01 14 Handover

14 HandoverAbout This Chapter

Handover is one of the basic functions of the GSM. It enables calls to be set up in better cellsand reduces call drops. Handover also lowers cross interference. The handover procedureconsists of handover triggering, handover preparation, handover decision, and handoverexecution.

14.1 Handover ProcedureThe handover procedure is categorized into the intra-BSC handover procedure, inter-BSChandover procedure, inter-MSC handover procedure, and inter-system handover procedure.14.2 Internal BSC Signaling Procedure of HandoverThe internal BSC signaling procedure of handover is categorized by intra-BSC handover, inter-BSC handover, inter-MSC handover, and inter-system handover.14.3 Abnormal Handover CasesThis topic describes the abnormal handover cases.

HUAWEI BSC6000 Base Station SubsystemBSS Signaling Analysis Guide 14 Handover

Issue 01 (2007-11-26) Huawei Technologies Proprietary 14-1

14.1 Handover ProcedureThe handover procedure is categorized into the intra-BSC handover procedure, inter-BSChandover procedure, inter-MSC handover procedure, and inter-system handover procedure.

14.1.1 Intra-BSC Handover ProcedureThis topic describes the intra-BSC handover procedure.14.1.2 Inter-BSC Handover ProcedureThis topic describes the inter-BSC handover procedure.14.1.3 Inter-MSC Handover ProcedureThis topic describes the inter-MSC handover procedure.14.1.4 Inter-System Handover ProcedureThis topic describes the inter-system handover procedure.

14.1.1 Intra-BSC Handover ProcedureThis topic describes the intra-BSC handover procedure.Figure 14-1 shows the intra-BSC handover procedure.

14 HandoverHUAWEI BSC6000 Base Station Subsystem

BSS Signaling Analysis Guide

14-2 Huawei Technologies Proprietary Issue 01 (2007-11-26)

Figure 14-1 Intra-BSC handover procedure

MS BTS2 BTS1BSC

Channel Activation(3)

EstablishIndication(10)

SACCH

MS MSC

MeasurementReport(1)Measurement

Result(2)

HandoverCommand(5)

Channel ActivationAcknowledge(4)

FACCHStart T3103

HandoverAccess(6)

FACCH

Start T3124HandoverAccess

FACCHHandover Detect(7)

PHY INFO(8)FACCH

Stop T3124

SABM(9)FACCH

UA(11)FACCH HandoverComplete(12)

FACCHStop T3103

HandoverPerfomed(13)

1. The MS sends BTS1 a measurement report on an SACCH on the Um interface.2. BTS 1 forwards the measurement report to the BSC.3. Based on the measurement report, the BSC determines whether to hand over the MS to

another cell. If a handover is required, the BSC sends BTS 2 a Channel Activation message.4. Upon receiving the Channel Activation message, if the channel type is correct, BTS 2 turns

on the power amplifier on the specified channel to receive information on the uplink, andsends a Channel Activation Acknowledge message to the BSC.

5. Upon receiving the Channel Activation Acknowledge message, the BSC sends BTS 1 aHandover Command message. Then BTS 1 forwards the command to the MS. The messageis sent on an FACCH on the Um interface. BTS 1 starts timer T3103. The HandoverCommand message has all the information to be transferred on the new channel and thedata required for accessing the MS. It also indicates whether the handover is synchronousor asynchronous.

6. Upon receiving the Handover Command message, the MS attempts the access and sendsBTS 2 a Handover Access message on the FACCH.



7. Upon receiving the Handover Access message, BTS 2 sends the BSC a Handover Detectmessage.

8. Whether a PHY INFO message is sent or not depends on the handover type.l Asynchronous handover

After the channels of BTS 2 are activated, BTS 2 waits for the access of the MS on adedicated channel until timer T3103 expires. Upon detecting the Handover Accessmessage from the MS, BTS 2 sends the BSC a Handover Detect message and at thesame time to the MS a PHY INFO message on an FACCH. The PHY INFO messagecarries the new TA calculated by BTS 2. Upon receiving the PHY INFO message, theMS sends BTS 2 an SABM frame.

l Synchronous handoverBTS2 does not send the PHY INFO message. Upon sending the Handover Accessmessage, the MS sends BTS 2 the SABM frame.

9. When the PHY INFO message is sent depends on the handover type.l Asynchronous handover

The MS starts timer T3124 upon sending the Handover Access message for the firsttime. It stops timer T3124 upon receiving the PHY INFO message, and sends BTS 2an SABM frame on an FACCH.

l Synchronous handoverUpon sending the Handover Access message, the MS sends BTS 2 an SABM frame.

