OMF001003 GSM BSS Communication Flow

Course ContentCase Analysis Location Update Flow Call SetupOriginating Intra-BSC Handover Inter-BSC Handover BSC Normal Release BSC Local-end Release... SABM: Set asynchronous balanced

OMF001003 GSM BSS Communication Flow

OMF001003 GSM BSS Communication Flow

Huawei Wireless Training Department

• 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.

Course Content

Case Analysis

Location Update Flow Call Setup(Originating) Intra-BSC Handover Inter-BSC Handover BSC Normal Release BSC Local-end Release

I Location Update Flow

• Purpose of Location Update:

to facilitate paging.

When need the MS perform

Location Update ？

Location Update Flow

• Reference

 A-bis_logon saved

trace result file.)

• Note:

 The message num

ber in this page is t

he SN of the mess

ages in the result fil

e.

SABM: Set asynchronous balanced

mode command Initial access frame

UA: Unnumbered acknowledgement

IMMEDIATE ASSIGN COMMAND (5)

Establish_IND(Location Updating Req) (6)

CR(Complete_L3_

information)

Location Updating Accepted (10)

TMSI Reallocation Complete(NOTE 2) (11)

CC (NOTE 1)

Clear_CMP First SABM

• After establishing the SCCP connection between BSC and MSC t

here might be authentication and encryption in an intacte location update flow

 For Huawei MSC, authentication and encryption are all optional And n ormally authentication is mandatory and encryption is optional in practi cal 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 wil

l report “Identity Response” message which involves IMSI

 NOTE 1:Authentication, Encryption, Identity

 NOTE 1:Authentication, Encryption, Identity

To know more detailed refer to the BSC Normal Release Flow.

is optional in MSC.

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

 NOTE 2:

 NOTE 2:

 NOTE3:

 NOTE3:

Abnormal Occasion

message from MSC The possible reason is listed below:

II Call Setup Flow (Originating)

IMMEDIATE ASSIGN COMMAND (3)

Establish_IND(CM Service Req) (5)

(NOTE 3)

Disconnect (200) Release (201) Release Complete (203)

(NOTE 4) Clear_CMD Clear_CMP Conversation

(3) (4)

(18) (19) (21) (24) (25) (28) (32) (33)

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 Assignment TCH is directly ass igned When MSC sends “Assignment _Req” message, BSC will not assign new channel b

ut perform “Mode Modify” flow After “Mode Modify” BSS will report “Assignment _CMP” message to MSC.

Very Early Assignment

Abnormal Situation

nd BSC does not send “ASSIGNMENT CMD” but sends a

“Assignment_Fail” message Possible reason:

 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 BTS cannot receive the uplink signal correctly

message but MS sends “ASSIGNMENT FAIL” Possible re ason:

 Downlink bit error rate is too high

III Intra-BSC Handover

Establish_IND (128)

HANDOVER COMPLETE (132)

Handover_Performed

(NOTE 2)

• After reporting “Handover_Performed” message to MSC, BSC

 NOTE1:Asynchronous Handover and Synchronous Handover

 NOTE1:Asynchronous Handover and Synchronous Handover

 Intra-BSC asynchronous handover:

 BTS1 and BTS2 do not belong to the same base station

Thus the target BTS shall send PHY INFO message

 Intra-BSC synchronous handover:

 BTS1 and BTS2 belong to the same base station Actually they are too cells of one base site In such occasion there will not be PHY INFO message, After “Handover Access”

message, MS will send SABM very rapidly

 NOTE2 ：

 NOTE2 ：

Abnormal Occasion

• In case of intra-BSC asynchronous handover,

when BTS1 has sent PHY INFO message to the maximum time but still can’t the MS 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 channel in BTS1

 Abnormal Occasion

 Abnormal Occasion

IV Inter-BSC Handover

Measure Report from MS

BSC2(NOTE 1)

Handover_Required Handover_Request

Channel_Active Channel_Active_Ack Handover_Request_Ack

Handover_CMD

Handover_Detect Handover_Detect

PHY INFO PHY INFO First SABM Establish_INO

HANDOVER COMPLETE Handover_Cmp

Clear_CMD Clear_CMP

(NOTE 2)

 NOTE1: Two different handover occasions

will start local-end release flow for the previously occupied channel in BTS1.

wo occasions above.

V BSC Normal Release Flow

CHANNEL RELEASE (204)

Clear_CMD Clear_CMP

First DISC UA

Deactive_SACCH (205)

Release_IND (206)

RF_Release_Req (207)

RF_Release_Req_ACK (208)

• NSS originating release

• MS originating release

• In these two different occasions all the

messages are same except the direction of the first 3 messages.

 NOTE1 ： Two occasions of release

 NOTE1 ： Two occasions of release

VI Local-end Release Flow

VII Case Analysis

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 Description

 Case Description

Case Analysis

• By analyzing the messages we can find that MS has reported several EST IND message 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 frame 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

 Case Analysis

 Case Analysis

Case Analysis

• Conclusion:

 In the field when it appears many ASSIGNMENT FAIL

messages, 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 ASSIGNMENT FAIL via the previous SDCCH and the causing value is

“Protocol error unspecified”

Appendix: Power Class

Power Control Level

l evel

output power (dBm)

Tolerance _normal (dB)

Tolerance _extreme (dB)

Power control level

output power (dBm)

Tolerance _normal (dB)

Tolerance _extreme (dB)