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16Directed RetryAbout This Chapter

Directed retry is a special type of handover. It refers to the process of assigning an MS to a TCH

in a cell other than the serving cell when no radio resources are available. It is triggered by the

assignment procedure and employs internal or external handover procedures.

16.1 Directed Retry Procedure

The directed retry procedure is categorized into the intra-BSC directed retry procedure, inter-

BSC directed retry procedure, and inter-MSC directed retry procedure.

16.2 Internal BSC Signaling Procedure of DirectedRetry

The internal BSC signaling procedure of directed retry is categorized by intra-BSC directed

retry, inter-BSC directed retry, and inter-MSC directed retry.

16.3 Abnormal Directed Retry Cases

This topic describes the abnormal directed retry cases.

HUAWEI BSC6000 Base Station Subsystem

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16.1 Directed Retry Procedure

The directed retry procedure is categorized into the intra-BSC directed retry procedure, inter-

BSC directed retry procedure, and inter-MSC directed retry procedure.

16.1.1 Intra-BSC Directed Retry Procedure

This topic describes the intra-BSC directed retry procedure.

16.1.2 Inter-BSC directed retry procedure

This topic describes the inter-BSC directed retry procedure.

16.1.3 Inter-MSC directed retry procedure

This topic describes the inter-MSC directed retry procedure.

16.1.1 Intra-BSC Directed Retry ProcedureThis topic describes the intra-BSC directed retry procedure.

Figure 16-1shows the intra-BSC directed retry procedure.

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Figure 16-1Intra-BSC directed retry procedure

Assignment

Request(14)

Handover

Command(17)

Handover

Access(18)Handover

Detect(19)

PHY INFO(20)

SABM(22)

Establish

Indication(23)

UA(24)Handover

Complete(25)

Assignment

Complete(26)

Channel

Request(1) ChannelRequired(2)

Channel

Activation(3)

Immediate

Assignment

Command(5)

UA(7)

SABM(6)

Establish Indication

(CM Service

Request)(8)

CR(Complete

L3 Information)(9)

CC(10)

Channel Activation

Acknowledge(16)

MS BTS1 MSCBSC BTS2 MS

Channel Activation

Acknowledge(4)

Start

T3103

FACCH

RACH

AGCH

SDCCH

CM Service

Accepted(11)

SDCCH

Setup(12)

SDCCHCall Proceeding(13)

SDCCHFACCH

PHY INFO(21)

FACCH

FACCH

FACCH

Stop

T3103

1. The MS sends the BTS a Channel Request message on the RACH.

2. The BTS sends the BSC a Channel Required message.

3. Upon receiving the Channel Required message, the BSC assigns a signaling channel andsends the BTS a Channel Activation message.





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4. Upon receiving the Channel Activation message, if the channel type is correct, the BTS

enables the power amplifier on the AGCH, starts receiving the uplink information, and

sends the BSC a Channel Activation Acknowledge message.

5. Upon receiving the Channel Activation Acknowledge message, the BSC sends an

Immediate Assignment Command message on the AGCH to notify the MS of channelaccess.

6. After the MS successfully accesses the channel, the MSC sends the BSC an Assignment

Request message requesting for TCH radio resources. The Assignment Request message

carries the information on the circuits between the MSC and the BSC. If no TCH is available

in the serving cell, the BSC attempts to hand over the MS to a neighbor cell that provides

voice services. If the target cell and the serving cell are controlled by the same BSC, the

directed retry is an intra-BSC directed retry.

The intra-BSC directed retry procedure is as follows:

1. Steps 1 through 8 in the procedure shown in Figure 16-1are the random access and

immediate assignment procedures, in which the BSS assigns a signaling channel to the MS.2. Steps 14 through 24 in the procedure shown in Figure 16-1are TCH directed retry

procedure, in which the BSS assigns a speech channel to the MS.

16.1.2 Inter-BSC directed retry procedure

This topic describes the inter-BSC directed retry procedure.

Figure 16-2shows the inter-BSC directed retry procedure.

