01-12 Direct Retry Procedure

M900/M1800 Base Station Subsystem Signaling Analysis Manual Contents

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

Contents

12 Direct Retry Procedure..........................................................................................................12-1 12.1 Overview.......................................................................................................................................................12-2 12.2 Normal Procedure.........................................................................................................................................12-2

12.2.1 Intra-BSC Direct Retry .......................................................................................................................12-2 12.2.2 Inter-BSC Direct Retry Procedure......................................................................................................12-5 12.2.3 Inter-MSC Direct Retry Procedure.....................................................................................................12-6

12.3 Abnormal Cases............................................................................................................................................12-6

Figures M900/M1800 Base Station Subsystem

Signaling Analysis Manual

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

Figures

Figure 12-1 Intra-BSC direct retry procedure.....................................................................................................12-2

Figure 12-2 Inter-BSC direct retry procedure.....................................................................................................12-5

Figure 12-3 Inter-MSC direct retry procedure ....................................................................................................12-6

M900/M1800 Base Station Subsystem Signaling Analysis Manual 12 Direct Retry Procedure

Issue 01 (2007-03-15) Huawei Technologies Proprietary 12-1

12 Direct Retry Procedure

About This Chapter

The following table lists the contents of this chapter.

Title Description

12.1 Overview Introduces direct retry procedures.

12.2 Normal Procedure Describes normal direct retry procedures.

12.3 Abnormal Cases Describes abnormal direct retry procedures and offers guidance to troubleshooting

12 Direct Retry Procedure M900/M1800 Base Station Subsystem


12-2 Huawei Technologies Proprietary Issue 01 (2007-03-15)

12.1 Overview Direct retry procedure is a type of special handover procedure actually. In the assignment procedure, if there is not available radio resource in the selected cell, the MS is handed over to the adjacent cell. It is one type of handover from signaling channel to traffic channel.

12.2 Normal Procedure Based on the BSC and MSC to which the service cell and destination cell belong, direct retry can classify 3 types: Intra-BSC direct retry, inter-BSC direct retry and inter-MSC direct retry.

12.2.1 Intra-BSC Direct Retry

Signaling procedure

Figure 12-1 Intra-BSC direct retry procedure

BTS1 BSC MS MS BTS2MSC

Handover CMD

Channel ACT ACK (16)

(17)

Handover Access (18)

SABM (21)

EST IND (22)

PHY INFO (20)Handover Detect (19)

Handover Complete (24)

Channel ACT

Channel ACT ACK

IMM ASS CMD

Channel REQ

SABM

EST IND( CM Service REQ)CR(CM Service REQ)

CC

SetupCall Proceeding

ASS REQ

CM Service Accepted

UA

(1)

(3)

(4)

(5)

6)

(7)(8)

(9)

(10)

(11)

(12)(13)

(14)

Channel ACT (15)

Ass CMP (25)

(23)UA

Channel RQD (2)

Step 1 MS sends Channel Request to BTS on the access channel of the Um interface, and then BTS will transfer the message to BSC.

Step 2 BSC receives the Channel Required, allocates a signaling channel, and sends Channel Activation back to BTS.



Step 3 BTS receives the Channel Activation, turns on the power amplifier on the specified channel to receive information in the uplink direction if the channel type is correct, and sends Channel Activation Acknowledge to BSC.

Step 4 BSC receives the Channel Activation Acknowledge and sends IMM ASS CMD to MS informing MS of the access.

Step 5 When MS has been accessed successfully, MSC sends Assignment Request to BSC to request for allocation of the TCH resource. If a land circuit is needed between MSC and BSC, the Assignment Request message will contain the related land circuit information. If there is no TCH satisfying the requirement allocated to the current cell at this moment, BSC will retry a handover to hand MS over to the neighbor cell, which will provide speech service. If the destination cell is located in the same BSC with the current cell, this handover is an intra-BSC direct retry.

----End

Procedure Explanation Step 1 (1) ~ (8) in Figure 12-1 illustrates a random access and immediate assignment procedure, in

which BSS allocates signaling channel to MS.

Step 2 (14)~(24) in Figure 12-1 illustrates a TCH direct retry procedure, in which BSS allocates such resources as speech channel and A interface circuit for MS through handover.

