Handover Flow Process

第2章 Signaling Analysis Manual

第3章 M900/M1800 Base Station Subsystem 第4 章 Chapter 9 Handover Procedure

第5 章

Chapter 9 Handover Procedure

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

I. Signaling procedure

2.

Figure 9-1 Intra-BSC handover procedure

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第5 章II. Procedure explanation

1) MS sends Measurement Report to BTS1 on SACCH on interface Um, and BTS1

will transfer the message to BSC.

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.

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.

4) BSC receives the CHANNEL ACTIVATION ACKNOWLEDGE from BTS2 and

sends HANDOVER COMMAND to BTS1, which will transfer the command to

MS. The message is sent on FACCH on the Um interface.

5) MS receives the HANDOVER COMMAND and sends HANDOVER ACCESS on

FACCH to BTS2 for access attempt.

6) BTS2 receives the HANDOVER ACCESS from MS and sends HANDOVER

DETECT to BSC notifying that the HANDOVER ACCESS message bas been

received.

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.

8) 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.

9) BTS2 receives the first SABM, and sends EST IND to BSC, notifying it that the

radio link has been established.

10) At the same time, BTS2 sends UA frames on FACCH to MS, notifying that the

radio link layer has been established.

11) Then, MS sends HANDOVER COMPLETE on FACCH to BTS2, which transfers

the command to BSC notifying handover completion.

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.

III. Internal handling of BSC

The internal handling of the BSC in an internal handover procedure is given below:

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第5 章1) Huawei BSC performs handover decision at GLAP. Upon detecting that a call

complies with the handover initiation conditions, the GLAP sends a handover

request that carries a CGI list of the neighbor cells to the GMPU. The GMPU

then selects a neighbor cell from the list based on the cell priority (from high to

low). If the selected neighbor cell is under management of the same BSC based

on the CGI of the cell and that in the [Cell module information table] but there is

no channel available, the GMPU shall select the next neighbor cell. If the

selected neighbor cell is under management of another BSC based on the CGI

of the cell and that in the [Cell module information table], the GMPU shall initiate

an inter-BSC handover procedure that is omitted here. If the selected neighbor

cell with the highest priority is under management of the same BSC and there is

a channel available, the GMPU shall initiate an intra-BSC handover procedure

and this neighbor cell is the target cell. In this case, the source cell sends an

internal handover request to the target cell and starts the 2-second timer to await

the handover preparation completion.

2) Upon receipt of the internal handover request, the target cell allocates a channel

and sends a CHANNEL ACTIVATION message to BTS2 to activate the allocated

channel.

3) Upon completion of the channel activation, the target cell sends a CHANNEL

ACTIVATION ACKNOWLEDGE message to the source cell, informing the

source cell that the channel is available. Then it starts timer T3103B1 (see [Cell

Call Control Parameter Table]) to await the HANDOVER DETECT message.

4) Upon receipt of the CHANNEL ACTIVATION ACKNOWLEDGE message, the

source cell sends a HANDOVER COMMAND message to BTS1, stops the 2-

second timer and starts timer T3103A (see [Cell Call Control Parameter Table]).

5) Upon receipt of the HANDOVER DETECT message, the target cell stops timer

T3103B1 and starts timer T3103B2 (see [Cell Call Control Parameter Table]) to

await the HANDOVER COMPLETE message.

6) Upon receipt of the HANDOVER COMPLETE message from the MS, the target

cell stops timer T3103B2 and notifies such relevant modules as AIE and AIR to

update the RR connection number. At the same time, it sends an “Internal

Handover Success” message to the source cell.

7) Upon receipt of the “Internal Handover Success” message, the source cell

initiates a local release procedure to release the old channel.

8) The target cell sends a HANDOVER PERFORMED message to the MSC,

informing the MSC that an intra-BSC handover procedure has been completed.

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第5 章9.2.2 Inter-BSC Handover Procedure


3.

Figure 9-2 Normal inter-BSC handover procedure

II. Procedure explanation

Compared with the intra-BSC handover procedure, more A interface signaling are

added to the inter-BSC handover procedure. The added A interface signaling will be

described here. For the descriptions of other signaling, please see 9.2.1 Intra-BSC

Handover Procedure.

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第5 章1) When an MS needs to be handed over to the cell under management of the

target BSC (BSC2), the source BSC (BSC1) sends a HANDOVER REQUIRED

message to the MSC to request outgoing BSC handover.

2) Upon receipt of the HANDOVER REQUIRED message, the MS sends a

HANDOVER REQUEST message to BSC2.

3) BSC2 activates a new channel and sends HANDOVER REQUEST

ACKNOWLEDGE to MSC, notifying MSC the channel is now available.

