be disconnected.

2a, b. The iMSC server send the LCLS Status Change Request message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation" to the preceding node and to the succeeding node.

3. a) The oMSC server sends to the oBSS the LCLS-Connect-Control message with the LCLS-Connection-Status-Control IE set to "Release LCLS".

b) The tMSC server sends to the tBSS the LCLS-Connect-Control message with the LCLS-Connection-Status-Control IE set to "Release LCLS".

4. a) The oBSS confirms the reception of the LCLS release request but does not change the LCLS-BSS status since LCLS release request is not yet received for the associated call leg.

b) The tBSS reports the LCLS disconnection by sending the LCLS-Connect-Control Acknowledge message with the LCLS-BSS-Status IE set to "the call is no longer locally switched" to the tMSC server.

c) The oBSS reports the LCLS disconnection by sending the LCLS-Notification message with the LCLS-BSS-Status IE set to "the call is no longer locally switched" to the oMSC server.

5. a) On the receipt of the LCLS-Connect-Control Acknowledge message with the LCLS-BSS status still indicating local switching the oMSC server sends to the succeeding node the LCLS Status Change Request Acknowledge message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation" and the Result code IE set to LCLS Status Change Request accepted.

b) On the receipt of the LCLS-Connect-Control Acknowledge message the tMSC server sends to the preceding node the LCLS Status Change Request Acknowledge message with the LCLS-Status-Change IE set to "LCLS-Disconnection-Preparation" and the Result code IE set to LCLS Status Change Request accepted.

NOTE1: Since the LCLS disconnection is ordered by the iMSC server it does not forward the LCLS Status Change Request Acknowledge message to the succeeding node.

6. a) At reception of the LCLS-Connect-Control Acknowledge message indicating LCLS disconnection the tMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Not Connected" to the preceding node.

b) On the receipt of the LCLS-Notification message the oMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Not Connected" to the succeeding node.

NOTE2: Since the LCLS disconnection is ordered by the iMSC server it does not forward the LCLS Status Update message to the preceding/succeeding node.

7.2.4.6 MSC server Initiated when Access Side Termination is isolated in MGW

Figure 7.2.4.6.1 shows the message sequence example for the MSC server initiated LCLS Break for the case when the LCLS negotiation through the Core Network enabled the MSC server to use the option to isolate access side termination from the network side termination.

In this example the oMSC server moves the originating UE (access side termination T1) back to the context oC with network side termination T2 and requests the oMGW to be bothway through-connected after the oMSC server determined that local switching should be disconnected and sent to the succeeding node the LCLS Status Change Request message indicating LCLS disconnection preparation.

Figure 7.2.4.6.1: MSC initiated LCLS break, access side termination isolated in MGW

1. The oMSC server determines that local switching should be disconnected.

2. The oMSC server sends to:
a) the succeeding node the LCLS Status Change Request message with the LCLS-Status-Change IE set to
"LCLS-Disconnection-Preparation";

b) the oMGW request to move the access side termination T1 to context oC with the network side termination T2;

NOTE 1: Steps 2a and 2b can be performed in parallel.

NOTE 2: If the MSC server has previously used the Change Through-Connection procedure and requested the MGW to change the through-connection of the bearer to inactive instead of using the Isolate Bearer termination procedure then the MSC server will use the Change Through-Connection procedure to request the MGW to change the through-connection of the bearer to be both-way through-connected.

3. The further steps are performed as defined in clause 7.2.4.2, steps 3 – 12.