8.2.2 Intra-MSC UMTS to GSM Handover

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

8.2.2.1 Intra-MSC UMTS to GSM Handover that establishes Local Switching

8.2.2.1.1 General

When LCLS is not established for a call and an intra-MSC UMTS to GSM handover occurs that makes the call local, the call can be locally switched in the Target BSS. The following clauses describe the additional requirements for intra-MSC UMTS to GSM handover that establish LCLS.

8.2.2.1.2 Relocation Required

When the MSC server receives the Relocation Required message from the serving RNC, it requests the MGW to seize a TDM circuit if AoTDM or an IP termination if AoIP for the termination to the Target BSS as for the normal handover procedure.

The MSC server sends the Handover Request message to the Target BSS as for the normal case but shall include the GCR IE, the LCLS-Configuration IE and the LCLS-Connection-Status-Control IE set to "Connect".

8.2.2.1.3 Handover Request Acknowledge

If the Target BSS supports LCLS feature then it shall include the LCLS-BSS-Status IE in the Handover Request Acknowledge message in order to inform the MSC Server that the BSS supports the LCLS feature.

8.2.2.1.4 Handover Complete

The target BSS sends Handover Complete including the LCLS-BSS-Status IE, which indicates to the MSC server that the call is locally switched.

NOTE: The target BSS will send LCLS-Notification message to the MSC at the other call leg indicating the call is locally switched.

8.2.2.1.5 Example

8.2.2.1.5.1 Connection Model

Figure 8.2.2.1.5.1.1 shows the network model for the Basic Intra-MSC UMTS to GSM handover when LCLS is established as a result of the handover. The dashed line in green represents call control signalling and the dashed line in blue represents the user plane connection path via the core network, which should be used if LCLS is not established or after LCLS is broken. The non-dotted lines represent the bearer carrying real user plane data. In MGW-1 the bearer termination TS is used for the bearer towards RNC, bearer termination TA is used for the bearer towards the succeeding/preceding MGW, that is MGW-2 and bearer termination TT is used towards the Target BSS. In MGW-2 the bearer termination T2 is used for the bearer towards BSS-2 and bearer termination T1 is used for the bearer towards MGW-1.

In this example scenario the Handover Device is located in MGW-1 selected for the call establishment by the MSC-1 server, which controls the call and mobility management.

Connection Model 1: Before Handover

Connection Model 2: During Handover, TT is isolated from TS, TA is one-way connected to TT

Connection Model 3: After Handover, LCLS is established and both call legs are in Target BSS (=BSS-2)

Figure 8.2.2.1.5.1.1: Basic Intra-MSC UMTS to GSM Handover (network model)

8.2.2.1.5.2 Basic Sequence for Intra-MSC UMTS to GSM Handover that establishes Local Switching

Figure 8.2.2.1.5.2.1: Intra-MSC UMTS to GSM Handover that establishes Local Switching

1. Iu Relocation Required message is received from the RNC requesting an intra-MSC UMTS to GSM handover. The call is currently not locally switched.

2. The MSC-1 server requests the MGW-1 to reserve circuit or Connection Point towards the Target-BSS

3. The MSC-1 server sends the Handover Request message to the Target BSS with the GCR IE, the LCLS-Configuration IE and the LCLS-Connection-Status-Control IE indicating "connect" to through-connect the local call.

4. Target BSS performs call leg correlation with GCR to determine if another call leg is active with the same GCR. The Target BSS reports in Handover Request Acknowledge message that the local call was found but LCLS is not yet established.

4a. The BSS-2 notifies MSC-2 server the LCLS status is changed by sending the LCLS_Notification message with the LCLS-BSS-Status IE set to "Call not yet locally switched".

4b. If the call has been answered and MSC-2 server permits LCLS to be connected, then the MSC-2 server sends to the BSS-2 the LCLS_Connect_Control message with the LCLS-Connection-Status-Control IE set to "connect".

4c. The BSS-2 returns the LCLS_Connect_Control_ACK message with the LCLS-BSS-Status IE set to "Call not yet locally switched".

5. The MSC-1 server triggers the Relocation Command message.

6. The UE-1 is detected at the target BSS. Then the Target BSS/BSS-2 can internally transmit the user plane data.

7. In the Handover Complete message the Target-BSS indicates to the MSC-1 server in the LCLS-BSS-Status IE that the call has been locally switched.

8. The MSC-1 server requests the old serving RNC to release the old call leg.

9. The Serving BSS-2 informs the MSC-2 server that the call has been locally switched via LCLS_Notification message.

10. Releasing of the old call leg to the RNC is completed.

11. The termination Ts to the old RNC is removed from the Access MGW-1.

12. The MSC-1 server informs succeeding CN nodes that LCLS is connected.

NOTE : When BICC is used as the call control protocol the APM message is sent. When SIP-I is used the INFO request with the encapsulated APM message is sent.

8.2.2.2 Intra-MSC UMTS to GSM Handover that does not establish LCLS

Intra-MSC UMTS to GSM Handover that does not establish LCLS follows the procedures in 8.2.2.1. The differences are:

– in the step 7, the Target BSS informs MSC-1 server that the call is not locally switched in the Handover Complete message.

– steps 4a, 4b, 4c, 9 and step 12 are not triggered.