gsm bss communication flow.ppt

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GSM BSS Communication Flow

Foreword• Course Purpose:

– Understand the typical communication flow– Grasp the means of interface trace and trace review– Grasp the method of analyzing communication flow to aid us for fault

handling.

• How to learn this course:– With the help of stored tracing files, use tracing review function in the

maintenance console to study.

• Attention:– It is strictly forbidden to use the interface tracing function when the system

is under heavy traffic.

MS-MSC Signaling Stack

CM

MM

RR

LAPDm

SigL1

L3

L2

L1

MS

RR BTSM

LAPDm

SigL1

LAPD

SigL1

LAPD

SigL1

BTSM

RR

SCCPMTP

BSSMAP

CM

MM

BSSMAP

SCCPMTP

MSCBSCBTS

Um接口 A接口Abis接口Um Abis A

BSSAP Functional Structure• BSSAP ： BSS Application Part

– BSSMAP ： BSS Management Application Part– DTAP ： Direct Transfer Part

• CM• MM

• SCCP ： Signaling Connection Control Part• MTP ： Message Transfer Part

BSSAP

SCCP

MTP-3

MTP-2

MTP-1

Contents1. Location Update 2. Call Setup 3. Handover 4. Direct Retry 5. Short Message6. Release7. Case Analysis

Location Update Basis

• Purpose of location update:– To inform the system the LA in which the MS is going to

facilitate paging

When does the MS need to perform location update？ Getting into a new location area

Periodical location update timer expire.

Location Update FlowMS 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

CC

(NOTE 1)

(NOTE 3)

Clear_CMD

Clear_CMP

First SABM

UA

(NOTE 2)

NOTE 1: Authentication, Encryption, Identification

• 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.

Authentication Flow

MS BTS BSC MSC

Authentication Request

Authentication Response

NOTE 2

• To reallocate TMSI or not during location update is decided in MSC side configuration.

• If MSC chooses “Reallocate TMSI during location update” to “No”, TMSI will not be refreshed during location update.

Abnormal Occasion• BSC receives “Location Updating Rejected”

message from MSC. The possible reasons are:– The CGI which has been configured in BSC side

is not configured in MSC side.(wrong CGI)– Communication failure between MSC and VLR.– The MS is not registered in HLR.

Call Setup Flow (MOC)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

DisconnectRelease

Release Complete

(NOTE 3)Clear_CMDClear_CMP

Conversation

UA

UA

Call Setup Flow (MTC)MS BTS BSC MSC



Establish_IND(Paging RESP)First SABM

CM Service Accepted


(NOTE 1)

SetupCall Confirmed



(NOTE 2) Assignment_Req

Assignment_CMPASSIGNMENT CMPAlertingConnect

Connect Ack

DisconnectRelease

Release CompleteClear_CMDClear_CMP

Conversation

UA

UA

PagingPaging CMDPaging CMD

NOTE 1: Class Mark Update Flow

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 AssignmentMSC sends “Assignment_Req” before “Alerting”.

• Late AssignmentMSC sends “Assignment_Req” after ”Alerting”.

NOTE 2: Assignment Method• Very Early Assignment

MS BTS BSC MSCChannel_req Channel_Required

Channel_Active

Channel_Active_Ack IMMEDIATE ASSIGN COMMAND (TCH)

Establish_IND(CM Service Req)First SABM

CM Service Accepted

CR(Complete_L3_information)

CC

Setup

Call Processing

Channel Mode Modify

Mode_Modify

Mode_Modify_Ack

Assignment_Req

Assignment_CMPChannel Mode Modify ACKAlerting

UA

NOTE 2: Assignment Method• Late Assignment (MOC)

MS BTS BSC MSCChannel_req Channel_Required


IMMEDIATE ASSIGN COMMANDEstablish_IND(CM Service Req)First SABM

CM Service Accepted


SetupCall Processing



Assignment_Req

Assignment_CMPASSIGNMENT CMP

Alerting

ConnectConnect Ack

Conversation

UA

UA

NOTE 2: Assignment Method• Late Assignment (MTC)

MS BTS BSC MSC



Establish_IND(Paging RESP)First SABM

CM Service Accepted


SetupCall Confirmed



Assignment_Req

Assignment_CMPASSIGNMENT CMP

AlertingConnect

Connect AckConversation

UA

UA

PagingPaging CMDPaging CMD

Abnormal SituationMS BTS BSC MSC

Call Processing

Assignment_Req

Assignment_Fail

ASSIGNMENT COMMAND

Channel_Active_Ack

ASSIGNMENT FAIL

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 COMPLETEHandover_Performed

(NOTE 2)

(NOTE 1)

UA

NOTE1:Asynchronous/Synchronous Handover

• Intra-BSC asynchronous handover:– cell1 and cell2 do not belong to the same BTS.