10. Upon receiving the first SABM frame, BTS 2 sends an Establishment Indication messageto notify the BSC of radio link establishment.

11. At the same time, BTS2 sends a UA frame on an FACCH to notify the MS of radio linkestablishment.

12. The MS sends BTS 2 a Handover Complete message on the FACCH. BTS 2 forwards themessage to notify the BSC of the handover success. The Handover Complete messagecarries only the handover complete indication. The MS does not return to the old channelafter sending this message. If the MS does not receive the PHY INFO message or the UAframe from the target cell, it sends a Handover Failure message on the old channel.

13. Upon receiving the Handover Complete message, the BSC stops timer T3103 and sends aHandover Performed message to notify the MSC of the handover completion. TheHandover Performed message carries the handover type. At the same time, the BSC sendsan RF Channel Release message to notify BTS 1 of channel release. The target cell forwardsto the BSC the Handover Complete message it receives from the MS. Upon receiving theHandover Complete message, the BSC sends an RF Channel Release message to notifyBTS 1 of channel release. Upon receiving the RF Channel Release message, BTS 1 sendsan RF Channel Release Acknowledge message. The message indicates that the radiochannel is released and ready for reallocation.

14.1.2 Inter-BSC Handover ProcedureThis topic describes the inter-BSC handover procedure.Figure 14-2 shows the inter-BSC handover procedure.




Figure 14-2 Inter-BSC handover procedureMS BTS1 MSCBSC1 BSC2 BTS2

MeasurementReport from MS(1) HandoverRequired(2)SACCH

FACCH

MS

HandoverRequest(3) ChannelActivation(4)

Start T7 Channel ActivationAcknowledge(5)Handover RequestAcknowledge(6)Handover

Command(7)

Stop T7Handover

Command(8)

Start T8

HandoverCommand(9)

HandoverAccess(9)

StartT3124

HandoverAccess(10)

FACCHHandoverDetect(11)Handover

Detect(12)PHY

INFO(13)FACCH

SABM(14)FACCH

StopT3124


UA(16)FACCH

FACCH

HandoverComplete(17)HandoverComplete(18)HandoverComplete(19)Clear

Command(20)

Stop T8Clear

Complete(21)

The inter-BSC handover procedure involves more A-interface signaling procedures than theintra-BSC handover procedure. These signaling procedures are described subsequently. Fordetails about other signaling messages, refer to 14.1.2 Inter-BSC Handover Procedure.1. When an MS needs to be handed over to a cell controlled by BSC 2, BSC 1 sends the MSC

a Handover Required message and starts timer T7. This message carries the CELL IDs ofthe target cell group and the serving cell, and the handover cause.

2. Upon receiving the Handover Required message, if the MSC finds that the location area ofthe target cell is under itself, it sends BSC 2 a Handover Request message. The HandoverRequest message carries the information, transmission mode, MS classmark, and requiredchannel classmark of the serving and the target cells. Upon receiving the Handover Requestmessage, BSC 2 sends the MSC an SCCP Connection Confirm message, indicating that its



connection to the MSC is established. The A interface messages are sent through this SCCPconnection.

3. BSC 2 activates a new channel and sends a Handover Request Acknowledge message tonotify the MSC that the channel is now available. The Handover Request Acknowledgemessage carries the information of the allocated handover resources and the handovercommand indicating that the BSC is ready.

4. Upon receiving the Handover Request Acknowledge message, the MSC sends BSC 1 aHandover Command message. Then BSC 1 stops timer T7. BSC 1 sends the MS a HandoverCommand message and starts timer T8, requesting the MS to access the new channel. TheHandover Command message carries the cell ID, channel type, and handover reference.

5. Upon receiving the Handover Complete message from BSC 2, the MSC sends BSC 1 aClear Command message. The Clear Command message carries the clearance causes. BSC1 stops timer T8, initiates a channel release procedure, and sends the MSC a Clear Completemessage.

NOTEIf the serving cell and the target cell are in different location areas, the MS initiates a location updateprocedure after a call is complete.

14.1.3 Inter-MSC Handover ProcedureThis topic describes the inter-MSC handover procedure.Figure 14-3 shows the inter-MSC handover procedure.