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Figure 16-2Inter-BSC directed retry procedure

Assignment

Request(14)

Handover

Required(15)Handover

Request(16)

Channel

Activation(17)Handover

RequestAcknowledge(19)Handover

Command(20)

Handover

Command(21)

Handover

Complete(31)Clear

Command(32)

Channel

Request(1) Channel

Required(2)

Channel

Activation(3)

Immediate

Assignment

Command(5)

SABM(6)Establish

Indication

(CM Service

Request)(8)

CM Service

Accepted(11)

Setup(12)

Call

Proceeding(13)

MS BTS1 MSCBSC1 BSC2 BTS2

RACH

AGCH

MS

Start T10

Start T8

Channel Activation

Acknowledge(18)

SDCCH

UA(7)CR (Complete

Layer3

Information)(9)

CC(10)

SDCCH

SDCCH

SDCCH

SDCCH

Handover

Access(22)

FACCH

PHY

INFO(25)

FACCH

SABM(27)

FACCH

UA(29)

FACCH

FACCH

Handover

Complete(30)

PHY

INFO(26)

FACCH

EstablishIndication(28)

Stop T10

Stop T8

ClearComplete(33)





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The inter-BSC directed retry procedure involves more A-interface signaling procedures than the

intra-BSC directed retry procedure. These signaling procedures are described subsequently. For

details about other signaling messages, refer to 16.1.1 Intra-BSC Directed Retry Procedure.

1. When the MS must be handed over to a cell under BSC 2, BSC 1 sends the MSC a Handover

Required message requesting the MS to initiate an outgoing inter-BSC handover.

2. Upon receiving the Handover Required message, the MSC sends BSC 2 a Handover

Request message.

3. BSC 2 activates a new channel and sends a Handover Request Acknowledge message to

notify the MSC that the channel is now available.

4. Upon receiving the Handover Request Acknowledge message, the MSC sends BSC 1 a

Handover Command message. BSC 1 forwards the Handover Command message on a TCH

to notify the MS of channel access.

5. Upon receiving the Handover Complete message from BSC 2, the MSC sends BSC 1 a

Clear Command message. BSC 1 initiates a local release procedure and sends the MSC a

Clear Complete message.

16.1.3 Inter-MSC directed retry procedure

This topic describes the inter-MSC directed retry procedure.

Figure 16-3shows the inter-MSC directed retry procedure.

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Figure 16-3Inter-MSC directed retry procedure

Assignment

Request(1)

Handover

Required(2) Perform

Handover(3) Handover

Request(4)

Channel

Activation(5)

Channel

Activation

Acknowledge(6)

Handover

Request

Acknowledge(7)Radio Channel

Acknowledge(8)

IAM(9)

ACM(10)Handover

Command(11)

Handover

Detect(17)

Send End

Signal(23)Clear

Command(24)

Handover

Access(13)Handover

Detect(14)

PHY INFO(15)

SABM(18)Establish

Indication(19

UA(20Handover

Complete(21)

MS BTS1 MSC1

BSC1 MSC2 BSC2

Start T10

FACCH

SDCCH

FACCH

PHY INFO(16)

FACCH

FACCH

FACCH

BTS2 MS

Start T8

FACCHStop T10

Handover

Complete(22)

Stop T8

Clear

Complete(25)

Hand

Command(12)

For details about the inter-MSC directed retry procedure, refer to 16.1.2 Inter-BSC directedretry procedure.

16.2 Internal BSC Signaling Procedure of Directed Retry

The internal BSC signaling procedure of directed retry is categorized by intra-BSC directed

retry, inter-BSC directed retry, and inter-MSC directed retry.

16.2.1 Internal BSC Signaling Procedure of the Intra-BSC Directed Retry

This describes the internal BSC signaling procedure of the intra-BSC directed retry.

16.2.2 Internal BSC Signaling Procedure of the Inter-BSC Directed RetryThis describes the internal BSC signaling procedure of the inter-BSC directed retry.





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16.2.3 Internal BSC Signaling Procedure of the Inter-MSC Directed Retry

This topic describes the internal BSC signaling procedure of the inter-MSC directed retry.

16.2.1 Internal BSC Signaling Procedure of the Intra-BSC Directed

RetryThis describes the internal BSC signaling procedure of the intra-BSC directed retry.

After the BSC receives the Assignment Request message from the MSC, if the cell where the

call is originated has no available TCHs or Assignment Cell Load Judge is enabled, and the

Directed Retry Allowedin the Cell Attributeswindow is selected, a directed retry procedure

is triggered.

NOTE

Directed retry is a special type of handover.

