01-09 Handover Procedure

M900/M1800 Base Station Subsystem Signaling Analysis Manual Contents

Issue 01 (2007-03-15) Huawei Technologies Proprietary i

Contents

9 Handover Procedure..................................................................................................................9-1 9.1 Overview...........................................................................................................................................................9-2 9.2 Normal Procedure.............................................................................................................................................9-2

9.2.1 Intra-BSC Handover Procedure..............................................................................................................9-2 9.2.2 Inter-BSC Handover Procedure..............................................................................................................9-4 9.2.3 Inter-MSC Handover Procedure .............................................................................................................9-5 9.2.4 Inter-system Handover............................................................................................................................9-6

9.3 Abnormal Procedure.........................................................................................................................................9-7

Figures M900/M1800 Base Station Subsystem

Signaling Analysis Manual

ii Huawei Technologies Proprietary Issue 01 (2007-03-15)

Figures

Figure 9-1 Intra-BSC handover procedure............................................................................................................9-2

Figure 9-2 Normal inter-BSC handover procedure...............................................................................................9-4

Figure 9-3 Normal inter-MSC handover procedure..............................................................................................9-5

Figure 9-4 Normal inter-system handover procedure ...........................................................................................9-6

M900/M1800 Base Station Subsystem Signaling Analysis Manual 9 Handover Procedure

Issue 01 (2007-03-15) Huawei Technologies Proprietary 9-1

9 Handover Procedure About This Chapter

The following table lists the contents of this chapter.

Title Description

9.1 Overview Introduces the classification of handover procedures.

9.2 Normal Procedure Describes normal handover procedures, which consists of intra-BSC handover procedure, inter-BSC handover procedure, inter-MSC handover procedure, and inter-RAT handover procedure.

9.3 Abnormal Procedure Describes abnormal handover procedures and offers guidance to troubleshooting.

9 Handover Procedure M900/M1800 Base Station Subsystem


9-2 Huawei Technologies Proprietary Issue 01 (2007-03-15)

9.1 Overview Handover procedure includes intra-BSC handover procedure, inter-BSC handover procedure and inter-MSC handover procedure.

9.2 Normal Procedure A handover procedure can be divided into 3 types according to different ranges involved in handover: intra-BSC handover procedure, inter-BSC handover procedure and inter-MSC handover procedure.

9.2.1 Intra-BSC Handover Procedure

Figure 9-1 Intra-BSC handover procedure

BTS2 BSC MSC MS

Measurement Reports From MS (1)

(12)

MS

Handover CMD

Handover AccessHandover Detect)

Handover Complete

Channel ACT (2)

Channel ACT ACK (3)

(4)

(5)(6)

SABM (8)EST IND (9)

PHY INFO (7)

BTS1

Handover Performed

(10)UA

(11)

Step 1 MS sends Measurement Report to BTS1 on SACCH of the Um interface, and BTS1 will transfer the message to BSC.

Step 2 BSC receives the Measurement Report. If it judges that the MS should be handed over to another cell, it will send Channel Activation to BTS2 of the target cell to activate the channel.

Step 3 BTS2 receives the Channel Activation. If the channel type is correct, it will turn on the power amplifier on the specified channel to receive information in the uplink direction, and send Channel Activation Acknowledge to BSC.

Step 4 BSC receives the Channel Activation Acknowledge from BTS2, and sends Handover CMD to BTS1, which will transfer the command to MS. The message is sent on FACCH on the Um interface.



Step 5 MS receives the Handover CMD, and sends Handover Access on FACCH to BTS2 for access attempt.

Step 6 BTS2 receives the Handover Access from MS, and sends Handover Detect to BSC notifying that the handover access message bas been received.

Step 7 In case of asynchronous handover, i.e., when BTS1 and BTS2 are located in different BTSs, BTS2 will send PHY INFO on FACCH to MS while sending Handover Detect to BSC. The PHY INFO includes such contents as the synchronous information for correct access of MS. In case of synchronous handover, i.e., when BTS1 and BTS2 are located in the same BTS, the PHY INFO message will not be delivered.

Step 8 As for the asynchronous handover, MS receives the PHY INFO, and sends SABM on FACCH to BTS2. While for the synchronous handover, MS will send SABM soon after sending Handover Access.

Step 9 BTS2 receives the first SABM, and sends EST IND to BSC, notifying it that the radio link has been established.