Figure 14-3 Inter-MSC handover procedureMS BTS1 MSC1BSC1 MSC2 BSC2

MeasurementReport

from MS(1) HandoverRequired(2)SACCH

FACCH

BTS2

PerformHandover(3) Channel

Activation(5)Channel Activation

Acknowledge(6)Handover RequestAcknowledge(7)

HandoverCommand(12)

HandoverAccess(13)

FACCHHandoverDetect(14)Handover

Detect(15) PHYINFO(16)

FACCH

SABM(18)FACCH


UA(20)FACCH

FACCH

HandoverComplete(21)Handover

Complete(22)HandoverComplete(23)Send

End Signal(24)

ClearComplete(26)

MS

HandoverRequest(4)

RadioChannel Ack(8)

IAM(9)ACM(10)HandoverCommand(11)

PHYINFO(17)

FACCH

ClearCommand(25)

For the inter-MSC handover procedure, refer to 14.1.1 Intra-BSC Handover Procedure and14.1.2 Inter-BSC Handover Procedure.The messages on the E interface are as follows:1. Perform Handover: MAP layer message. It contains the CGI of the serving cell and the

target cell and the required channel type. It informs MSC2 that a handover will be initiated.2. Radio Channel Acknowledge: MAP layer message. It carries the information of the new

channel in the target cell and the handover number.3. IAM: initial address message. It is a TUP/ISUP message.4. ACM: address complete message. It is a TUP/ISUP message.5. Send End Signal: MAP layer message.

14.1.4 Inter-System Handover ProcedureThis topic describes the inter-system handover procedure.



Figure 14-4 shows the inter-system handover procedure.

Figure 14-4 GSM-to-UMTS handover procedure

MS BSS RNCMSC

Handover Required(1)

FACCH

MS

Inter System to UTRANHandover Command(2)

Start T3121 Handover toUTRAN Complete(4)

HandoverCommand(3)

HandoverComplete(5)

Stop T3121

Clear Command(6)

Clear Complete(7)

FACCH

Figure 14-5 UMTS-to-GSM handover procedure

MS BSS RNCMSC MS

HandoverCommand(4)

FACCH

HandoverRequest(1)

FACCHHandoverAccess(7)

FACCH

Handover Detect(6)

Handover RequestAcknowledge(2) Handover Request

Acbowledge(3)HandoverAccess(5)

HandoverComplete(8) Handover

Complete(9)

The GSM-to-UMTS handover procedure is as follows:1. The BSS sends a Handover Required message to the MSC.2. Upon receiving the Handover Required message, the MSC sends the BSS an Inter System

To UTRAN Handover Command message and starts timer T3121.3. Upon receiving the Inter System To UTRAN Handover Command message, the BSS sends

the MS a Handover Command message on an FACCH.4. After the connection is successfully established, the MS sends the RNC a Handover to

UTRAN Complete message on a UTRAN channel.




5. The RNC forwards the Handover Complete message to the MSC.6. Upon receiving the Handover Complete message, the MSC stops timer T3121 and initiates

a channel release procedure.NOTE

For the UMTS-to-GSM handover procedure, refer to the GSM-to-UMTS handover procedure mentionedearlier.

14.2 Internal BSC Signaling Procedure of HandoverThe internal BSC signaling procedure of handover is categorized by intra-BSC handover, inter-BSC handover, inter-MSC handover, and inter-system handover.

14.2.1 Internal BSC Signaling Procedure of the Intra-BSC HandoverThis topic describes the internal BSC signaling procedure of the intra-BSC handover.14.2.2 Internal BSC Signaling Procedure of the Inter-BSC HandoverThis topic describes the internal BSC signaling procedure of the inter-BSC handover.14.2.3 Internal BSC Signaling Procedure of the Inter-MSC HandoverThis topic describes the internal BSC signaling procedure of the inter-MSC handover.14.2.4 Internal BSC Signaling Procedure of the Inter-System HandoverThis describes the internal BSC signaling procedure of the inter-system handover.

14.2.1 Internal BSC Signaling Procedure of the Intra-BSC HandoverThis topic describes the internal BSC signaling procedure of the intra-BSC handover.1. If the BSC considers that the call meets the handover conditions, it selects a candidate cell

from the candidate cell list based on the priority in a descending order. If the candidate cellis internal BSC, the BSC notifies the candidate cell of initiating an intra-BSC handoverprocedure, and the candidate cell will be the target cell.

2. The target cell allocates a channel and notifies BTS 2 of channel activation.3. After BTS 2 activates the channel, BTS 1 sends the MS a Handover Command message.

