Handover Signalling

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Handover Signalling in BSC

dn9814132Issue 8-0 en

# Nokia CorporationNokia Proprietary and Confidential

1 (41)

2003319

Nokia BSC/TCSM S11 Product Documentation



The information in this documentation is subject to change without notice and describes only theproduct defined in the introduction of this documentation. This documentation is intended for theuse of Nokia's customers only for the purposes of the agreement under which the documentationis submitted, and no part of it may be reproduced or transmitted in any form or means without theprior written permission of Nokia. The documentation has been prepared to be used byprofessional and properly trained personnel, and the customer assumes full responsibility whenusing it. Nokia welcomes customer comments as part of the process of continuous developmentand improvement of the documentation.

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Contents

Contents 3

List of tables 5

List of figures 6

Summary of changes 7

1 Overview of Handover Signalling in BSC 9

2 TCH handover signalling in BSC 112.1 Internal intra-cell handover in BSC 112.2 Internal inter-cell handover in BSC 13

2.3 External handover in BSC (source cell side) 152.4 External handover in BSC (target cell side) 16

3 SDCCH handover signalling in BSC 193.1 Internal intra-cell handover 193.2 Internal inter-cell handover 213.3 External handover (source cell side) 243.4 External handover (target cell side) 25

4 Statistics, parameters and alarms for TCH and SDCCH handovers 274.1 Parameters for TCH and SDCCH handovers 274.2 Alarms for TCH and SDCCH handovers 28

5 Handover signalling phases in BSC 315.1 Start of handover in BSC 315.1.1 Handover type determination in BSC 315.1.2 Candidate cell determination in BSC 315.2 Handover type reselection in BSC 325.2.1 External handover after unsuccessful internal handover BSC 325.2.2 Internal handover after unsuccessful external handover in BSC 345.2.3 Handover type reselection due to Directed Retry in BSC 365.2.4 Changing A interface circuit in BSC internal handover between half rate

and full rate channels in BSC 365.3 Handover request in BSC 365.4 Handover inquiry in BSC 37

5.5 Physical context request in BSC 375.6 Channel reservation and queueing in BSC 375.7 Channel activation in BSC 385.8 Connection of unidirectional speech path in BSC 385.9 Start of message buffering in BSC 395.10 Handover command / assignment command in BSC 395.11 Handover detection in BSC 405.12 Indication of handover establishment in BSC 405.13 Connection of bidirectional speech path in BSC 405.14 Handover / assignment complete in BSC 41



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Contents



5.15 Release of the old channel in BSC 41






List of tables



5 (41)

List of tables



List of figures

Figure 1. BSC internal intra-cell handover from TCH to TCH 12

Figure 2. BSC internal inter-cell handover from TCH to TCH 14

Figure 3. BSC external handover (source cell) from TCH to TCH 16

Figure 4. BSC external handover (target cell) from TCH to TCH 17

Figure 5. BSC internal intra-cell handover from SDCCH to SDCCH 20

Figure 6. BSC internal inter-cell handover from SDCCH to SDCCH 22

Figure 7. BSC external handover (source cell) from SDCCH to SDCCH 24

Figure 8. BSC external handover (target cell) from SDCCH to SDCCH 26






Summary of changes

Summary of changes

Changes between document issues are cumulative. Therefore, the latest document

issue contains all changes made to previous issues.

Changes made between issues 8 and 7 1

The document has been revised throughout to comply with the latest

documentation standards. No changes have been made in the document due to

new features.

Changing A interface circuit in BSC internal handover between half rate

and full rate channels in BSC

Program block information removed.

Changes made between issues 7 1 and 7

General changes

Link corrections.

Changes made between issues 7 and 6

Chapter Alarms

In a description of alarm ADJ_CELL_ID_ERROR_A the layer level number has

been corrected.

Chapter Detailed description, section Internal handover

In a paragraph Starting of transparent procedure during handover the mention of

message buffering has been corrected.



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Summary of changes



1 Overview of Handover Signalling in BSC

This is a description of the Handover Signalling in the DX 200 Base Station

Controller (BSC).