Thus the target BTS shall send PHY INFO message.

• Intra-BSC synchronous handover:– cell1 and cell2 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.

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.

Inter-BSC HandoverMS 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_IND

HANDOVER COMPLETEHandover_CmpClear_CMD

Clear_CMP(NOTE 2)

UA

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.

Direct Retry – Intra-BSCMS BTS1 BSC MSC


Channel_Active

Channel_Act_Ack

IMMEDIATE ASSIGN CMD

Est_IND

First SABM

CM Service Accepted

CR(CMP_L3_info)

CC

Setup

Call Processing

Handover CMD

PHY INFO

Establish_IND

Channel_Active

Channel_Active_Ack

Assignment_Req

Assignment_CMP

Handover CMP

UA

BTS2 MS

Handover AccessHandover Detect

First SABM

UA

Direct Retry – Inter-BSCMS BTS1 BSC1 MSC

CM Service Accepted

Setup

Call Processing

Handover CMD

PHY INFO

Channel_Active

Channel_Act_Ack

Assignment_Req

Handover RQD

Handover CMP

BSC2 BTS2

Handover Access

First SABM

MS

Clear CMP

Handover REQ

Handover REQ AckHandover CMD

Handover DetectHandover Detect

EST_IND

Handover CMPClear CMD

UA

Short Message – MO on SDCCHMS BTS BSC MSCChannel_req Channel_Required

Channel_Active


EST_IND(CM Service Req)

First SABM

CP DATA


CP ACK

Disconnect

Release Complete

UA

First SABM(SAPI 3)UA EST_IND(SAPI 3)

Release

Clear CMDChannel Release

Disconnect

From SC (Service Centre )

Deactive SACCH Release IND

RF Channel REL RF Channel REL ACK Clear CMP

RLSDRLSD CMP

CP DATACP ACK

To SC (Service Centre )

Short Message – MT on SDCCHMS BTS BSC MSC



IMMEDIATE ASSIGN COMMAND

EST_IND (Paging Response)

First SABM

CP DATA

CR(Complete_L3_infor)CC

CP ACK

Disconnect

Release Complete

UA

First SABMUA

EST_REQ(SAPI 3)

Release

Clear CMD

Paging CMDPaging CMD

EST_CNF(SAPI 3)

Clear CMP

CP DATA

From SC

To SCCP DATACP ACK

Short Message – MO on SACCHMS BTS BSC MSC

ACTIVE CALL

CP DATA

CP ACK

First SABM(SAPI 3)UA EST_IND(SAPI 3)

CM Service ACC (FACCH)

CM Service REQ (FACCH)

CP DATA

CP ACK

From SC

To SC

ACTIVE CALL

Short Message – MT on SACCHMS BTS BSC MSC

ACTIVE CALL

CP DATA

CP ACK

First SABM(SAPI 3)UA

EST_IND(SAPI 3) CP DATA

CP DATA

CP ACK

EST_CNF(SAPI 3)

From SC

To SC

ACTIVE CALL

Contents1. Location Update 2. Call Setup 3. Handover 4. Direct Retry 5. Short Message6. Call Release7. Case Analysis

BSC Normal Release FlowMS BTS BSC MSC

(NOTE 1) Disconnect

Release

Release Complete

CHANNEL RELEASEClear_CMD

Clear_CMP

First DISC Deactive_SACCH

Release_IND

RF_Release_Req

RF_Release_Req_ACK

UA

RLSD

RLSD CMP

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.

Local-end Release FlowMS 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 an 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 caused the BTS to report many EST IND messages.

Case Analysis

• Conclusion:

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

– Connection between TRX and (CDU).(ie duplexer in case of BTS30X….series)

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”.

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

gsm bss communication flow.ppt

Documents

tmsi authentication

intact location update

location updategetting

contentslocation update

huawei msc

identification flow

setup flow mocpage

setup handover