Then BSC 2 starts timer T3103 to wait for the Handover Detect message.4. After receiving the Handover Detect message, BTS 2 continues to wait for the

Establishment Indication message and the Handover Complete message.5. After BTS 2 receives the Handover Complete message, BSC 2 stops timer T3103 and

considers that the call is set up in the target cell.6. BSC 1 notifies BTS 1 of initiating a local release procedure. At the same time, it sends a

Handover Performed message to notify the MSC of the handover completion.

14.2.2 Internal BSC Signaling Procedure of the Inter-BSC HandoverThis topic describes the internal BSC signaling procedure of the inter-BSC handover.1. If the BSC considers that the call meets the handover conditions, it selects a candidate cell

from the candidate cell list based on the priority in a descending order. If the candidate cellis an external cell, the inter-BSC handover procedure is directly triggered. For the servingcell, the external cell is the target cell, and the BSC where the target cell is located is thetarget BSC, namely BSC 2.



2. Upon handover initiation, BTS 1 sends the MSC a Handover Required message and startstimer T17 to wait for a Handover Command message.

3. Upon receiving the Handover Ruest message from the MSC, BSC 2 allocates and activateschannels according to the target cell ID in the message. After channel activation, BTS 2sends the MSC a Handover Request Acknowledge message that carries the HandoverCommand message.

4. BTS 1 forwards to the MS the Handover Command message that BSC 1 receives and atthe same time starts timer T8 to wait for the handover completion. If the Clear Commandmessage with the cause value Handover Success is received before T8 expires, BSC 1considers that the handover is successful. If the MS returns to the old channel before timerT8 expiry, BSC 1 stops timer T8 and sends the MSC a Handover Failure message. If timerT8 expires, BSC 1 considers that the call drops and sends the MSC a Clear Request message.

5. BTS 2 forwards the Handover Detect message to the MSC and continues to wait for theEstablishment Indication message and the Handover Complete message.

6. Upon receiving the Handover Complete message, BTS 2 sends a Handover Completemessage to notify the MSC of handover completion.

7. Upon receiving the Clear Command message, BTS 1 releases the radio resources the calloccupies and returns the MSC a Clear Complete message.

14.2.3 Internal BSC Signaling Procedure of the Inter-MSCHandover

This topic describes the internal BSC signaling procedure of the inter-MSC handover.The internal BSC signaling procedure of the inter-MSC handover is the same as that describedin 14.2.2 Internal BSC Signaling Procedure of the Inter-BSC Handover.

14.2.4 Internal BSC Signaling Procedure of the Inter-SystemHandover

This describes the internal BSC signaling procedure of the inter-system handover.Compared with the internal system handover, the BSC in the inter-system handover procedureperforms more actions as described subsequently:l Obtaining the UTRAN neighbor cell information and the UTRAN system parameters from

the Idle Mode, Handover Data, and Other Attributes in the Cell Attributes window.l Recording the capability of the MSThe internal BSC signaling procedure of the inter-system handover is the same as that describedin 14.2.2 Internal BSC Signaling Procedure of the Inter-BSC Handover.

14.3 Abnormal Handover CasesThis topic describes the abnormal handover cases.

14.3.1 No Available Channels in the Target CellThis describes the fault, probable causes, and handling suggestions.14.3.2 Old BSS Not Receiving a Specified Message upon T8 ExpiryThis describes the fault, probable causes, and handling suggestions.




14.3.3 Inter-Cell Handover Failure (MS Returning to the Old Channel)This topic describes the fault, probable causes, and handling suggestions.14.3.4 BSC Receiving a Connection Failure Indication MessageThis topic describes the fault, probable causes, and handling suggestions.14.3.5 Forced Handover FailureThis topic describes the fault, probable causes, and handling suggestions.14.3.6 Handover Failure Caused by CIC MalfunctionThis topic describes the fault, probable causes, and handling suggestions.14.3.7 Handover Failure Caused by MS Access FailureThis topic describes the fault, probable causes, and handling suggestions.

14.3.1 No Available Channels in the Target CellThis describes the fault, probable causes, and handling suggestions.

DescriptionNo channels are available in the target cell.

AnalysisThe probable causes are as follows:l During an inter-cell handover, no TCHs are available in the target cell.l During a cross-cell handover, no SDCCHs are available in the target cell.l During an intra-cell handover due to bad quality, no TCHs are available in the cell.