Handover signalling depends on the handover type, which are presented in

sections TCH handover signalling and SDCCH handover signalling . The

signalling consists of several phases, which are described in Handover signalling phases in BSC.

In addition to these, information about Statistics, parameters and alarms for TCH

and SDCCH handovers is provided.



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Overview of Handover Signalling in BSC



2 TCH handover signalling in BSC

The handovers are functionally divided into four types. The signalling used in the

handover depends on the handover type. The signalling consists of several

phases. These phases are described in the following sections.

Back to Overview of Handover Signalling in BSC.

2.1 Internal intra-cell handover in BSC

The signalling of the intra-cell handover is presented in the signalling diagram

below.



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TCH handover signalling in BSC



Figure 1. BSC internal intra-cell handover from TCH to TCH

MS BTS1 BTS2 BSC MSC

meas_rep measurement_result

> >

physical_context_request

<

physical_context_confirm>

channel_activation

<

channel_activation_ack>

Unidirectional speech

is connected andmessage bufferingis started

assignment_command<

assignment_command<

sabm establish_indication> >

Bidirectional speechpath is connected

ua<

assignment_complete>

rr assignment_complete< >

handover_performed>

message bufferingis stopped

rf_channel_release<

rf_channel_release_ack

>






The phases of the signalling are the following:

. start of handover

. physical context request

. channel reservation and queueing

. channel activation

. connection of a unidirectional speech path

. assignment command

. indication of establishment

. connection of a bidirectional speech path

. completion of assignment

. release of the old channel


2.2 Internal inter-cell handover in BSC

The signalling of the inter-cell handover is presented in the signalling diagram

below.



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Figure 2. BSC internal inter-cell handover from TCH to TCH


meas_rep measurement_result> >

channel_activation<


Unidirectional speechis connected andmessage bufferingis started

handover_command<

handover_command<

handover_access handover_detection> >

physical_info<


Bidirectional speechpath is connected

ua<

handover_complete>

rr handover_complete

< >

handover_performed>


rf_channel_release<


>







. start of handover

. channel reservation and queueing


. connection of a unidirectional speech path

. handover command

. handover detection



. completion of handover



2.3 External handover in BSC (source cell side)

The signalling of the external handover at the source cell side is presented in the

signalling diagram below.



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Figure 3. BSC external handover (source cell) from TCH to TCH


. start of handover

. handover inquiry

. handover command



2.4 External handover in BSC (target cell side)

The signalling of the external handover at the target cell side is presented in the




> >

handover_required

>

handover_command

<

handover_command

<

handover_command

<

rf_channel_release clear_command

< <


>

clear_complete

>






Figure 4. BSC external handover (target cell) from TCH to TCH


.

handover request . channel reservation and queueing



. detection of handover


handover_request

<

channel_activation

<

channel_activation_ack

>

Bidirectional speechis connected

handover_request_ack

>

handover_access handover_detection> >

handover_detection

>

physical_info

<

sabm establish_indication

> >

ua

<

handover_complete

>

rr handover_complete< >

handover_complete

>



17 (41)






For an overview, see Overview of Handover Signalling in BSC.






3 SDCCH handover signalling in BSC

The handovers are functionally divided into four types. The signalling used in the

handover depends on the handover type. The signalling consists of several

phases. These phases are described in the following sections.

SDCCH handover in BSC is an optional feature. Stand-alone Dedicated Control

Channel (SDCCH) handover signalling enables the feature.


3.1 Internal intra-cell handover

The signalling of the intra-cell handover is presented in the signalling diagram

below.



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SDCCH handover signalling in BSC



Figure 5. BSC internal intra-cell handover from SDCCH to SDCCH


. start of handover

. physical context request


meas_rep measurement_result> >

physical_context_request<

physical_context_confirm>

channel_activation<


message buffering

is started

assignment_command<

assignment_command<


ua<

assignment_complete>

rr assignment_complete< >

handover_performed>


rf_channel_release<


>






. channel reservation


.

start of message buffering . assignment command


. completion of assignment



3.2 Internal inter-cell handover

The signalling of the inter-cell handover is presented in the signalling diagram

below.



