8.4.5 BSS Internal Handover

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

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

8.4.5.1 General

The following procedures describe the specific handling compared to the basic principles described in 3GPP TS 23.205 [2] clause 8.4.5 to achieve BSS Internal Handover with LCLS for an A-interface User Plane over IP (AoIP).

If the call is not locally switched but both call legs have been correlated and an internal handover occurs that makes the call local, the call should be locally switched in the BSS.

If a call is currently locally switched and an internal handover occurs that makes the call not local, the local switching should be broken in the BSS and the user plane data shall be connected through the core network.

NOTE1: For A-interface User Plane over TDM (AoTDM), a BSS internal handover that results in LCLS break will trigger a BSS Initiated LCLS Break according to procedures in clause 7.2.2.

If an internal handover procedure occurs that does not modify the LCLS status of a call, the local switching should not be modified within the BSS.

8.4.5.2 Internal Handover Required

If the MSC Server accepts the Internal Handover Required message it shall send an Internal Handover Command message to the BSS. If the call is currently locally switched in the BSS, the MSC Server shall also signal LCLS-Status-Change Request message containing LCLS-Status-Change IE set to "LCLS Disconnection Preparation for Handover" through the core network to enable UL bi-casting during handover.

The MSC Server shall not wait for the LCLS-Status-Change Request Acknowledge message before proceeding with the Internal Handover.

8.4.5.3 Internal Handover Command

If local switching is permitted by the core network and the MSC Server has not previously requested that the BSS should connect the local call, (e.g. no previous LCLS-Connection-Status-Control = "Connect"), the MSC Server shall include the LCLS-Connection-Status-Control IE indicating "Connect" in the Internal Handover Command message.

Otherwise, the MSC Server shall send the Internal Handover Command message according to the procedures in 3GPP TS 48.008 [7].

8.4.5.4 Handover Complete

The BSS sends Handover Complete message including the LCLS-BSS-Status that indicates whether the call is locally switched (e.g. "Call is Locally Switched with requested LCLS configuration " or "the call is no longer locally switched").

The MSC server shall send to an adjacent call node the LCLS-Status-Update message with the LCLS-Status IE indicating the new LCLS Status (e.g. "LCLS connected" or "LCLS Not Connected").

8.4.5.5 Example BSS Internal Handover that Establishes Local Switching

8.4.5.5.1 Connection Model

Figure 8.4.5.5.1.1 shows the network model for the Intra-MSC BSS Internal Handover, where the call leg pertinent to the UE-1 is handed over from the serving BSS-1 to BSS-2. BSS-1 is the same as BSS-2 for BSS Internal Handover. The bearer termination T₂ is used for the bearer towards BSS-2, which is not affected by this handover. Bearer termination T_S is used for the bearer towards BSS-1 and the bearer terminations T₁ and T_A are used for the bearer towards the succeeding/preceding MGW. Bearer termination T_T is for the bearer termination towards the BSS after internal handover. The colours and line types used in the figure are defined differently from 3GPP TS 23.205 [2] to indicate LCLS specific issues.

Connection Model 1: Before BSS Internal Handover – Call is not locally switched

Connection Model 2: During BSS Internal Handover

Connection Model 3: After BSS Internal Handover – Call is Locally Switched

Figure 8.4.5.5.1.1: BSS Internal Handover Connection Model that Establishes Local Switching

8.4.5.5.2 Basic Sequence for BSS Internal Handover that Establishes Local Switching

Figure 8.4.5.5.2.1 shows the message sequence example for the BSS Internal Handover that Establishes Local Switching.

In the example, the MSC server receives the Internal Handover Required message and requests the MGW to reserve an RTP bearer termination (T_T) using the Reserve and Configure RTP Connection Point procedure with specific flow directions.

Figure 8.4.5.5.2.1: BSS Internal Handover for AoIP that Establishes Local Switching

1 – 2. As for normal Internal Handover, see TS 23.205 [2] clause 8.4.5.

3. The MSC-1 Server determines that local switching is permitted by the core network and sends the Internal Handover Command message, including the LCLS-Connection-Control-Status message indicating "Connect" if not previously indicated to BSS-1.

