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Content for  TS 23.284  Word version:  17.0.0

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8 Handover/Relocation  8.1 UMTS to UMTS  8.2 UMTS to GSM  8.2.1 General  8.2.2 Intra-MSC UMTS to GSM Handover  8.2.2.1 Intra-MSC UMTS to GSM Handover that establishes Local Switching  8.2.2.1.1 General  8.2.2.1.2 Relocation Required  8.2.2.1.3 Handover Request Acknowledge  8.2.2.1.4 Handover Complete  8.2.2.1.5 Example  8.2.2.2 Intra-MSC UMTS to GSM Handover that does not establish LCLS 
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8  Handover/Relocation  p. 64

8.1  UMTS to UMTS  p. 64

For Inter-MSC Handover UMTS to UMTS the Anchor MSC server shall include the GCR IE, the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE in IAM to the Target MSC server. The Target MSC server may modify the LCLS-Configuration-Preference IE based on its own LCLS configuration requirements. The rules for LCLS Configuration settings are defined in clause 4.2. The Target MSC server shall include the final LCLS-Configuration-Preference IE and the LCLS-Negotiation Response IE in the APM or ACM or CPG. The Target MSC server may then use the LCLS parameters to enable LCLS if a subsequent UTRAN to GERAN handover occurs.
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8.2  UMTS to GSM  p. 64

8.2.1  General  p. 64

The procedures specified in TS 23.205 for BICC based CS Core Network and 3GPP 23.231 [3] for SIP-I based CS Core Network shall be followed. The following clauses describe the additional requirements for LCLS functionality.

8.2.2  Intra-MSC UMTS to GSM Handover  p. 64

8.2.2.1  Intra-MSC UMTS to GSM Handover that establishes Local Switching  p. 64

8.2.2.1.1  General  p. 64
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  p. 64
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  p. 65
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  p. 65
The target BSS sends Handover Complete including the LCLS-BSS-Status IE, which indicates to the MSC server that the call is locally switched.
8.2.2.1.5  Example  p. 65
8.2.2.1.5.1  Connection Model  p. 65
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.
Copy of original 3GPP image for 3GPP TS 23.284, Fig. 8.2.2.1.5.1.1: Basic Intra-MSC UMTS to GSM Handover (network model)
Figure 8.2.2.1.5.1.1: Basic Intra-MSC UMTS to GSM Handover (network model)
(⇒ copy of original 3GPP image)
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8.2.2.1.5.2  Basic Sequence for Intra-MSC UMTS to GSM Handover that establishes Local Switching  p. 68
Copy of original 3GPP image for 3GPP TS 23.284, Fig. 8.2.2.1.5.2.1: 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
(⇒ copy of original 3GPP image)
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Step 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.
Step 2.
The MSC-1 server requests the MGW-1 to reserve circuit or Connection Point towards the Target-BSS
Step 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.
Step 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.
Step 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".
Step 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".
Step 4c.
The BSS-2 returns the LCLS_Connect_Control_ACK message with the LCLS-BSS-Status IE set to "Call not yet locally switched".
Step 5.
The MSC-1 server triggers the Relocation Command message.
Step 6.
The UE-1 is detected at the target BSS. Then the Target BSS/BSS-2 can internally transmit the user plane data.
Step 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.
Step 8.
The MSC-1 server requests the old serving RNC to release the old call leg.
Step 9.
The Serving BSS-2 informs the MSC-2 server that the call has been locally switched via LCLS_Notification message.
Step 10.
Releasing of the old call leg to the RNC is completed.
Step 11.
The termination Ts to the old RNC is removed from the Access MGW-1.
Step 12.
The MSC-1 server informs succeeding CN nodes that LCLS is connected.
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8.2.2.2  Intra-MSC UMTS to GSM Handover that does not establish LCLS  p. 69

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.

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