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Figure 6. BSC internal inter-cell handover from SDCCH to SDCCH




> >

channel_activation

<


>

message buffering

is started

handover_command

<

handover_command

<

handover_access handover_detection

> >

physical_info

<


> >

ua

<

handover_complete

>rr handover_complete

< >

handover_performed

>

message buffering

is stopped

rf_channel_release

<


>






. start of handover


.

channel activation. start of message buffering

. handover command





Starting of transparent procedure during handover

Because the MSC is not involved in handover signalling, a new transparent

procedure (authentification, identification or setup) may be started during the

handover. All transparent messages coming from the MSC are buffered in the

BSC and handled at the end of the handover.

The buffering of messages starts after a new channel has been activated.

Therefore it is possible that the handover attempt has been started but the message

buffering has not. During this time the BSC handles all transparent procedures in

the normal way.

Starting of ciphering procedure during handover

If a ciphering procedure starts during a handover, it is buffered in the BSC and

handled at the end of the handover.

Starting of assignment or clear procedures during handover

If an assignment or clear procedure starts before the MS is assigned to move to a

new channel, the BSC handles that procedure immediately and the handover

attempt is terminated. During the message buffering the assignment and clear

procedure are buffered in the BSC and handled at the end of the handover.

Starting of handover during signalling procedure

If a handover attempt is started during the signalling procedures (authentification,

identification, setup, ciphering), the BSC handles the signalling procedure in the

normal way.

Starting of handover during assignment procedure



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If a handover attempt is started during an assignment procedure, BSC handles the

assignment procedure in the normal way and the handover attempt is terminated.


3.3 External handover (source cell side)

The signalling of the external handover on the source cell side is presented in the


Figure 7. BSC external handover (source cell) from SDCCH to SDCCH


. start of handover

. handover inquiry

. handover command


Starting of transparent procedure during handover


meas_rep meas urement_result> >

handover_required>

handover_command<

handover_command<

handover_command<

rf_channel_release clear_command< <

rf_channel_release_ack>

clear_complete>






Because the MSC is involved in the handover signalling, it does not start any new

transparent procedures which affect the started handover attempt after receiving

the HANDOVER_REQUIRED message. If a transparent procedure is started

before that, the BSC handles it in the normal way.

Starting of ciphering during handover

If ciphering starts during the handover attempt, the MSC rejects the handover

attempt by sending a HANDOVER_REQUIRED_REJECT message and the

BSC terminates the handover attempt. The BSC handles the started ciphering

procedure in the normal way.

Starting of assignment procedure during handover

If assignment procedure occurs during handover attempt, the parameter dr

supported with any cause value determines which one of the

procedures is executed, handover or assignment. Parameter dr supported

with any cause value is situated on the Base Station System Application

Part (BSSAP) and it can have values 'supported' and 'not supported'. If the

parameter is set to the value 'supported', the handover is continued and the

assignment procedure starts after handover. If the parameter is set to the value 'not

supported', the assignment procedure is started and the handover is terminated. A

new handover attempt is started after assignment, if needed.

Starting the handover attempt during transparent procedure

If a handover attempt starts during a transparent procedure (authentification,identification, setup, ciphering), the BSC handles the transparent procedure in the

normal way.

Starting the handover attempt during a ciphering or assignment procedure

If a handover attempt starts during a ciphering or assignment procedure, the

handover attempt is terminated immediately and the ciphering or assignment

procedure is handled.


3.4 External handover (target cell side)

The signalling of the external handover on the target cell side is presented in the




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Figure 8. BSC external handover (target cell) from SDCCH to SDCCH


. handover request








handover_request

<

channel_activation

<


>

handover_request_ack

>

handover_access handover_detection

> >

handover_detection

>

physical_info<


> >

ua

<

handover_complete

>

rr handover_complete

< >

handover_complete

>






4 Statistics, parameters and alarms for

TCH and SDCCH handovers

The handover performance counters are collected from every cell in the BSC and

they are divided into six groups:

.

counters for external handovers (source cell side)

. counters for external handovers (target cell side)

. counters for internal inter-cell handovers (source cell side)

. counters for internal inter-cell handovers (target cell side)

. counters for internal intra-cell handovers

. counters for causes of starting handovers

The handover observation data is collected on all handovers. The target of one

handover observation is one cell. The handover observations are as follows:

. internal handover observation

. incoming handover observation

. outgoing handover observation

For more information on the counters and the handover observation, see BSC

Counters: SDCCH Observation.