----End

Internal handling of BSC Upon receiving the ASSIGNMENT REQUEST message from the MSC, if there is no TCH available in the cell where the MS is located and if the “Direct retry” field in the [Cell call control table] is set to “Yes”, the BSC shall initiate a directed retry handover procedure.

The GMPU in the BSC first start the 8-second timer to await the HANDOVER INDICATION message from the LAPD and then initiate a forced handover procedure, as described below:

Step 1 The GMPU sends a “Forced HANDOVER COMMAND” to the LAPD to initiate a forced handover procedure.

Step 2 The LAPD sends a HANDOVER INDICATION message that contains a CGI list of the neighbor cells to the BSC. Upon receipt of this message, the BSC stops the 8-second timer.

Step 3 The BSC selects a neighbor cell from the CGI list based on the cell priority (from high to low). If the selected neighbor cell is under management of the same BSC but there is no channel available, the BSC 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 BSC shall initiate a directed retry inter-BSC handover procedure (see the next section). If the selected neighbor cell with the highest priority is under management of the same BSC based on the CGI of the cell and that in the [Cell module information table] and there is a channel available, the BSC shall initiate a directed retry intra-BSC handover procedure. In this case, the selected neighbor cell is the target cell for the directed retry handover. The source cell where the call is initiated sends an “Internal HANDOVER REQUEST” to the target cell and starts the 2-second timer to await the handover preparation completion.

Step 4 Upon receipt of the “Internal HANDOVER REQUEST”, the target cell allocates a channel and notifies the target BTS (BTS2) to activate the allocated channel.




Step 5 Upon completion of 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.

Step 6 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].

Step 7 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.

Step 8 Upon receipt of the HANDOVER COMPLETE message from the MS, the target cell stops timer T3103B2 and notifies the relevant modules such 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.

Step 9 The target cell sends the MSC an ASSIGNMENT COMPLETE message that contains target cell information, informing the MSC that the assignment has been completed.

Step 10 Upon receipt of the “Internal Handover Success” message, the source cell initiates a local release procedure to release the relevant resources.

----End



12.2.2 Inter-BSC Direct Retry Procedure

Signaling procedure

Figure 12-2 Inter-BSC direct retry procedure

Clear CMD

Clear CMP

(31)

(32)

BTS1 BSC2 MS MS BTS2MSC

Handover CMD (21)

Channel ACT

Channel ACT ACK

ASS CMD

Channel REQ

SABM

EST IND( CM Service REQ)CR(CMP L3 information)

CC

Setup

Call ProceedingASS REQ

CM Service Accepted

UA

(1)

(3)

(4)

(5)

(6)

(7)(8)

(9)

(10)(11)

(12)

(13)(14)

Channel ACT ACK (18)

EST IND (27)


SABM (26)PHY INFO (25)

Handover Detect) (23)

Handover Complete (29)

Channel ACT (17)

Handover REQ

Handover REQ ACKHandover CMD

(16)(15)Handover RQD

(20)

Handover Detect (24)

(19)

BSC1

HandoverCMP (30)(28)UA

Channel REQ (2)

Procedure Explanation The explanation of the procedure is omitted.




12.2.3 Inter-MSC Direct Retry Procedure

Signaling procedure

Figure 12-3 Inter-MSC direct retry procedure

BTS1 BSC1MS MSC1

(2)Handover RQD

(5)Channel ACT

BTS2 MS

Handover REQ (4)

BSC2

Channel ACT ACK (6)


Handover REQ ACK (7)

SABMEstablish 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)

Ass REQ (1)

(20)UA

Procedure Explanation The explanation of the procedure is omitted.

12.3 Abnormal Cases The handover procedure might be incapable to work normally due to radio interface message loss, call interruption, user on-hook, transmission, or NSS/BSS running abnormally.

Since there are many causes for abnormal procedures, only the typical cases will be described here.

Inter-BSC direct retry failure due to no available channel in the destination cell When there is no available channel in the destination cell, this cell will respond with a handover failure message and the original cell will respond to MSC with Assignment Failure.



Intra-BSC handover retry failure due to channel activation failure In case of channel activation failure in the destination cell, BSC responds to MSC with Assignment Failure.

01-12 Direct Retry Procedure

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