Step 10 At the same time, BTS2 sends UA frames on FACCH to MS, notifying that the radio link layer has been established.

Step 11 Then, MS sends Handover Complete on FACCH to BTS2, which transfers the command to BSC notifying handover completion.

Step 12 BSC sends Handover Performed to MSC, notifying that the handover has been completed. At the same time, BSC initiates a local release procedure to BTS1 to release the old channel occupied.

----End




9.2.2 Inter-BSC Handover Procedure

Figure 9-2 Normal inter-BSC handover procedure

(4)Channel ACT

Channel ACT ACK

BTS1 BSC1MS

SABM

Establish IND

Handover AccessHandover Detect

PHY INFO

PHY INFO

MSC BTS2 MS

Measure Report from MS

HANDOVER COMMAND

Handover REQ

Handover REQ ACKHandover CMD

Handover Detect

Handover CMPClear CMD

Clear CMP

(3)(2)Handover RQD

(1)

(12)

BSC2

(5)

(7)(8)

(9)(10)

(11)

(6)

(13)

(14)

(15)

HANDOVER CMP (17)(18)(19)

(20)

(16)UA

Compared with intra-BSC handover procedure, there are A interface signaling added to the inter-BSC handover procedure. These A interface signaling will be described here. As for the descriptions of other signaling, please see 9.2.1 BSC Handover Procedure.

Step 1 When MS needs to be handed over to the cell to which BSC2 belongs, BSC1 sends Handover Required to MSC to request for outgoing BSC handover.

Step 2 MS receives Handover Required and sends Handover Request to the target BSC2.

Step 3 BSC2 activates a new channel, and sends Handover REQ ACK to MSC, notifying MSC the channel is now available.

Step 4 MSC receives the Handover REQ ACK, and sends Handover CMD to BSC1, which will transfer the message to MS, notifying MS to access in the new channel.

Step 5 MSC receives the Handover CMP from BSC2, sends Clear CMD to BSC1 which will initiate local release. Then BSC1 responds to MSC with Clear CMP, indicating the clearance has been completed.

----End



9.2.3 Inter-MSC Handover Procedure

Figure 9-3 Normal inter-MSC handover procedure

BTS1 BSC1MS MSC1

Measure Report from MS

(2)Handover RQD

(1)

(5)Channel ACT

BTS2 MS

Handover REQ (4)

BSC2

Channel ACT ACK (6)

Handover Access (13)

Handover REQ ACK (7)

SABM

Establish IND

Handover_Detect

PHY INFO

PHY INFO

Handover Detect

Handover CMP

(15)

(14)

(23)

(16)

(18)

(19)

HANDOVER CMP (21)(22)

Clear CMD

Clear CMP

(24)

(25)

MSC2

(3)Perform Handover

HANDOVER COMMANDHandover CMD (11)

(12)

(8)Radio Channel ACK

(9)IAM

(10)ACM

Send End Signal

(17)

(20)UA

As for the descriptions of this procedure, please see "Inter-BSC Handover Procedure" and "Intra-BSC Handover Procedure".

Here introduces briefly the messages on E interface:

Step 1 Perform handover: It is the message on MAP layer, including the CGI of original cell and destination cell and the required channel type, inform MSC2 that a handover will be initiated.

Step 2 Radio channel ACK: It is the message on MAP layer, including the information of the new channel in destination cell and handover number.

Step 3 IAM: Initial address message, it is TUP/ISUP message.

Step 4 ACM: address complete message, it is TUP/ISUP message.

Step 5 Send end signal: it is MAP message, send end signal.

----End




9.2.4 Inter-system Handover

Signaling procedure

Figure 9-4 Normal inter-system handover procedure

MS CN

1. MEAS Report 2. MEAS Result

BTS BSC

GSM=>WCDMA

RNC UE

3.HO Required

5. CC

6. Reloc Req AckHO To

Utran Cmd

7. HOCommand

HO ToUtran Cmd8. DR(HO toUtran Cmd)9. HO to Utran Cmd

10. RL Sync

Node B

11. RL RestoreIND12. Reloc Detect

13. HO to Utran Complete14. RelocComplete15. Clear Cmd

16. ClearComplete

4.CR(Reloc Req)RL SETUP

RL SETUP RESP

Procedure explanation For details about the flow, see 9.2.2 Inter-BSC Handover Procedure.

The following is a brief description about the messages at the 3G side.