4.1 Parameters for TCH and SDCCH handovers

TCH



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Statistics, parameters and alarms for TCH and SDCCH handovers



The parameter genHandoverReqMessage is available in the Operation and

Maintenance Center (OMC) or MMI (for more information, see Base Transceiver

Station Handling in BSC command group EQ ). This parameter determines how

many candidate cells are included in the cell identifier list sent to the MSC in the

external handover.

The parameter disableIntHo is available in the MMI. This parameter forces

all handovers to be controlled by the MSC. For more information, see BSS Radio

Network Configuration Database (BSDATA) .

SDCCH

The parameter EnableSDCCHHandover is available in the OMC or MMI (for

more information, see Base Transceiver Station Handling command group EQ ).

It determines if a handover in the signalling phase of the SDCCH is allowed or

not.

The parameter disableIntHo is available in the MMI. It forces all handovers

to be controlled by the MSC. For more information, see BSS Radio Network

Configuration Database (BSDATA).

The parameter genHandoverReqMessage is available in the OMC or MMI

(for more information, see Base Transceiver Station Handling command group

EQ ). This parameter determines how many candidate cells are included in the cell

identifier list sent to the MSC in the external handover.

For an overview, seeOverview of Handover Signalling in BSC.

4.2 Alarms for TCH and SDCCH handovers

The 2725 ADJ_CELL_ID_ERROR_A alarm indicates that the source cell's

adjacent cell information in the BSS Radio Network Configuration Database is

incorrect. The source BSC is able to notice this before it sends a

HANDOVER_COMMAND to the MS. The MSC sends layer 3 information in

the HANDOVER_COMMAND message to the BSC. Layer 3 information

contains the BCCH frequency and the BSIC of the target cell.

The source BSC compares the BCCH frequency and BSIC with the original

information it has on the handover candidates. If these do not match, there has

been a failure in candidate cell identification.

The alarm gives information on the source cell whose adjacent cell definitions are

incorrect.

Actions needed to cancel the alarm






After the operator has checked all adjacent cell definitions of the cell given in the

alarm, the alarm can be cancelled with an MML command.




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Statistics, parameters and alarms for TCH and SDCCH handovers



5 Handover signalling phases in BSC

5.1 Start of handover in BSC

The BSC receives measurements (the MEASUREMENT_RESULT message)

from the BTS. It processes the measurements and decides on the handover algorithm (for more information, see RF Power Control and Handover Algorithm

). If thresholds have been triggered for the handover, the BSC generates a

candidate cell list and starts the handover attempt.


5.1.1 Handover type determination in BSC

If the parameter disableIntHo is set, an external handover is started.

If another BSC controls the first cell of the candidate cell list, an external

handover is started.

If the first cell of the candidate cell list is the same cell as the serving one, an

internal intra-cell handover is started.

If none of the above-mentioned requirements are met, an internal inter-cell

handover is started.


5.1.2 Candidate cell determination in BSC

In an internal intra-cell handover, the serving cell is used as the candidate cell.

In an internal inter-cell handover, several cells can be used as candidate cells.

In an external handover, the parameter genHandoverReqMessage indicates

how many cells are passed to the MSC in a handover inquiry.



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Handover signalling phases in BSC




5.2 Handover type reselection in BSC

With the help of handover type reselection, the candidate cell list is used more

thoroughly to find a new channel. Handover type reselection is used only once for

each failed handover attempt.

There are four types of handover type reselection:

. An external handover is attempted after an unsuccessful internal handover

(due to congestion).

.

An internal handover is attempted after an unsuccessful external handover (due to congestion).

. An internal directed retry is started after an unsuccessful external directed

retry attempt. For more information, see Directed Retry in BSC.

. When there is a need to change the A interface circuit in an internal

handover when handovers between half rate and full rate channels are

carried out. For more information, see Half Rate in BSC.


