OMF001003 GSM BSS Communication Flow Training 20060803 a 2.0

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OMF001003 GSM BSS Communication Flow ISSUE 2.0

HUAWEI TECHNOLOGIES CO., LTD.

www.huawei.com

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 Course Purpose:

 Grasp the means of interface trace and the method of analyzing it to aid us for fault handling.  How to learn this course:

 Copy the interface trace result files.  Use the maintenance console’s interface trace review function to study,  Attention:

 It is strictly forbidden to use the interface tracing function when the system is busy or the system is under heavy duty.


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Chapter 1 Location Update Flow Chapter 2 Call Setup (Originating) Chapter 3 Intra-BSC Handover

Chapter 4 Inter-BSC Handover Chapter 5 BSC Normal Release Chapter 6 BSC Local-end Release Chapter 7 Case Analysis


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Location Update Flow  Purpose of location update:

 To inform the system the LA in which the MS is to facilitate paging



When need the MS perform location update？ 

Getting into a new LA



Periodical location update


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Location Update Flow MS

BTS

Channel_req

command——initial access frame response

MSC

Channel_Required Channel_Active

SABM: Set asynchronous balanced mode

UA: Unnumbered acknowledgement

BSC

Channel_Active_Ack IMMEDIATE ASSIGN COMMAND

First SABM

Establish_IND(Location Updating Req)

UA

CR(Complete_L3_ information) CC

Location Updating Accepted

(NOTE 1)

TMSI Reallocation Complete(NOTE 2) (NOTE 3)

Clear_CMD Clear_CMP


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Notes 

NOTE 1:Authentication, Encryption, Identity

 After establishing the SCCP connection between BSC and MSC

there might be authentication and encryption in an intact location update flow.  For Huawei MSC, authentication and encryption are all optional. And normally authentication is mandatory and encryption is optional in practical situation.  Huawei BSS system supports encryption algorithm A5/1 and A5/2.  Identification: The ID in Est_IND message reported by the MS is

TMSI. When VLR cannot identify the TMSI or TMSI authentication fails, MSC will start Identification flow. That means MSC will send transparent transfer message “Identity Request” to MS and MS will report “Identity Response” message which involves IMSI. HUAWEI TECHNOLOGIES CO., LTD.

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Notes NOTE 2:



 To reallocate TMSI or not during location update is

optional in MSC.  If MSC chooses “Reallocate TMSI during location

update” to “No”, MS will not report “TMSI Reallocation Complete” message. 

NOTE3:  In this flow we have simplified the release flow.

To know more detailed refer to the BSC Normal Release Flow. HUAWEI TECHNOLOGIES CO., LTD.

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Abnormal Occasion  BSC receives “Location Updating Rejected” message from

MSC. The possible reason is listed below:  A. MSC has not configured the CGI which has been configured in BSC.  B. Communication failed between MSC and VLR.  C. MS has not been registered in HLR.


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Call Setup Flow (Originating) MS

BTS

BSC MSC Channel_Required Channel_Active Channel_Active_Ack IMMEDIATE ASSIGN COMMAND First SABM Establish_IND(CM Service Req) CR(Complete_L3_information) UA CC (NOTE 1) CM Service Accepted Setup Call Processing (NOTE 2) Assignment_Req Channel_Active Channel_Active_Ack ASSIGNMENT COMMAND First SABM Establish_IND UA ASSIGNMENT CMP Assignment_CMP Alerting Connect Connect Ack Conversation (NOTE 3) Disconnect Release Release Complete Clear_CMD (NOTE 4) Clear_CMP

Channel_req


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NOTE 1: Class Mark Update Flow

1. Authentication, encrytion, identification are optional. See Note 1 in Location Update Flow.

2. Class mark update flow: For dual-band MS, after the SCCP connection is established and before receiving message "CM Service Accepted" there will be a class mark update flow. CR message reports the information of class mark 2. And in the class mark update flow, MS will report class mark 3 information which is related to the dual-band capabilty.


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NOTE 2: Assignment Method Very Early Assignment

In “IMMEDIATE ASSIGN COMMAND ” ,TCH is directly assigned. When MSC sends “Assignment _Req” message, BSC will not assign new channel but perform “Mode_Modify” flow. After that, MS will report “Assignment _CMP” message to MSC. Early Assignment

MSC sends “Assignment_Req” before “Alerting”.

Very Early Assignment

Late Assignment

MSC sends “Assignment_Req” after ”Alerting”.


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Abnormal Situation  Abnormal Occasion 1: MSC has sent “Assignment_Req” and BSC

does not send “ASSIGNMENT CMD” but sends a “Assignment_Fail” message. Possible reasons:  A. BSC has not available TCH.  B. The CIC assigned by the MSC is not idle in BSC.  C. There are transmission faults or interference in the uplink, so BTS cannot receive the uplink signal correctly.  Abnormal Occasion2: BSC has sent “ASSIGNMENT CMD message

but MS sends “ASSIGNMENT FAIL”. Possible reason:  Downlink bit error rate is too high.


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Intra-BSC Handover MS

BTS1

BSC

BTS2

MS

Measure Report from MS Channel_Active Channel_Active_Ack HANDOVER COMMAND Handover Access (NOTE 1)

Handover_Detect

PHYINFO PHYINFO First SABM UA

Establish_IND

HANDOVER COMPLETE Handover_Performed (NOTE 2)


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MSC

Notes 

NOTE1:Asynchronous Handover and Synchronous Handover

 Intra-BSC asynchronous handover:

 BTS1 and BTS2 do not belong to the same BTS. Thus the target BTS shall send PHY INFO message.  Intra-BSC synchronous handover:

 BTS1 and BTS2 belong to the same BTS. Actually they are two cells of one BTS. In such occasion there will not be PHY INFO message, After “Handover Access” message, MS will send SABM directly.



