8.2.3 Inter-MSC UMTS to GSM Handover

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

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

8.2.3.1 Inter-MSC UMTS to GSM Handover that establishes Local Switching

8.2.3.1.1 General

When LCLS is not established for a call and an inter-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 inter-MSC handovers that establish LCLS.

8.2.3.1.2 MSC-1 / MGW-1

8.2.3.1.2.1 Relocation Required

When MSC-1 Server receives the Relocation Required message from the serving RNC and determines that the call shall be handed over to the Target MSC Server, it shall send the GCR of the call, the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE to the Target MSC Server in a MAP Prepare-Handover_Request message.

8.2.3.1.2.2 Handover Request Acknowledge

When MSC-1 Server receives the MAP Prepare_Handover_Response including Handover_Request_Acknowledgement message with a LCLS-BSS-Status IE the Anchor MSC-1 Server configures the bearer terminations in MGW-1 and sends the GCR IE, the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE to the target MSC-Server.

8.2.3.1.2.3 Bearer establishment between MGW-1 and Target MGW

The handling of the bearer establishment between MGW-1 and Target MGW is as described in clause 6.1 for a Basic Mobile Originating Call. The MSC server shall also use the Change Flow Direction procedure to request the MGW-1 to set the Handover Device to the initial state.

8.2.3.1.2.4 MGW Flow Direction Control

In accordance with the normal handover case the MGW-1 isolates the termination towards the Target MGW (T₂) from the termination to the RNC(T_S) and configures the Anchor termination (T₁) one-way DL towards the Target MGW termination (T₂). Termination to the RNC (T_S) is both-way connected to Anchor termination (T₁) since it is also receiving UL user data from termination to the RNC (T_S).

8.2.3.1.2.5 Relocation Command/Handover Detect

The MSC-1 Server uses the Change Flow Direction procedure to requests the MGW-1 to set the Handover Device to intermediate state.

8.2.3.1.2.6 Handover Complete

When the MSC-1 Server receives the Handover Complete message, it releases the related Iu-interface connection towards RNC. The MSC-1 Server also requests MGW-1 to set the Handover Device to its final state by removing the bearer termination towards the RNC.

The MSC-1 server shall send to the adjacent call node the LCLS-Status-Update message with the LCLS-Status IE indicating that LCLS is established.

8.2.3.1.3 Target MSC Server / Target MGW

8.2.3.1.3.1 Prepare Handover Request message and MGW selection

The Target MSC server selects the Target MGW when it receives MAP Prepare Handover Request message. The Target 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".

8.2.3.1.3.2 Handover Request Acknowledge

If the Target BSS supports the LCLS feature it shall include the LCLS-BSS-Status IE in the Handover Request Acknowledge message in order to inform the Target MSC Server that the BSS supports the LCLS feature. The Target MSC Server sends the same information in the MAP Prepare Handover Response message to the MSC-1 Server.

8.2.3.1.3.3 Bearer establishment towards Target BSS

When the Target MSC Server has selected the Target MGW it requests the Target MGW to seize a TDM circuit if AoTDM using the Reserve Circuit procedure, or an IP termination if AoIP using the reserve Connection Point procedure as for the normal handover procedure. The Target MSC Server sends the Handover Request message to the Target BSS containing the CIC for AoTDM or the IP addresses and UDP ports received from the target MGW if AoIP.

8.2.3.1.3.4 Bearer establishment between MGW-1 and Target MGW

The handling of the bearer establishment between MGW-1 and Target MGW is as described for basic mobile terminating call in clause 6.2.

8.2.3.1.3.5 Handover Complete

When LCLS has been established during the handover procedure, the target BSS informs the target MSC-Server that the call has been locally switched in the Handover Complete message.

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

8.2.3.1.4 Example of Inter-MSC UMTS to GSM Handover that establishes Local Switching

8.2.3.1.4.1 Connection Model

Figure 8.2.3.1.4.1.1 shows the network model for the Basic Inter-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 T_S is used for the bearer towards RNC, bearer termination T₁ is used for the bearer towards the succeeding/preceding MGW, that is MGW-2 and bearer termination T₂ is used towards the Target MGW. In MGW-2 the bearer termination T₄ is used for the bearer towards BSS-2 and bearer termination T₃ is used for the bearer towards MGW-1. In Target-MGW the bearer termination T_T is used towards the Target-BSS and bearer termination T₅ is used 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, T₂ is isolated from T_S, T₁ is one-way connected to T₂

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

Figure 8.2.3.1.4.1.1: Basic Inter-MSC UMTS to GSM Handover (network model)

8.2.3.1.4.2 Basic Sequence for Inter-MSC UMTS to GSM Handover that establishes Local Switching

Figures 8.2.3.1.4.2.1 and 8.2.3.1.4.2.2 show the message sequence example for the Basic Inter-MSC UMTS to GSM Handover shown in the corresponding network model Figure 8.2.3.1.4.1.1. The Handover Device is located in MGW-1 selected for the call establishment by the MSC-1 server, which controls the call and the mobility management. The description is based on 3GPP TS 23.009 [9], 3GPP TS 23.205 [2] and 3GPP TS 23.231 [3].

