7.3.4 LCLS Re-establishment Example Call Flows

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

7.3.4.1 LCLS Re-establishment Connection Model for LCLS

Figure 7.3.4.1.1 shows the network model for a LCLS Re-establishment 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 Re-establishment

Connection Model 2: After LCLS Re-establishment

Figure 7.3.4.1.1: LCLS Re-establishment (Network model)

7.3.4.2 MSC server Initiated Example Call Flow

Figure 7.3.4.2.1 shows the message sequence example for the MSC server initiated LCLS Re-establishment. In the example the MSC server trigger the LCLS negotiation in the CN. The BSS establishes local switching when both legs are informed LCLS is allowed.

Figure 7.3.4.2.1: MSC server Initiated LCLS Re-establishment

1. The oMSC server determines that local switching should be re-established. The oMSC server sends to the succeeding node the LCLS Status Change Request message with the LCLS-Status-Change IE set to "LCLS-Connection-Preparation".

2. The iMSC server transfers the LCLS Status Change Request message to the tMSC server.

3. The tMSC server sends LCLS Status Change Request Acknowledge message to the preceding node.

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

NOTE: Step 3 and 4 can be performed paralleled.

5. The iMSC server transfers the LCLS Status Change Request Acknowledge message to the oMSC server.

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

7. On receipt of LCLS Status Change Request Acknowledge message, the oMSC server sends to the oBSS the LCLS-Connect-Control message with the LCLS-Connection-Status-Control IE set to "connect".

8. Because LCLS connect requests are received for the associated call leg, the oBSS/tBSS re-establish the LCLS.

9. The oBSS reports the LCLS connection by sending the LCLS-Connect-Control Acknowledge message to the oMSC server.

10. The tBSS reports the LCLS connection by sending the LCLS-Notification message to the tMSC server.

11. The oMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Connected" to the succeeding node.

12. The iMSC server transfers the LCLS Status Update message to the tMSC server.

13. The tMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Connected" to the preceding node. The iMSC server does not forward the LCLS Status Update message to the oMSC server since the same LCLS Status is already received from the oMSC server.

7.3.4.3 Intermediate Node / GMSC Server Initiated Example Call Flow

Figure 7.3.4.3.1 shows the message sequence example for the Intermediate Node / GMSC Server initiated LCLS Re-establishment.

Figure 7.3.4.3.1: Intermediate Node / GMSC Server Initiated LCLS Re-establishment

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

2. The iMSC server sends the LCLS Status Change Request message with the LCLS-Status-Change IE set to "LCLS-Connection-Preparation" to the oMSC server.

3. The iMSC server sends the LCLS Status Change Request message with the LCLS-Status-Change IE set to "LCLS-Connection-Preparation" to the tMSC server.

4. The oMSC server sends LCLS Status Change Request Acknowledge message to the succeeding node.

5. The tMSC server sends LCLS Status Change Request Acknowledge message to the preceding node.

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

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

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

9a. The tBSS reports the LCLS connection by sending the LCLS-Connect-Control Acknowledge message to the tMSC server.

9b. The oBSS reports the LCLS connection by sending the LCLS-Notification message to the oMSC server.

10. The tMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Connected" to the preceding node.

11. The oMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Connected" to the succeeding node.

NOTE: Since LCLS re-establishment is ordered by the iMSC server it does not forward the LCLS Status Update message to the preceding/succeeding node.

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

Figure 7.3.4.4.1 shows the message sequence example for the MSC server initiated LCLS Re-establishment 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 requests the oMGW to interrupt the communication on the bearer by using the Isolate Bearer Termination Procedure (i.e. to isolate access side termination T1 from the network side termination T2) after the oBSS re-established local switching.

Figure 7.3.4.4.1: MSC server Initiated LCLS Re-establishment, access side termination isolated in MGW

1 – 10. When the oMSC server determines that local switching should be re-established it initiates the LCLS Re-establishment procedure specified in clause 7.3.4.2, steps 1 – 10.

11. a) The oMSC server send to the oMGW request to isolate the access side termination T1 from the network side termination T2.

NOTE 1: 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].

b) The oMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Connected" to the succeeding node.

NOTE 2: Steps 11a and 11b can be performed in parallel.

12. The iMSC server transfers the LCLS Status Update message to the tMSC server.

13. The tMSC server sends the LCLS Status Update message with the LCLS-Status IE set to "LCLS Connected" to the preceding node. The iMSC server does not forward the LCLS Status Update message to the oMSC server since the same LCLS Status is already received from the oMSC server.