8.4.1 Intra-MSC Inter-BSS GSM to GSM Handover

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

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

8.4.1.1 Intra-MSC Inter-BSS GSM to GSM Handover that breaks Local Switching

8.4.1.1.1 General

When a call is locally switched through the BSS and an intra-MSC inter-BSS GSM to GSM handover occurs then the LCLS shall be broken and the user plane shall be connected through the core network. The intra-MSC inter BSS GSM to GSM handover procedures specified in 3GPP TS 23.205 [2] and 3GPP TS 23.231 [3] shall be followed. The following clauses describe the additional requirements for intra-MSC inter BSS GSM to GSM handovers of LCLS related calls.

During a Locally Switched (intra-BSS) Connection when no bicasting occurs there is no data transmission through the core network. In this release the use plane is kept active and therefore does not need to be re-activated when the LCLS is broken due to inter-BSS handover out of LCLS.

8.4.1.1.2 Handover Required

When the MSC server receives the Handover Required message from the serving BSS, 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 shall use the Change Flow Direction procedure to request the MGW to set the Handover Device to the initial state.

8.4.1.1.3 MGW Flow Direction Control

The MSC Server may perform the MGW Flow Direction Control in the following ways:

– In accordance with the normal handover case by isolating the termination to the Target BSS (T_T) from the termination to the Serving BSS (T_S) and configuring the Anchor termination (T_A) one-way DL to the Target BSS (T_T). Termination to the Serving BSS (T_S) is bothway connected to Anchor termination (T_A) since it is also receiving UL user data from termination to the Serving BSS (T_S).

Or:

– The MSC server may request the MGW to set termination to Target BSS (T_T) to bothway connected to Anchor termination (T_A) and isolate termination to Serving BSS (T_S) completely. This improves the user plane switching and saves a signalling step to the MGW at Handover Detect message. 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".

8.4.1.1.4 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 anchor MSC Server that the BSS supports the LCLS feature, and therefore the MSC Server shall not act upon the status indicated, i.e. no signalling of LCLS-Status IE through the core network.

Upon receipt of the Handover Request Acknowledge message the MSC Server shall send to the adjacent call node the LCLS-Status-Change-Request message to indicate "LCLS Disconnection-Preparation-for handover".

When the far end MSC server receives the LCLS-Status-Change-Request message indicating LCLS Disconnection preparation-for-handover it shall send to the BSS the LCLS_Connect_Control message with the LCLS-Connection-Status-Control IE indicating "BicastatHandover". When the LCLS_Connect_Control acknowledge message is received from the BSS, the far end MSC server shall return the LCLS Status Change Request Acknowledge message indicating "LCLS Disconnection-Preparation-for-handover" and a Result code indicating LCLS Status Change Request accepted.

8.4.1.1.5 Handover Command/Handover Detect

When the MSC server sends the Handover Command message or alternatively if it receives the Handover Detect message, if the MSC server followed the MGW control procedures for a non-LCLS call and kept the Termination to the Serving BSS connected then it shall use the Change Flow Direction procedure to requests the MGW to set the Handover Device to intermediate state however if the MSC server isolated T_S and set T_T to bothway through-connected then no MGW control procedure is required at this point.

Upon receipt of the Handover Detect message the MSC Server shall send to the adjacent call node the LCLS-Status-Change-Request message with the LCLS-Status-Change-Request IE set to "Indicate DL data after Handover".

When the far end MSC server receives the LCLS-Status-Change-Request message with the LCLS-Status-Change-Request IE set to "Indicate DL data after Handover" it shall send to the BSS the LCLS_Connect_Control message with the LCLS-Connection-Status-Control IE indicating "DL Data_at_Handover" and after reception of the LCLS_Connect_Control acknowledge message from the BSS, return the LCLS-Status-Change-Request-Acknowledge message with "Indicate DL data after Handover" and a Result code indicating LCLS Status Change Request accepted.

NOTE: this informs this call leg in the BSS that the other UE has left the BSS and has been detected in the new BSS and as such DL data from the CN is now coming from that UE.

8.4.1.1.6 Handover Complete

When the MSC server receives the Handover Complete message, it releases the A-interface line towards the serving BSS. The MSC server also requests the MGW to set the Handover Device to its final state by removing the bearer termination (T_S) towards the serving BSS.

The MSC server shall send to the adjacent call node the LCLS-Status-Update message with the LCLS-Status IE indicating the LCLS disconnection.

When the serving BSS receives Clear Command it shall release any local switch path. The serving BSS shall inform the far end MSC server that LCLS is broken with the LCLS_Notification message.

NOTE: The LCLS_Notification message does not need to be sent to the Anchor MSC Server since the Clear Command message received from the serving BSS also means LCLS is disconnected.