5.2.1 External handover after unsuccessful internal handover BSC

The BSC generates the candidate cell list in the handover algorithm process. On

the basis of this list, the BSC decides what kind of a handover is attempted. The

candidate cell list may be, for example, the following:

. (pos 1) a cell belongs to the serving BSC

. (pos 2) a cell belongs to another BSC







. (pos 9-16) ...






Since the first cell candidate in the list belongs to the serving BSC, an internal

handover is attempted. The BSC's own cells are searched for on the list, and the

following candidate cell list (maximum length 16 cells) is generated next. The list

is used for requesting a channel in the serving BSC.





If all the candidate cells have radio channel congestion, the internal handover

attempt is stopped.

If the internal handover fails due to a lack of resources (none of the requestedchannels is available and queueing is not used), a new handover attempt will be

started immediately according to a BSC external handover procedure. Candidate

cells belonging to another BSC are searched for on the candidate cell list and the

following list is sent to the MSC (the number of the candidate cells in the list is

equal to the parameter genHandoverReqMessage ; in this example the

parameter is set to be 3):



.

(pos 7) a cell belongs to another BSC

If an internal handover fails because of lack of resources and unsuccessful

queueing, the handover attempt is stopped and the handover algorithm is inquired

to start an external handover. If suitable target cells are found, an external

handover attempt is started immediately.

Handover type reselection is not used:

. When there are no suitable target cells.

.

In the case of Intelligent Underlay-Overlay handovers. For moreinformation, see Intelligent Underlay-Overlay.

. When the starting cause for the handover attempt is pre-emption. For more

information, see Radio Resource Pre-emption and Queueing in BSC.

. When the starting cause for the handover attempt is directed retry, and the

parameter disableExtDr is set. For more information, see Directed

Retry in BSC.



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. After queueing when the reason for the handover is either directed retry or

a forced handover. For more information, see Radio Network

Administration.

.

During FACCH call setup. For more information, see FACCH Call Set-up.

In these cases the call continues on the old channel. However, in the case of a

directed retry, the BSC starts to clear the directed retry attempt. The next

handover attempt may be started after the parameterized timer

MinIntBetweenUnsuccHoAttempt has expired. For more information,

see RF Power Control and Handover Algorithm.


5.2.2 Internal handover after unsuccessful external handover in BSC

The BSC generated the candidate cell list in the handover algorithm process. On

the basis of this list, the BSC decides what kind of handover is attempted. The

candidate cell list may be, for example, the following:









. (pos 9-16) ...

Since the first cell candidate belongs to another BSC, an external handover is

attempted. The BSC sends as many candidate cells as the parameter

genHandoverReqMessage allows to be sent to MSC at most. If this

parameter is set to be, for example, 3, the candidate cell list sent to the MSC is the

following:


. (pos 2) a cell belongs to the same BSC







If an external handover fails because of lack of resources, the handover attempt is

stopped and the handover algorithm is inquired to start an internal handover.

Target cells belonging to the serving BSC are searched for. If suitable target cells

are found, an internal handover attempt is started immediately. A list of those

internal candidates that are still valid is sent to the Radio Resource Management

process of the BSC:





Handover type reselection is not used:

. when there are no suitable target cells

. if the handover attempt is started because of

- pre-emption (for more information, see Radio Resource Pre-emption

and Queuing in BSC )

- directed retry (for more information, see Directed Retry in BSC )

- forced handover (for more information, see Radio Network

Administration ) or

- traffic reason (for more information, see Traffic Reason Handover

Procedure in BSC ).. when parameter disableIntHo is set.

In these cases when handover type reselection is not used, the call continues on

the old channel. The next handover attempt may be started after the

parameterized timer MinIntBetweenUnsuccHoAttempt has expired.

However, in the case of a directed retry, the MSC starts to clear the directed retry

attempt.

Also if timer T7 has expired, handover type reselection is attempted and an

internal handover may be started. For more information, see Parameter

Administration. If the handover type reselection is not possible, an externalhandover restart is attempted immediately.




35 (41)




5.2.3 Handover type reselection due to Directed Retry in BSC

If the MSC in the network does not support the Directed Retry feature, the

parameter disableExtDr must be set in the BSC.