3a. 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".

3b. 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".

3c. The BSS-2 returns the LCLS_Connect_Control_ACK message with the LCLS-BSS-Status IE set to "Call not yet locally switched".

4 – 5. As for normal Internal Handover, see TS 23.205 [2] clause 8.4.5.

6. The Handover Complete message includes the LCLS-BSS-Status IE indicating that call is locally switched.

NOTE: alternatively the BSS-1 could indicate LCLS-BSS-Status IE in LCLS-Notification message.

7. The LCLS Status is propagated through the Core Network.

8. The BSS-2 sends the LCLS_Notification message to MSC-2 server with the LCLS-BSS-Status IE set to "call is locally switched with requested LCLS configuration".

9. The termination (T_S) is removed from the Access MGW-1.

8.4.5.6 Example BSS Internal Handover that Breaks Local Switching

8.4.5.6.1 Connection Model

Figure 8.4.5.6.1.1 shows the network model for the Intra-MSC BSS Internal Handover, where the call leg pertinent to the UE-1 is handed over from the serving BSS-1 to BSS-2. BSS-1 is the same as BSS-2 for BSS Internal Handover. The bearer termination T₂ is used for the bearer towards BSS-2, which is not affected by this handover. Bearer termination T_S is used for the bearer towards BSS-1 and the bearer terminations T₁ and T_A are used for the bearer towards the succeeding/preceding MGW. Bearer termination T_T is for the bearer termination towards the BSS after internal handover. The colours and line types used in the figure are defined differently from 3GPP TS 23.205 [2] to indicate LCLS specific issues.

Connection Model 1: Before BSS Internal Handover – Call is locally switched

Connection Model 2: During BSS Internal Handover

Connection Model 3: After BSS Internal Handover – Call is not Locally Switched

Figure 8.4.5.6.1.1: BSS Internal Handover Connection Model that Breaks Local Switching

8.4.5.6.2 Basic Sequence for BSS Internal Handover that Breaks Local Switching

Figure 8.4.5.6.2.1 shows the message sequence example for the BSS Internal Handover that Breaks Local Switching.

Figure 8.4.5.6.2.1: BSS Internal Handover for AoIP that Breaks Local Switching

1 – 2. As for normal Internal Handover, see TS 23.205 [2] clause 8.4.5.

3. MSC-1 Server indicates preparation for disconnection due to handover through the Core Network.

3a. MSC-2 Server indicates to BSS-2 to start UL bicasting.

3b. The BSS-2 sends the LCLS_Connect_Control_Ack message with the LCLS-BSS-Status IE set to "the call is locally switched with requested LCLS configuration ".

3c. MSC-2 Server sends LCLS-Status-Change-Request-Acknowledgement.

4. The MSC-1 Server determines that local switching is permitted by the core network and sends the Internal Handover Command message, optionally including the LCLS-Connection-Control-Status message indicating "Connect".

4a BSS-1 may indicate Handover Detected.

5. BSS-1 sends the Handover Complete message to MSC-1 Server to indicate LCLS-BSS-Status is set to "Call is no longer locally switched".

6. MSC-1 Server propagates the change of the LCLS Status through the Core Network.

NOTE: If the Internal Handover did not in the end result in LCLS break then the MSC Server will send LCLS-Status-Update message indicating that the call is locally switched.

7. BSS-2 indicates that the call is no longer locally switched in the LCLS_NOTIFICATION message.

8. The old termination (T_S) is removed and the call is normally switched through the Core Network.

8.4.5.7 Example BSS Internal Handover that Does Not Modify LCLS Status of a Locally Switched Call

8.4.5.7.1 Connection Model

Figure 8.4.5.7.1.1 shows the network model for the Intra-MSC BSS Internal Handover, where the call leg pertinent to the UE-1 is handed over from the serving BSS-1 to BSS-2. BSS-1 is the same as BSS-2 for BSS Internal Handover. The bearer termination T₂ is used for the bearer towards BSS-2, which is not affected by this handover. Bearer termination T_S is used for the bearer towards BSS-1 and the bearer terminations T₁ and T_A are used for the bearer towards the succeeding/preceding MGW. Bearer termination T_T is for the bearer termination towards the BSS after internal handover. The colours and line types used in the figure are defined differently from 3GPP TS 23.205 [2] to indicate LCLS specific issues.