8.4.1.1.7 Example

8.4.1.1.7.1 Connection Model

Figure 8.4.1.1.7.1.1 shows the network model for the Intra-MSC Inter-BSS GSM to GSM Handover, where the call leg pertinent to the UE-1 is handed over from the serving BSS-1 to the Target BSS. BSS-1 is the same as BSS-2 when LCLS is established for the call. 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 Target BSS. 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: The call is locally switched

Connection Model 2: MGW-1 is both-way connected between Ta and Tt, Ts is isolated; BSS-2 starts to bicast data UL

Connection Model 3: UE has moved to Target BSS but HO Detect has not yet been received by MSC-S-1

Connection Model 4: LCLS is released in BSS-2, old serving Termination Ts is removed

Figure 8.4.1.1.7.1.1: Intra-MSC Inter-BSS Handover Connection Model that breaks LCLS

8.4.1.1.7.2 Basic Sequence for Inter-BSS Handover that breaks Local Switching

Figure 8.4.1.1.7.2.1: Intra-MSC Inter-BSS Handover that breaks Local Switching

1. The Handover Required message is received from the BSS-1 requesting an inter-BSS handover. The call is currently locally switched so the MSC-1 server can know that the inter-BSS handover at one end will break local switch (the local switch is not broken in the serving BSS (BSS-1) until the UE-1 has moved out of the BSS-1 and the MSC-1 server sends the Clear Command message).

2. In this example the Anchor MSC-1 server requests from its MGW-1 the seizure of the bearer termination Tt towards the Target BSS and through-connects it bothway to Ta. Additionally it isolates the old serving Termination Ts. This makes the handover much more efficient than even current non-LCLS handover as immediately the UE-1 moves into the new target BSS it will be able to send UL user data to the UE-2.

NOTE 1: This flow shows the termination to the Target BSS as always connected bothway. This is a change to the existing call handling which would normally connect the termination as one-way and then change to bothway after receiving the Handover Detect message. However the termination does not need to be connected one-way and will in fact make the break in speech worse since UL data cannot be sent from the UE-1 until the MGW topology is modified, also it saves the additional intermediate H.248 modification step.

3. The Anchor 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. The Target BSS returns the Handover Request Acknowledge message and also indicates that call is not possible to be locally switched.

5. The Anchor MSC-1 server sends the change in LCLS to the succeeding MSC server and the Anchor MSC-1 server asks it to prepare for the LCLS disconnection for Handover to trigger sending of the LCLS-Connect-Control message at the far end MSC-2 server.

NOTE 2: 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.

5a. The far end MSC-2 server requests the BSS-2 to start sending data UL with the LCLS_Connect_Control message and the LCLS-Connection-Status-Control IE indicating "BicastatHandover", see Figure 8.4.1.7.1.1 Connection Model 2. This triggers the BSS-2 to bicast the user plane data in the same way as the Access MGW-1 would be doing in a non-LCLS inter-BSS handover. At this point the BSS-2 shall send any DL data it receives directly to the served UE. Since the BSS-2 cannot receive DL data at the same time as it receives local data (Ts is isolated) this will minimise the break in user plane data even more than for existing non-LCLS handover.

NOTE 3: The Serving BSS-1 shall forward the user plane data from the UE-1 to the UE-2 while the UE-1 is served by the BSS-1. The UL user plane data from UE-2 are bi-cast to both MGW2 and local path by the BSS-2. The MGW-2 transmits the user plane data to the MGW-1, and the MGW-1 will transmit the user plane data to the target BSS. When the UE-1 leaves the serving BSS-1 and begins sending UL data from the Target BSS, that data will then be received via the A-interface leg at the serving BSS-2.

NOTE 4: Possible bicasting may have been activated earlier when LCLS was established in the BSS-1 /BSS-2 (not shown in the figure 8.4.1.8.2.1) and was indicated with the LCLS-Configuration IE in step 3 and applies to both call legs. If LCLS bicasting was not activated the LCLS-Configuration value is "Connect" (i.e. no bicasting) in step 3, but the value of the LCLS-Connection-Status-Control in step 5 is "BicastatHandover", which applies only for this call leg.

5b. 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".

6. The Anchor MSC-1 server triggers the Handover Command message. When the UE-1 moves to the Target BSS in this example it can immediately send UL data through the CN to the UE-2 and also can receive DL data from the UE-2 via the CN since the MGW-1 topology for Ta, Tt is already bothway connected. This is a change from the current non-LCLS solution but is more efficient since the non-LCLS solution needs to set this to one-way DL only until it receives Handover Detect message.

7. MSC-2 Server sends LCLS-Status-Change-Request-Acknowledgement.

8. The UE-1 is detected at the target BSS. The BSS-1/BSS-2 may continue to send the user plane data locally until the Clear Command message is received.

8a. The MSC-1 Server sends LCLS-Status-Change-Request to indicate that UE-1 has been detected in the target BSS and user data is now being sent through the CN and DL to the distant UE-2.

8b. The MSC-2 Server signals to the BSS-2 that DL data received from the CN is now real user data coming from the UE-1.

8c. 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".

8d. Acknowledgement back through the CN that the indication for DL data after Handover Detect has been delivered.

9. In the Handover Complete message the Target-BSS indicates to the MSC-1 server in the LCLS-BSS-Status IE that the call is not possible to be locally switched.

