OMF001003 GSM BSS Communication Flow Training 20060803 a 2.0

HUAWEI TECHNOLOGIES CO., LTD.

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OMF001003 GSM BSS Communication Flow

ISSUE 2.0

HUAWEI TECHNOLOGIES CO., LTD. Page 2All rights reserved

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.


Chapter 1 Chapter 1 Location Update FlowLocation Update Flow

Chapter 2 Chapter 2 Call Setup (Originating)Call Setup (Originating)

Chapter 3 Chapter 3 Intra-BSC Handover Intra-BSC Handover

Chapter 4 Chapter 4 Inter-BSC HandoverInter-BSC Handover

Chapter 5 Chapter 5 BSC Normal ReleaseBSC Normal Release

Chapter 6 Chapter 6 BSC Local-end ReleaseBSC Local-end Release

Chapter 7 Chapter 7 Case AnalysisCase Analysis


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


Location Update Flow

SABM: Set asynchronous balanced mode

command——initial access frame

UA: Unnumbered acknowledgement

response

MS BTS BSC MSC

Channel_req Channel_Required

Channel_Active

Channel_Active_Ack

IMMEDIATE ASSIGN COMMAND

Establish_IND(Location Updating Req)

CR(Complete_L3_information)

Location Updating Accepted

TMSI Reallocation Complete(NOTE 2)

CC

(NOTE 1)

(NOTE 3) Clear_CMD

Clear_CMP

First SABM

UA


Notes

After establishing the SCCP connection between BSC and MSC ther

e 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 optio

nal 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 TM

SI. When VLR cannot identify the TMSI or TMSI authentication fails,

MSC will start Identification flow. That means MSC will send transpar

ent transfer message “Identity Request” to MS and MS will report “I

dentity Response” message which involves IMSI.

NOTE 1:Authentication, Encryption, Identity NOTE 1:Authentication, Encryption, Identity


Notes

In this flow we have simplified the release flow.

To know more detailed refer to the BSC Normal

Release Flow.

NOTE 2: NOTE 2:

NOTE3: NOTE3:

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.


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.










Call Setup Flow (Originating)MS BTS BSC MSC

Channel_req Channel_Required Channel_Active

Channel_Active_Ack IMMEDIATE ASSIGN COMMAND

Establish_IND(CM Service Req)First SABM

CM Service Accepted

CR(Complete_L3_information)CC

(NOTE 1)SetupCall Processing

ASSIGNMENT COMMANDFirst SABM Establish_IND

Channel_ActiveChannel_Active_Ack

(NOTE 2) Assignment_Req

Assignment_CMPASSIGNMENT CMPAlertingConnect

Connect Ack(NOTE 3)

DisconnectRelease

Release Complete

(NOTE 4)Clear_CMDClear_CMP

Conversation

UA

UA


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.


NOTE 2: Assignment Method

Very Early Assignment

In “IMMEDIATE ASSIGN COMMAND ” ,TCH i

s directly assigned. When MSC sends “Assign

ment _Req” message, BSC will not assign new

channel but perform “Mode_Modify” flow. After

that, MS will report “Assignment _CMP” messa

ge to MSC.

Early Assignment

MSC sends “Assignment_Req” before “Alertin

g”.

Late Assignment

MSC sends “Assignment_Req” after ”Alerting”.

Very Early Assignment


Abnormal Situation

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

s not send “ASSIGNMENT CMD” but sends a “Assignment_Fail” mess

age. 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 BT

S cannot receive the uplink signal correctly.

Abnormal Occasion2: BSC has sent “ASSIGNMENT CMD message bu

t MS sends “ASSIGNMENT FAIL”. Possible reason:

Downlink bit error rate is too high.




Chapter 3 Chapter 3 Intra-BSC HandoverIntra-BSC Handover






Intra-BSC HandoverMS BTS1 BSC BTS2 MS MSC

Channel_Active

Channel_Active_Ack

Measure Report from MS

HANDOVER COMMAND

Handover AccessHandover_Detect

PHYINFO

PHYINFO

First SABM

Establish_IND

HANDOVER COMPLETE

Handover_Performed

(NOTE 2)

(NOTE 1)

UA


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.

NOTE1:Asynchronous Handover and Synchronous Handover

Notes

After reporting “Handover_Performed” message to MSC, BSC will

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

NOTE2 ：


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.

Abnormal Occasion










Inter-BSC Handover

MS BTS1 BSC1 BTS2 MSMSC

Measure Report from MS

BSC2

(NOTE 1)

Handover_Required Handover_RequestChannel_Active

Channel_Active_AckHandover_Request_Ack

Handover_CMD

Handover_Detect

Handover_Detect

HANDOVER COMMAND Handover Access

PHY INFO

PHY INFO

First SABMEstablish_INO

HANDOVER COMPLETEHandover_CmpClear_CMD

Clear_CMP(NOTE 2)

UA


NOTE1: Two different handover occasions of inter-BSC handover

NOTE2 ：

Notes

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

end release flow for the previously occupied channel in BTS1.

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.










BSC Normal Release Flow

MS BTS BSC MSC(NOTE 1) Disconnect

Release

Release Complete

CHANNEL RELEASE

Clear_CMD

Clear_CMP

First DISC Deactive_SACCH

Release_IND

RF_Release_Req

RF_Release_Req_ACK

UA


Notes

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.

NOTE1 ： Two occasions of release










Local-end Release Flow

MS BTS BSC MSC

Deactive_SACCH

Release_Confirm

RF_Release_Req

RF_Release_Req_ACK

Release_Req(LOCAL END) (NOTE 1)










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.


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.


Case Analysis

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).

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 newl

y assigned TCH but can’t succeed. And then MS sends ASSIG

NMENT FAIL via the previous SDCCH and the causing value is

“Protocol error unspecified”.


Appendix: Power Class

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

power class

GSM900 Max peak power

DCS1800 Max peak power

Tolerance(dB)_normal

Tolerance(dB)_extreme

1 ------ 1W(30dBm) +/-2 +/-2.5

2 0.25W(24dBm) 2 2.5

3 5W(37dBm) 4W(36dBm) 2 2.5

4 2W(33dBm) 2 2.5

5 0.8W(29dBm) 2 2.5


Power Control Level

Phase 2 MS

Power control

level and the

exact power

relations.

power control level

output power (dBm)

Tolerance_normal

(dB)

Tolerance _extreme

(dB)

Power control level

output power (dBm)

Tolerance_normal

(dB)

Tolerance _extreme

(dB)

0-2 39 2 2.5 29 36 2 2.5

3 37 3 4 30 34 3 4

4 35 3 4 31 32 3 4

5 33 3 4 0 30 3 4

6 31 3 4 1 28 3 4

7 29 3 4 2 26 3 4

8 27 3 4 3 24 3 4

9 25 3 4 4 22 3 4

10 23 3 4 5 20 3 4

11 21 3 4 6 18 3 4

12 19 3 4 7 16 3 4

13 17 3 4 8 14 3 4

14 15 3 4 9 12 4 5

15 13 3 4 10 10 4 5

16 11 5 6 11 8 4 5

17 9 5 6 12 6 4 5

18 7 5 6 13 4 4 5

19-31 5 5 6 14 2 5 6

15-28 0 5 6

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OMF001003 GSM BSS Communication Flow Training 20060803 a 2.0