Figure 8.2.3.1.4.2.1: Initial phase of Inter-MSC UMTS to GSM Handover establishing Local Switching

1. Iu Relocation Required Request is received from RNC requesting an inter-MSC handover. The call is currently not locally switched.

2. The MSC-1 server determines that inter-MSC handover is required and sends the MAP Prepare-Handover Request message to target MSC-Server which includes LCLS Negotiation Request IE, the LCLS-Configuration-Preference IE and GCR IE.

3a, b. The Target-MSC-Server requests the target MGW to reserve circuit or Connection Point towards the Target-BSS

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

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

5a. 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".

5b. 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".

5c. The BSS-2 returns the LCLS_Connect_Control_ACK message with the LCLS-BSS-Status IE set to "Call not yet locally switched".

6a, b. (These signalling steps are only applicable to AoIP.) When the Target MSC-Server receives the BSSMAP Handover Request-Ack message, it sends the BSC-B IP address and UDP Port number to the MGW-B using the Configure RTP Connection Point procedure.

7. The Target MSC-Server sends the MAP Prepare Handover Response message to MSC-1 server.

8a, b. In accordance with normal handover the MSC-1 server requests MGW-1 to isolate the termination towards Target MGW (T₂) from the termination to the Serving BSS-1 (T_S) and to configure the Anchor termination (T₁) one-way DL towards the Target MGW termination (T₂).

9. MSC-Server 1 sends IAM (Initial Address Message) to Target MSC-Server including GCR, the LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE.

NOTE 1: Corresponding SIP-I signalling is specified in 3GPP TS 23.231 [3].

NOTE 2: The LCLS-Negotiation Request IE and the LCLS-Configuration-Preference IE in step 9 can be different from LCLS Negotiation Request IE and the LCLS-Configuration-Preference IE in step 2, because step 9 is BICC signalling and the LCLS Negotiation Request and the LCLS-Configuration-Preference IE values can be changed by intermediate MSC-Servers.

10a, b. Target MSC-Server reserves bearer connection T₅towards MGW-1.

11. After Target MGW has replied with the bearer address and the binding reference (Step 10b), the Target MSC-Server returns APM with selected codec, the LCLS-Negotiation Response IE and the LCLS-Configuration-Preference IE.

Figure 8.2.3.1.4.2.2: Completion phase of Inter-MSC UMTS to GSM Handover establishing Local Switching

12. Target MSC-Server sends ACM (Address Complete Message). Target MSC-Server awaits the capturing of the UE-1 on the radio path when the ACM is sent and MSC-1 server initiates the handover execution when receiving ACM.

13 – 18. When the local switching has been established during the handover procedure, the target BSS shall inform the target MSC-Server that the call has been locally switched in HANDOVER COMPLETE, and the target BSS shall also send a new message LCLS-Notification with LCLS-BSS-Status IE to inform the MSC-2 server that the local switching has been established. In steps 16a and 16b the MSC-1 server configures MGW-1 for the completion of the handover.

19. A-HO-DETECT/COMPLETE when received, is included in the MAP-Send-End-Signal request and send back to the MSC-1 server.

20. Target MSC-Server sends ANSWER when A-HO-DETECT/COMPLETE is received.

21a, b. MSC-1 Server releases the call leg in RNC.

22a, b. MSC-1 server releases the bearer termination towards RNC.

23. Target MSC-Server informs the MSC-1 server about the LCLS Status.

24. MSC-1 server (Anchor MSC-Server) sends LCLS-Status-Update message to the far end MSC-2 server.

NOTE 3: 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.

25. Local switching is established in the BSS.

8.2.3.2 Inter-MSC UMTS to GSM Handover that does not establish Local Switching

Inter-MSC UMTS to GSM Handover that does not establish Local Switching follows the procedures in 8.2.3.1. The differences are:

– in the step 17, the target BSS informs target MSC that the call is not locally switched in the Handover Complete.

– steps 5a, 5b, 5c 18, step 23, step 24 and step 25 are not triggered.