1. If the BSC considers that the call meets the handover trigger conditions, it selects a

candidate cell from the candidate cell listbased on the priority in a descending order. If the

candidate cell is under the same BSC as the serving cell, the BSC notifies the candidate

cell of initiating an intra-BSC directed retry procedure, and the candidate cell will be the

target cell.

2. A channel in the target cell is assigned and the BTS2 that serves the target cell is notified

to activate the channel.

3. After BTS2 activates the channel, the BSC notifies the MS to send a Handover Command

message to BTS1. Meanwhile, BSC2 starts timer T3103 to wait for the Handover Detect

message.

4. On receiving the Handover Detect message, BTS2 continues to wait for the Establishment

Indication message and the Handover Complete message.

5. After BTS2 receives the Handover Complete message, BSC2 stops timer T3103 and

considers that the call is set up in the target cell.

6. The BSC sends an Assignment Complete message to notify the MSC of the assignment

completion. The Assignment Complete message carries the information of the target cell.

At the same time, BSC1 notifies BTS1 to initiate a local release procedure.

16.2.2 Internal BSC Signaling Procedure of the Inter-BSC DirectedRetry

This describes the internal BSC signaling procedure of the inter-BSC directed retry.

After the BSC receives the Assignment Request message from the MSC, if the cell where the

call is originated has no available TCHs or Assignment Cell Load Judge is enabled, and the

Directed Retry Allowedin the Cell Attributeswindow is selected, a directed retry procedure

is triggered. If the target cell of the directed retry is an external cell, the inter-BSC directed retry

procedure is triggered.

NOTE

Directed retry is a special type of handover.

The BSC starts an eight-second timer awaiting the Handover Detect message from the BTS. At

the same time, it initiates a forced handover procedure.

1. If the BSC considers that the call meets the handover trigger conditions, it selects acandidate cell from the candidate cell list based on the priority in a descending order. If the

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candidate cell is an external cell, the outgoing BSC directed retry procedure, namely the

inter-BSC handover procedure is triggered.

2. Upon handover initiation, BTS1 sends the MSC a Handover Required message and starts

timer T17 to wait for a Handover Command message.

3. On receiving the Handover Request message from the MSC, BSC1 allocates and activatesa channel according to the target cell ID in the message. After channel activation, BTS2

sends the MSC a Handover Request Acknowledge message that carries the Handover

Command message.

4. BTS1 forwards to the MS the Handover Command message that BSC1 receives and at the

same time starts timer T8 to wait for the handover completion. If the Clear Command

message with the cause value Handover Success or Call Control is received before T8

expires, BSC 1 considers that the handover is successful. If the MS returns to the old channel

before T8 expires, BSC1 stops T8 and sends the MSC a Handover Failure message. If T8

expires, BSC1 considers that the call drops and sends the MSC a Clear Request message.

5. BTS2 forwards the Handover Detect message to the MSC and continues to wait for the

Establishment Indication message and the Handover Complete message.

6. On receiving the Handover Complete message, BTS2 stops timer T3103B2 and sends a

Handover Complete message to notify the MSC of handover completion.

7. On receiving the Clear Command message, BTS2 releases the radio resources the call

occupies and returns the MSC a Clear Complete message.

16.2.3 Internal BSC Signaling Procedure of the Inter-MSC DirectedRetry

This topic describes the internal BSC signaling procedure of the inter-MSC directed retry.

The internal BSC signaling procedure of the inter-MSC directed retry is the same as that of theinter-BSC directed retry.

16.3 Abnormal Directed Retry CasesThis topic describes the abnormal directed retry cases.

16.3.1 No Available Channels in the Target Cell

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

16.3.2 Channel Activation Failure


16.3.1 No Available Channels in the Target Cell


Fault

When no channels are available in the target cell, BSC 2 sends BSC 1 a handover failure message,

and BSC 1 sends the MSC an Assignment Failure message.

Probable Causes

No TCHs are available in the target cell.





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Handling Suggestions

Check whether the target cell is congested. If yes, add TRXs to the target cell.

16.3.2 Channel Activation FailureThis topic describes the fault, probable causes, and handling suggestions.

Fault

When channel activation in the target cell fails, BSC 2 sends the MSC an Assignment Failure

message.

Probable Causes

The desired channel in the target cell fails to be activated due to TRX failure.

Handling Suggestions

Locate the channel number in the Channel Activation Negative Acknowledge message, and

check whether the corresponding TRX fails. If the TRX fails, replace it.

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01-16 directed retry

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