When the source BSC notices that disableExtDr is set and that the first

candidate cell belongs to another BSC, the source BSC goes through the

candidate list. Cells belonging to the serving BSC are searched for and a BSC

internal DR is attempted according to this new list.

For more information on Directed Retry, see Directed Retry in BSC .


5.2.4 Changing A interface circuit in BSC internal handover between half rate and full rate channels in BSC

In call setup, the BSC sends an internal assignment request which includes the A

interface circuit type. The circuit type is stored. This information is sent internally

in the BSC in a BSC internal handover start message.

If the old A interface circuit does not support the new channel type of the new

cell, there is a need to change the A interface circuit in a BSC internal handover.

This is known at the earliest when the BSC tries to allocate a TCH from the

target cell.

If TCH allocation fails due to a contradiction between the type of free TCH

resources and the circuit type CIP, the BSC starts a BSC external handover to the

same cell. The candidate cell list contains only this one cell.

For more information on half rate, see Half Rate in BSC .


5.3 Handover request in BSCAfter receiving a HANDOVER_REQUIRED message from the BSC, the MSC

determines the candidate cell and sends a HANDOVER_REQUEST message to

the target BSC. When the BSC receives this message, the handover attempt is

started. The message contains, for example, information on the cell identifier of

the candidate cell.







5.4 Handover inquiry in BSC

If the handover is allowed at the current phase of the call, the BSC sends the

HANDOVER_REQUIRED message to the MSC. Otherwise the handover

attempt is terminated and the call continues on the old channel.

If the MSC does not allow the handover or, for example, there are no free

channels available in the target cell, the MSC sends a

HANDOVER_REQUIRED_REJECT message to the BSC. The handover

attempt is terminated and the call continues on the old channel.


5.5 Physical context request in BSC

The BSC requests the physical context from the BTS by sending a

PHYSICAL_CONTEXT_REQUEST message to the BTS. If a

PHYSICAL_CONTEXT_CONFIRM message is not received from the BTS

before time supervision elapses, the handover attempt is terminated and the call

continues in the old channel.


5.6 Channel reservation and queueing in BSC

The BSC tries to reserve a new channel for a handover attempt.

In an internal intra-cell handover, the serving cell is used as a candidate cell.

In an internal inter-cell handover, it is possible to use several candidate cells. Thesearch for a free channel is carried out according to the candidate cell list.

In an external handover, the candidate cell is presented in a general form. This

means that the CGI, LAC+CI or CI are possible.



37 (41)




The priority, queueing, interference band, channel type information and type of A

interface circuit pool are also used when reserving a channel (for more

information, see Radio Channel Allocation ). In an internal handover, the MSC

sends this information in the ASSIGNMENT_REQUEST message at the

beginning of the call. In an external handover, the MSC sends the information in

the HANDOVER_REQUEST message.

If a free channel is found from the candidate cell(s), the handover attempt

continues. Otherwise, if queueing is allowed, the BSC starts to queue for a free

channel (in an external handover the BSC sends a QUEUEING_INDICATION

message to the MSC). If queueing is not allowed or if no free channel is found by

queueing, the handover attempt is terminated (in an external handover, the BSC

sends a HANDOVER_FAILURE message to the MSC) and the call continues on

the old channel.


5.7 Channel activation in BSC

The BSC requests the BTS to activate a new channel by sending a

CHANNEL_ACTIVATION message. If a

CHANNEL_ACTIVATION_ACKNOWLEDGE message is received from the

BTS before time supervision elapses, the handover attempt continues. The

HANDOVER_REQUEST_ACKNOWLEDGE message is sent to the MSC in an

external SDCCH handover.

If the activation of a channel fails and the BTS sends a

CHANNEL_ACTIVATION_NEGATIVE_ACKNOWLEDGE message to the

BSC, the handover attempt is terminated (in an external handover, the BSC sends

a HANDOVER_FAILURE message to the MSC) and the call continues on the

old channel.


5.8 Connection of unidirectional speech path in BSC

If the handover attempt is allowed at the current phase of the call, the BSC

connects a unidirectional speech path so that the MS is immediately able to hear

the speech from the MSC. The BSC starts the buffering of messages from the

MSC.







