Content for TS 23.284 Word version: 18.0.0

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8.4.5 BSS Internal Handover 8.4.5.1 General 8.4.5.2 Internal Handover Required 8.4.5.3 Internal Handover Command 8.4.5.4 Handover Complete 8.4.5.5 Example BSS Internal Handover that Establishes Local Switching
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8.4.5 BSS Internal Handover p. 118

8.4.5.1 General p. 118

The following procedures describe the specific handling compared to the basic principles described in clause 8.4.5 of TS 23.205 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.

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 p. 118

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 p. 118

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 TS 48.008.

8.4.5.4 Handover Complete p. 119

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 p. 119

8.4.5.5.1 Connection Model p. 119

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 TS is used for the bearer towards BSS-1 and the bearer terminations T1 and TA are used for the bearer towards the succeeding/preceding MGW. Bearer termination TT is for the bearer termination towards the BSS after internal handover. The colours and line types used in the figure are defined differently from TS 23.205 to indicate LCLS specific issues.

Copy of original 3GPP image for 3GPP TS 23.284, Fig. 8.4.5.5.1.1: BSS Internal Handover Connection Model that Establishes Local Switching

Figure 8.4.5.5.1.1: BSS Internal Handover Connection Model that Establishes Local Switching
(⇒ copy of original 3GPP image)

8.4.5.5.2 Basic Sequence for BSS Internal Handover that Establishes Local Switching p. 120

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 (TT) using the Reserve and Configure RTP Connection Point procedure with specific flow directions.

Copy of original 3GPP image for 3GPP TS 23.284, Fig. 8.4.5.5.2.1: BSS Internal Handover for AoIP that Establishes Local Switching

Figure 8.4.5.5.2.1: BSS Internal Handover for AoIP that Establishes Local Switching
(⇒ copy of original 3GPP image)

Step 1 - 2.

As for normal Internal Handover, see clause 8.4.5 of TS 23.205.

Step 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.

Step 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".

Step 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".

Step 3c.

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

Step 4 - 5.

As for normal Internal Handover, see clause 8.4.5 of TS 23.205.

Step 6.

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

Step 7.

The LCLS Status is propagated through the Core Network.

Step 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".

Step 9.

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