Handling SuggestionsThe handling suggestions are as follows:l Check whether the target cell is congested. If yes, add TRXs to the target cell.l On the Cell Attributes tab page, click Channel Management. In the displayed dialog box,

click Advanced. On the Huawei Channel Allocation Algorithm II and Radio ChannelManagement and Control tab page, check whether TCH Traffic Busy Threshold (%)is set properly.

l On the Cell Attributes tab page, click Channel Management. In the displayed dialog box,check whether Enhanced TCH Adjust Allowed is set properly.

14.3.2 Old BSS Not Receiving a Specified Message upon T8 ExpiryThis describes the fault, probable causes, and handling suggestions.

DescriptionThe old BSS does not receive a Clear Command message from the MSC or a Handover Failuremessage from the MS upon T8 expiry.



AnalysisIf the old BSS does not receive a Clear Command message from the MSC or a Handover Failuremessage from the MS upon T8 expiry, it will release the dedicated channels and send the MSCa Clear Request message with the cause value Radio Interface Message Failure. Before a ClearCommand message is received, the terrestrial resources on the old BSS side remain allocated.On receiving the Clear Command message, the old BSS changes the state of the terrestrialresources to idle and sends a Clear Complete message to the MSC. Then the MSC releases theSCCP connection to the old BSS and thus terminates the connection with the BSS.

Handling SuggestionsOn the Cell Attributes tab page, click Other Attributes. Then click Advanced. On the CellTimer tab page, check whether T8 is set properly.

14.3.3 Inter-Cell Handover Failure (MS Returning to the OldChannel)

This topic describes the fault, probable causes, and handling suggestions.

FaultThe MS returns to the old channel after inter-cell handover failure.

Probable CausesThe probable causes are that the MS cannot access the old channel due to bad transmissionquality on the Um interface, low level, or incorrect data configuration of the external cells.

Handling SuggestionsCheck the transmission quality on the Um interface.

14.3.4 BSC Receiving a Connection Failure Indication MessageThis topic describes the fault, probable causes, and handling suggestions.

FaultCalls drop.

Probable CausesThe probable causes are as follows:l Radio link failurel Hardware faults

Handling Suggestionsl Check the resources on the Um interface.l Check the antenna system and the power amplifier.




14.3.5 Forced Handover FailureThis topic describes the fault, probable causes, and handling suggestions.

FaultForced handover fails.

Probable Causesl No neighbor celll Incorrect configuration of the neighbor celll No resources in the neighbor celll Bad radio conditions in the neighbor cell

Handling SuggestionsCheck the data configuration and the radio conditions of the neighbor cell, for example, the level.

14.3.6 Handover Failure Caused by CIC MalfunctionThis topic describes the fault, probable causes, and handling suggestions.

FaultHandover fails. The cause value of the Handover Failure message is Requested TerrestrialResource Unavailable.

Probable CausesIf the CIC allocated in the Handover Request message received by the target BSC is marked asBLOCK, the target BSC will respond to the MSC with a Handover Failure message with thecause value Requested Terrestrial Resource Unavailable.

Handling SuggestionsCheck the state of the CIC.

14.3.7 Handover Failure Caused by MS Access FailureThis topic describes the fault, probable causes, and handling suggestions.

FaultThe MS cannot be handed over.

Probable CausesRefer to the handling suggestions.



Handling Suggestionsl Check whether the handover trigger conditions are met.l Check whether there is a neighbor cell list in the measurement report.l Check whether the neighbor cell has available channels.l Check whether the data of the neighbor cell is incorrect, for example, whether there is

inconsistency in CGI.




14 Handover14.1 Handover Procedure14.1.1 Intra-BSC Handover Procedure14.1.2 Inter-BSC Handover Procedure14.1.3 Inter-MSC Handover Procedure14.1.4 Inter-System Handover Procedure

14.2 Internal BSC Signaling Procedure of Handover14.2.1 Internal BSC Signaling Procedure of the Intra-BSC Handover14.2.2 Internal BSC Signaling Procedure of the Inter-BSC Handover14.2.3 Internal BSC Signaling Procedure of the Inter-MSC Handover14.2.4 Internal BSC Signaling Procedure of the Inter-System Handover

14.3 Abnormal Handover Cases14.3.1 No Available Channels in the Target Cell14.3.2 Old BSS Not Receiving a Specified Message upon T8 Expiry14.3.3 Inter-Cell Handover Failure (MS Returning to the Old Channel)14.3.4 BSC Receiving a Connection Failure Indication Message14.3.5 Forced Handover Failure14.3.6 Handover Failure Caused by CIC Malfunction14.3.7 Handover Failure Caused by MS Access Failure

01 14 Handover

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