5.9 Start of message buffering in BSC

If the handover attempt is allowed at the current phase of the call, the BSC starts

the buffering of messages coming from the MSC.


5.10 Handover command / assignment command inBSC

In an internal handover, a HANDOVER_COMMAND (inter cell) or an

ASSIGNMENT_COMMAND (intra cell) message is sent to the MS on the old

channel.

In an external handover, the source BSC sends as many candidate cells as the

parameter genHandoverReqMessage allows to be sent to the MSC at most.

In the HANDOVER_REQUIRED message the candidate cells are identified in a

general form (CGI , LAC +CI or CI). This information is found for each

candidate cell from the BSS Radio Network Configuration Database using BCCH

and BSIC information received from the MS in measurement reports.

After the channel has been activated at the target BSC, the BSC sends a

HANDOVER_REQUEST_ACKNOWLEDGE message to the MSC. The

message includes information that is needed for the HANDOVER_COMMAND

message, including the BCCH and BSIC of the target cell. The MSC sends the

HANDOVER_COMMAND message to the source BSC which passes it to the

MS on the old channel.

If the cell identifier or the selected BCCH frequency and BSIC are not correct in

the source BSC's BSDATA, the source BSC selects a wrong cell as the candidate

cell. This has two possible consequences:

1. The MSC finds the candidate cell from the radio network and the handover

attempt is continued. The MSC sends the HANDOVER_COMMAND

message which was created in the target BSC to the MS via the source

BSC. However, because radio measurements belong to another cell, the

handover attempt fails and the call continues on the old channel.

2. The candidate cell is not found in the radio network, the MSC rejects the

handover attempt and the call continues on the old channel.



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If the BSC notices events like the ones mentioned above, it interrupts the

handover attempt and generates an alarm which indicates this failure.

If the MS fails to move into a new channel, it can return to the old channel by

sending a HANDOVER_FAILURE or ASSIGNMENT_FAILURE message to

the old BTS which passes it to the BSC. In an external handover, the BSC sends

the HANDOVER_FAILURE message to the MSC. The handover attempt is

terminated and the call continues in the old channel.

If the MS fails to move into the new channel and does not return to the old

channel, the handover attempt is terminated and the call is cleared.


5.11 Handover detection in BSC

After receiving an access burst from the MS, the BTS sends a

HANDOVER_DETECTION message to the BSC. In the case of an external

handover, the HANDOVER_DETECTION message is sent to the MSC.


5.12 Indication of handover establishment in BSC

After the BTS has received an SABM (Set Asynchronous Balanced Mode) frame,

it sends an ESTABLISH_INDICATION message to the BSC.


5.13 Connection of bidirectional speech path in BSCIn an internal handover, unidirectional and bidirectional speech paths are

switched through. If the connection fails, the handover attempt is terminated and

the call is cleared.






In an external handover, the bidirectional speech path is connected. The target

BSC sends a HANDOVER_REQUEST_ACKNOWLEDGE message to the

MSC. The message contains the handover command information which the MSC

sends to the source BSC. If the connection fails, the handover attempt is

terminated by sending the HANDOVER_FAILURE message to the BSC and the

call continues in the old channel.


5.14 Handover / assignment complete in BSC

After the MS has established the main signalling link successfully, it sends the

HANDOVER_COMPLETE (internal inter-cell) or the

ASSIGNMENT_COMPLETE (internal intra-cell) message in the new channel. Inan internal handover, the BSC sends the HANDOVER_PERFORMED to the

MSC. In an external handover, the BSC sends the HANDOVER_COMPLETE

message to the MSC.


5.15 Release of the old channel in BSCIn an internal handover, after the MS has completed the handover, the BSC

releases the old channel by sending an RF_CHANNEL_REL message to the

BTS. In an external handover, the MSC requests the source BSC to release the

old channel by sending a CLEAR_COMMAND message. The old channel is

released by sending an RF_CHANNEL_REL message to the BTS and a

CLEAR_COMPLETE message to the MSC.



Handover Signalling

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