Connection Model 1: Before BSS Internal Handover – Call is locally switched

Connection Model 2: During BSS Internal Handover

Connection Model 3: After BSS Internal Handover – Call is Locally Switched

Figure 8.4.5.7.1.1: BSS Internal Handover Connection Model that Does Not Modify LCLS Status of a Locally Switched Call

8.4.5.7.2 Basic Sequence for BSS Internal Handover that Does Not Modify LCLS Status of a Locally Switched Call

Figure 8.4.5.7.2.1 shows the message sequence example for the BSS Internal Handover that Does Not Modify LCLS Status of a Locally Switched Call.

Figure 8.4.5.7.2.1: BSS Internal Handover for AoIP that Does Not Modify LCLS Status of a Locally Switched Call

1 – 2. As for normal Internal Handover, see TS 23.205 [2] clause 8.4.5.

3. MSC-1 Server indicates preparation for disconnection due to handover through the Core Network.

3a. MSC-2 Server indicates to BSS-2 to start UL bicasting.

3b. The BSS-2 sends the LCLS_Connect_Control_Ack message with the LCLS-BSS-Status IE set to "the call is locally switched with requested LCLS configuration".

3c. MSC-2 Server sends LCLS-Status-Change-Request-Acknowledgement.

4a BSS-1 may indicate Handover Detected.

5. BSS-1 sends the Handover Complete message to MSC-1 Server to indicate LCLS-BSS-Status is set to "the call is locally switched with requested LCLS configuration".

6. The old termination (T_S) is removed and the call is normally switched through the Core Network.

8.4.5.8 Example BSS Internal Handover that Does Not Modify LCLS Status of a non-Locally Switched Call

8.4.5.8.1 Connection Model

Figure 8.4.5.8.1.1 shows the network model for the Intra-MSC BSS Internal Handover, where the call leg pertinent to the UE-1 is handed over from the serving BSS-1 to BSS-2. BSS-1 is the same as BSS-2 for BSS Internal Handover. The bearer termination T₂ is used for the bearer towards BSS-2, which is not affected by this handover. Bearer termination T_S is used for the bearer towards BSS-1 and the bearer terminations T₁ and T_A are used for the bearer towards the succeeding/preceding MGW. Bearer termination T_T is for the bearer termination towards the BSS after internal handover. The colours and line types used in the figure are defined differently from 3GPP TS 23.205 [2] to indicate LCLS specific issues.

Connection Model 1: Before BSS Internal Handover – Call is not locally switched

Connection Model 2: During BSS Internal Handover

Connection Model 3: After BSS Internal Handover – Call is not Locally Switched

Figure 8.4.5.8.1.1: BSS Internal Handover Connection Model that Does Not Modify LCLS Status of a non-Locally Switched Call

8.4.5.8.2 Basic Sequence for BSS Internal Handover that Does Not Modify LCLS Status of a non-Locally Switched Call

Figure 8.4.5.8.2.1 shows the message sequence example for the BSS Internal Handover that Does Not Modify LCLS Status of a Locally Switched Call.

Figure 8.4.5.8.2.1: BSS Internal Handover for AoIP that Does Not Modify LCLS Status of a non-Locally Switched Call

1 – 2. As for normal Internal Handover, see TS 23.205 [2] clause 8.4.5.

3. The MSC-1 Server determines that local switching is permitted by the core network and sends the Internal Handover Command, optionally including the LCLS-Connection-Control-Status message indicating "Connect".

3a BSS-1 may indicate Handover Detected.

4. BSS-1 sends the Handover Complete message to MSC-1 Server to indicate LCLS-BSS-Status is set to "Call is not locally switched".

5. The old termination (T_S) is removed and the call is normally switched through the Core Network.