omf001003 gsm bss communication flow-training-20060803-a-2.0.ppt

7/27/2019 OMF001003 GSM BSS Communication Flow-training-20060803-A-2.0.ppt

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HUAWEI TECHNOLOGIES CO., LTD. All rights reserved

www.huawei.com

Internal

OMF001003

GSM BSS Communication Flow

ISSUE 2.0


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

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 consoles interface trace review

function to study,

Attention:

It is strictly forbidden to use the interface tracing functionwhen 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

SABM: Set asynchronous balanced mode

commandinitial access frame

UA: Unnumbered acknowledgement

response

MS BTS BSC MSC

Channel_req Channel_Required

Channel_ActiveChannel_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


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Notes

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.

NOTE 1:Authentication, Encryption, Identity


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Notes

In this flow we have simplified the release flow.

To know more detailed refer to the BSC Normal

Release Flow.

NOTE 2:

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.


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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_req Channel_RequiredChannel_Active

Channel_Active_Ack

IMMEDIATE ASSIGN COMMAND

Establish_IND(CM Service Req)First SABM

CM Service Accepted

CR(Complete_L3_information)

CC(NOTE 1)

Setup

Call Processing

ASSIGNMENT COMMANDFirst SABM Establish_IND

Channel_Active

Channel_Active_Ack

(NOTE 2) Assignment_Req

Assignment_CMPASSIGNMENT CMPAlerting

Connect

Connect Ack(NOTE 3)

Disconnect

ReleaseRelease Complete

(NOTE 4)Clear_CMDClear_CMP

Conversation

UA

UA


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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 SCCPconnection 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.

Late Assignment

MSC sends Assignment_Req

after Alerting.

Very Early Assignment


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Abnormal Situation

Abnormal Occasion 1: MSC has sent Assignment_Req and BSCdoes 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 HandoverMS BTS1 BSC BTS2 MS MSC

Channel_ActiveChannel_Active_Ack

Measure Report from MS

HANDOVER COMMANDHandover Access

Handover_DetectPHYINFO

PHYINFO

First SABM

Establish_IND

HANDOVER COMPLETE

Handover_Performed

(NOTE 2)

(NOTE 1)

UA


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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


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Abnormal Occasion

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

sent PHY INFO message to the maximum time but the MS stillcant 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


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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 SABM

Establish_INO

HANDOVER COMPLETEHandover_Cmp

Clear_CMD

Clear_CMP(NOTE 2)

UA


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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.


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BSC Normal Release Flow

MS BTS BSC MSC

(NOTE 1) DisconnectRelease

Release Complete

CHANNEL RELEASEClear_CMDClear_CMP

First DISC

Deactive_SACCHRelease_IND

RF_Release_ReqRF_Release_Req_ACK

UA


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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.

NOTE1Two occasions of release


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Local-end Release Flow

MS BTS BSC MSC

Deactive_SACCH

Release_ConfirmRF_Release_Req

RF_Release_Req_ACK

Release_Req(LOCAL END) (NOTE 1)


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Case Description

BSCs 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

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

newly assigned TCH but cant succeed. And then MS sends

ASSIGNMENT FAIL via the previous SDCCH and the causing

value is Protocol errorunspecified.


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Appendix: Power Class

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

power

class

GSM900Max

peakpower

DCS1800Max

peakpower

Tolerance(dB)

_normal

Tolera

_extr

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

2 0.25W(24dBm)2 2.

35W(37dBm)4W(36dBm)2 2.

42W(33dBm) 2 2.

50.8W(29dBm) 2 2.


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Power Control Level

Phase 2 MS

Power control

level and the

exact power

relations.

power

controllevel

output

power(dBm)

Tolerance

_normal(dB)

Tolerance

_extreme(dB)

Power

controllevel

output

power(dBm)

Tolerance

_normal(dB)

Tolera

_extre(d)

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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