The original BSS in the GSM system judges that the UE shall hands over to the cell of UTRAN according to its handover algorithm and measurement information. BSS sends the handover request towards CN. MSC sends RNC the message "RANAP_RELOCATION_REQUEST", which carries the following information about UE: IMSI, CN ID, target cell ID, encryption information, IU signaling connection ID, handover cause, RAB configuration information, and user plane information.

RNC assigns radio resources for the migration and configures the NodeB through the "RL SETUP" process. Then NodeB transmits and receives the radio signals.

NodeB returns the message "RL SETUP RESPONSE" after succeed in establishing RL. RNC sends the radio resources and other parameter packets (including U-RNTI, RAB

information, transmission layer information and physical layer information) to CN through the "RELOCATION REQUEST ACKNOWLEGEIU" message in the UI interface. CN then sends them to the original BSS and the BSS sends them to the UE. UE withdraws the relevant access parameters (configured by the RNC) in the system messages and then implements downlink synchronization with NodeB. After the synchronization, UE sends the data through the uplink.

NodeB reports "RL RESTORE IND" to RNC after detect the uplink synchronization.



After receive the "RL RESTORE IND", RNC reports "RELOCATION DETECT" to CN to notify that UE accesses to 3G from 2G. This message contains no other content.

UE sends the "HANDOVER TO UTRAN COMPLETE" message to RNC to notify the handover is complete. The message can also contain the encryption serial number and activation time of each CN.

Inter-BSC Handover Procedure Refer to 9.2.2 Inter-BSC Handover Procedure.

9.3 Abnormal Procedure In case of such exceptions as radio interface message loss, call drop, user hangup, transmission, NSS, or even BSS equipment failure, the handover procedure might be incapable to run normally.

Since there are many reasons resulting in abnormal handover procedure, only the common cases will be described here.

Handover failure due to CIC exception Suppose the CIC allocated in the Handover REQ received by BSC is marked as BLOCK, BSC will respond to MSC with Handover Failure due to "Requested terrestrial resource unavailable".

Handover failure due to MS access failure When a MS accesses the new channel, BTS cannot decode the HANDOVER ACCESS or HANDOVER COMPLETE message correctly, and the handover will fail. The MS returns to the old channel and responds with a HANDOVER FAILURE message.

In the intra-BSC handover, if the BSC has not received the HANDOVER COMPLETE message on the new channel or HANDOVER FAILURE message on the old channel at expiry of timer T3103A, it shall consider the call as dropped and send a CLEAR REQUEST message to the MSC on the old channel. Upon receiving the CLEAR COMMAND message from the MSC, the BSC releases the old channel and notifies the target cell to release the new channel. If timer T3103B1 or T3103B2 times out, the target cell shall release the new channel.

In the inter-BSC handover procedure, if BSC1 has not received the HANDOVER COMPLETE message at expiry of timer T3103B2, it shall send a CLEAR REQUEST message to the MSC to release the call. If BSC2 has not received the HANDOVER DETECT or HANDOVER COMPLETE message, it shall send a CLEAR REQUEST message to the MSC for the same purpose

Handover Procedure Initiation Failure In case of handover procedure initiation failure, perform the following steps:

Step 1 Check whether the call complies with the handover initiation conditions

Step 2 Check whether there is a CGI list of neighbor cells in the measurement report

Step 3 Check whether there is any channel available for the selected neighbor cell




Step 4 Check whether the data about the selected neighbor cell is correct, for example, whether the CGI of the neighbor cell is consistent with that in the [Cell module information table].

Step 5 Log in to the BSC maintenance system. Click [Trace/GSM Trace/GSM Interface Trace] to display the [Interface Trace] dialog box. Check whether the LAPD has sent the GMPU a LAPD_GMPU_HO_IND message on the interface between the GMPU and LAPD. If yes, the call complies with the handover initiation conditions. If a neighbor cell with correct data already exists but there is still no HANDOVER COMMAND message on interface Um, check whether the handover procedure initiation failure is due to no channel available or another reason.

----End

ContentsFigures9 Handover ProcedureAbout This Chapter9.1 Overview9.2 Normal Procedure9.2.1 Intra-BSC Handover Procedure9.2.2 Inter-BSC Handover Procedure9.2.3 Inter-MSC Handover Procedure9.2.4 Inter-system Handover

9.3 Abnormal Procedure

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