10. The MSC-1 server requests the old serving BSS-1 to clear the old call leg. The BSS-1 now stops sending local the user data from UE-1, LCLS is finally broken.

11. The Serving BSS-2 informs the MSC-2 server that LCLS is broken via LCLS_Notification message.

12. Clearing of the old call leg to the Serving BSS-1 is completed.

13. The termination Ts to the old serving BSS-1 is removed from the Access MGW-1.

14. The Anchor MSC-1 server informs succeeding CN nodes that LCLS is finally disconnected.

NOTE 5: 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.

LCLS is impossible after an Inter-BSS handover which makes the call not local (as described above). While a handover is being performed for one call leg, it is possible that a handover also is started for the other call leg, possibly moving both call legs to the same target BSS, thereby creating a local call. The target BSS shall only establish LCLS for a local call when both call legs are connected and e.g. any handover process has been successfully completed on both call legs.

8.4.1.2 Intra-MSC Inter-BSS GSM to GSM Handovers that establishes Local Switching

8.4.1.2.1 General

When LCLS is not established for a call and an intra-MSC inter-BSS GSM to GSM handover occurs that makes the call local, the call should be locally switched in the BSS. The Intra-MSC inter-BSS GSM to GSM handover procedures specified in 3GPP TS 23.009 [9], 3GPP TS 23.205 [2] and 3GPP TS 23.231 [3] shall be followed. The following clauses describe the additional requirements for intra-MSC handovers that establish LCLS.

8.4.1.2.2 Handover Required

8.4.1.2.3 Bearer establishment towards Target BSS

When the 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 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.

8.4.1.2.4 MGW Flow Direction Control

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

8.4.1.2.5 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 anchor MSC Server that the BSS supports the LCLS feature.

The anchor MSC Server shall not act upon the status indicated, i.e. no signalling of LCLS-Status IE through the core network.

8.4.1.2.6 Handover Command/Handover Detect

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

8.4.1.2.7 Handover Complete

When the MSC-Server receives the Handover Complete message, it releases the A-interface line towards the serving BSS. The MSC-Server also requests the MGW to set the Handover Device to its final state by removing the bearer termination towards the serving BSS.

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

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

8.4.1.2.8 Example

8.4.1.2.8.1 Connection Model

Figure 8.4.1.2.8.1.1 shows the network model for the Intra-MSC Inter-BSS GSM to GSM Handover, where the call leg pertinent to the UE-1 is handed over from the serving BSS-1 to the Target BSS. Target BSS is the same as BSS-2 when LCLS is established for the call. 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 Target BSS. 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 handover

Connection Model 2: During handover, T_S and T_A are both-way connected, T_T is isolated from T_S

Connection Model 3: UE has moved to Target BSS but HO Detect has not yet been received by MSC-1-S

Connection Model 4: The call is locally switched

Figure 8.4.1.2.8.1.1: Connection Models for Inter-BSS Handover that establishes Local Switching

8.4.1.2.8.2 Basic Sequence for Inter-BSS Handover that establishes Local Switching

Figures 8.4.1.2.8.2.1 and 8.4.1.2.8.2.2 show the message sequence example for the Basic Intra-MSC GSM to GSM Handover shown in the corresponding network model Figure 8.4.1.2.8.1.1. 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 8.4.1.2.8.2.1: Inter-BSS Handover that establishes Local Switching

1. Handover Required message is received from BSS-1 requesting an inter-MSC handover. The call is currently not locally switched.

2. MSC-1 Server determines that an intra-MSC handover is required and checks that LCLS negotiation in the core network permitted LCLS. The MSC-1 Server reserves a new Termination for Target BSS and configures this as one-way connected to the Anchor Termination (as per existing handover procedures).

3. MSC-1 Server sends Handover Request message to target BSS with GCR and instructs the BSS to prepare to connect LCLS. The LCLS-Configuration IE can instruct the BSS to bi-cast user plane data, if applicable.

4. Target BSS performs call leg correlation with GCR to find if another call leg is active with the same GCR. The 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".

5a, b. (These signalling steps are only applicable to AoIP.) MSC-1 Server sends the IP address and UDP Port number of the Target BSS to MGW-1 using the Configure RTP Connection Point procedure.

6. MSC-1 Server sends the Handover Command message.

7. UE-1 gets connected to the Target BSS, which sends Handover Detect.

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

9. Target BSS indicates in the Handover Complete message that the call is locally switched.

10. 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".

11. MSC-1 Server requests the old serving BSS-1 to clear the old call leg.

12. Clearing of the old call leg to the Serving BSS-1 is completed.

13. The termination Ts to the old serving BSS-1 is removed from MGW-1.

14. 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.

LCLS becomes possible after an Inter-BSS handover which makes the call local (as described above). While a handover is being performed for one call leg, it is possible that a handover also is started for the other call leg, possibly moving that call leg to another BSS and in that case the call does not become local. The target BSS shall only establish LCLS for a local call when both call legs are connected and e.g. any handover process has been successfully completed on both call legs.