4) Upon receipt of the HANDOVER REQUEST ACKNOWLEDGE message, the

MSC sends a HANDOVER COMMAND message to BSC1, which will transfer

the message to MS, notifying MS to access in the new channel.

5) Upon receipt of the HANDOVER COMPLETE message from BSC2, the MSC

sends a CLEAR COMMAND message to BSC1, which will initiate a local release

procedure. Then BSC1 responds to MSC with CLEAR COMPLETE, indicating

the clearance has been completed.


The internal handling of the BSC in an inter-BSC handover procedure is given below:

1) Huawei BSC performs handover decision at GLAP. Upon detecting that a call

complies with the handover initiation conditions, the GLAP sends a HANDOVER

REQUEST message that contains a CGI list of neighbor cells to the GMPU. The

GMPU then selects a neighbor cell from the list based on the cell priority (from

high to low). If the selected neighbor cell is under management of the same BSC

based on the CGI of the cell and that in the [Cell module information table], but

there is no channel available, the GMPU shall select the next neighbor cell. If the

selected neighbor cell is under management of another BSC, the GMPU shall

initiate an inter-BSC handover procedure. For the source cell in this case, the

cell under management of another BSC is the target cell and the BSC that

manages the target cell is the target BSC.

2) Upon initiation of the intra-BSC handover procedure, the source cell sends a

HANDOVER REQUIRED message to the MSC and starts the 10-second timer

(T7) to await the HANDOVER COMMAND message.

3) Upon receipt of the HANDOVER REQUEST message from the MSC, BSC2

allocates a channel based on the target cell identify in this message and

activates the allocated channel. After the activation, the target cell sends the

MSC a HANDOVER REQUEST ACKNOWLEDGE message that carries the

HANDOVER COMMAND message. In this case, the target cell starts timer

T3103B1 (see [Cell Call Control Parameter Table]) to await the HANDOVER

DETECT message.

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第5 章4) When BSC1 receives the HANDOVER COMMAND message from the MSC, the

source cell sends the HANDOVER COMMAND to BTS1, which shall transfer this

message to the MS. At the same time, the source cell starts timer T3103A (see

[Cell Call Control Parameter Table]) to await handover completion. If the source

cell receives the CLEAR COMMAND message with cause “Handover Success”

before expiry of timer T3103A, BSC1 shall consider the handover procedure as

successful. If the MS returns to the old channel before expiry of this timer, BSC1

stops this timer and sends a HANDOVER FAILURE message to the MSC. If this

timer times out before reception of the CLEAR COMMAND message, BSC1 shall

consider the call in the handover procedure as dropped and send a CLEAR

REQUEST message to the MSC to release the call.

5) Upon receipt of the HANDOVER DETECT message, the target cell stops timer

T3103B1, sends a HANDOVER DETECT message to the MSC and starts timer

T3103B2 to await the HANDOVER COMPLETE message.

6) Upon receipt of the HANDOVER COMPLETE message from the MS, the target

cell stops timer T3103B2 and transfers this message to the MSC, informing the

MSC that an intra-BSC handover has been completed.

7) Upon receiving the CLEAR COMMAND message from the MSC before expiry of

timer T3103A, the source cell locally releases the radio resources and sends

back a MSC CLEAR COMMAND message.

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第5 章9.2.3 Inter-MSC Handover Procedure


4.

Figure 9-3 Normal inter-MSC handover procedure

II. Procedure explanation

For the descriptions of this procedure, refer to 9.2.1 Intra-BSC Handover Procedure

and 9.2.2 Inter-BSC Handover Procedure.

Here introduces briefly the messages on the E interface:

1) Perform Handover is the message on MAP layer. It contains the CGI of source

cell and target cell and the required channel type. It informs MSC2 that a

handover will be initiated.

2) RADIO CHANNEL ACK is the message on MAP layer. It contains the information

of the new channel in the target cell and handover number.

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第5 章3) INITIAL ADDRESS MESSAGE (IAM) is a TUP/ISUP message.

4) ADDRESS COMPLETE MESSAGE (ACM) is a TUP/ISUP message.

5) Send End Signal is a MAP message.


Refer to III. Internal handling of BSC.

9.3 Abnormal Cases

In case of such exceptions as radio interface message loss, call drop, user hangup,

transmission failure, NSS or even BSS equipment failure, the handover procedure

might be incapable to run normally.

There are many reasons resulting in abnormal handover procedure. Only the

common cases will be described here.

9.3.1 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".

9.3.2 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

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第5 章DETECT or HANDOVER COMPLETE message, it shall send a CLEAR REQUEST

message to the MSC for the same purpose.

9.3.3 Handover Procedure Initiation Failure

In case of handover procedure initiation failure, perform the following steps:

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

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

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

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].

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.

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Handover Flow Process

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