NOTE2：  After reporting “Handover_Performed” message to MSC, BSC will

start the local-end release flow for the previous channel (in BTS 2).


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Abnormal Occasion 

Abnormal Occasion

 In case of intra-BSC asynchronous handover, when BTS1 has

sent PHY INFO message to the maximum time but the MS still can’t access the new channel, BTS1 will send CONN FAIL IND message. Its causing value is “handover access failure”. When

BSC receives this message, it will start local-end release flow to release the new channel in BTS1.


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Inter-BSC Handover MS

BTS1

BSC1

MSC

BSC2

BTS2

MS

(NOTE 1) Measure Report from MS Handover_Required Handover_Request

Channel_Active

Channel_Active_Ack Handover_Request_Ack Handover_CMD HANDOVER COMMAND

Handover_Detect Handover Access Handover_Detect

PHY INFO PHY INFO First SABM Establish_INO

(NOTE 2)

Clear_CMD Clear_CMP


Handover_Cmp

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UA

HANDOVER COMPLETE

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Notes NOTE1: Two different handover occasions of inter-BSC handover



 Intra-MSC inter-BSC Handover:

 Two BSCs belong to the same MSC.  Inter-MSC Handover:

 Two BSCs do not belong to the same MSC.  The A-interface messages are the same for the two occasions above. 

NOTE2：

 When BSC1 receives Clear_CMD message from MSC, it will start local-

end release flow for the previously occupied channel in BTS1.


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BSC Normal Release Flow MS

BTS (NOTE 1)

BSC

MSC

Disconnect Release Release Complete Clear_CMD

CHANNEL RELEASE

Clear_CMP

Deactive_SACCH First DISC UA

Release_IND RF_Release_Req RF_Release_Req_ACK


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Notes 

NOTE1：Two occasions of release

 NSS originating release  MS originating release  In these two different occasions all the messages are

the same except the direction of the first 3 messages.


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Local-end Release Flow MS

BTS

BSC

MSC

Deactive_SACCH Release_Req(LOCAL END) (NOTE 1)

Release_Confirm RF_Release_Req RF_Release_Req_ACK


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Case Description  BSC’s ASSIGNMENT CMD message has been sent but the

BTS sends several EST IND messages. Later MS sends ASSIGNMENT FAIL message and the causing value is Protocol error unspecified.


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Case Analysis

 By analyzing the messages we can find that MS has reported several

EST IND messages before ASSIGNMENT FAIL message.  When BTS receives a SABM frame it will report an EST IND message

to BSC. So we know now that the MS must have sent many SABM frames to the BTS.  BTS can decode the SABM frame sent by MS but MS cannot decode

the UA frame sent by BTS. And this causes that the BTS reports many EST IND messages.


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Case Analysis 

Case realize:  In

the lab we can simulate. Disconnect the TX antenna to make the downlink signal pure. Trace the A-bis signaling we can find that BTS sends several EST IND messages to BSC for the newly assigned TCH but can’t succeed. And then MS sends ASSIGNMENT FAIL via the previous SDCCH and the causing value is “Protocol error unspecified”.

 Conclusion:

 In the practical field, when many ASSIGNMENT FAIL messages appear, the main reason might be the pure downlink receiving power level or receiving quality.

 Connection between TRX and SCU (CDU).


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Appendix: Power Class  Phase 2 MS output Power Class and the exact power relations.

p o w e r G S M 9 0 0 M a x D C S 1 8 0 0 M a x T o l e r a n c e ( d B ) T o l e r a n c e ( d B ) p e a k p o w e r p e a k p o w e r_ n o r m a l _ e x t r e m e c l a s s 1 2

1 W ( 3 0 d B m ) + / 2

+ / 2 . 5

0 . 2 5 W ( 2 4 d B m ) 2

2 . 5

3 5 W ( 3 7 d B m )4 W ( 3 6 d B m ) 2

2 . 5

4 2 W ( 3 3 d B m )

2

2 . 5

5 0 . 8 W ( 2 9 d B m )

2

2 . 5


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Power Control Level Phase 2 MS

Power control level and the exact power relations.

p o w e r o u t p u t T o l e r a n c eT o l e r a n c eP o w e ro u t p u tT o l e r a n c eT o l e r a n c e c o n t r o l p o w e r _ n o r m a l _ e x t r e m ec o n t r o l p o w e r _ n o r m a l _ e x t r e m e d B m ) ( d B ) ( d B ) l e v e l ( d B m ) ( d B ) ( d B ) e v e l ( l 0 2 3 9 2 2 . 5 2 9 3 6 2 2 . 5 3 3 7 3 4 3 0 3 4 3 4 4 3 5 3 4 3 1 3 2 3 4 5 3 3 3 4 0 3 0 3 4 6 3 1 3 4 1 2 8 3 4 7 2 9 3 4 2 2 6 3 4 8 2 7 3 4 3 2 4 3 4 9 2 5 3 4 4 2 2 3 4 1 0 2 3 3 4 5 2 0 3 4 1 1 2 1 3 4 6 1 8 3 4 1 2 1 9 3 4 7 1 6 3 4 1 3 1 7 3 4 8 1 4 3 4 1 4 1 5 3 4 9 1 2 4 5 1 5 1 3 3 4 1 0 1 0 4 5 1 6 1 1 5 6 1 1 8 4 5 1 7 9 5 6 1 2 6 4 5 1 8 7 5 6 1 3 4 4 5 1 9 3 1 5 5 6 1 4 2 5 6 1 5 2 8 0 5 6


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Thank You www.huawei.com

OMF001003 GSM BSS Communication Flow Training 20060803 a 2.0

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