OMF000404 Case Analysis-Call Drop ISSUE2.0

 In intra-BSC handover procedure, the target cell sends Inter Clear Request to the source cell when the timer for the target cell to wait for HANDOVER COMPLETE from MS expires, in this

OMF000404 Case Study – Call Drop

Issue 2.0

References

 31160978-BSC Traffic Statistic Manual Volume I

 31033203-BSS Troubleshooting Manual

Upon completion of this course, you are

supposed to be able to:

 Understand the principles of call drop.

 Analyze and solve call drop problems

Chapter 1 Principle of call drop

Chapter 2 Analysis of call drop Chapter 3 Call drop cases

Chapter 1 Principle of call drop

Section 1 Definition of call drop Section 2 TCH call drop

Section 3 SDCCH call drop

Definition of Call Drop

 There are two types of call drop: TCH call drop and SDCCH call drop:

 TCH call drop means TCH channel is released abnormally after it is occupied successfully

 SDCCH call drop means SDCCH channel is released abnormally after it

is occupied successfully

Chapter 1 Principle of call drop

Section 1 Definition of call drop Section 2 TCH call drop

Section 3 SDCCH call drop

Causes of TCH Call Drop

 Usually, the typical causes for sending the Clear_Request message are as follows:

 Radio interface failure

 O&M intervention

 Equipment failure

 Protocol error

 Preemption

Formula of TCH Call Drop Rate

 Formula of TCH call drop rate

 TCH call drop rate=TCH call drops / Successful TCH seizures (all)

Measurement Point of TCH Call Drop

Measurement Point of TCH Call Drop

 TCH serves as traffic channel, BSC receives ERROR INDICATION message from BTS

 TCH is seized as traffic channel, BSC receives CONNECT FAILURE INDICATION message from BTS

 In assignment procedure and handover procedure, failure of decoding HANDOVER DETECTION and HANDOVER COMPLETE message

 TCH serves as traffic channel, incoming BSC handover initiated and the timer for the target cell to wait for HANDOVER COMPLETE message expires

 TCH serves as traffic channel, outgoing BSC handover initiated and the timer for the source cell to wait for CLEAR COMMAND message from MSC expires (T8 expires)

 In intra-BSC handover procedure, the target cell sends Inter Clear Request

to the source cell when the timer for the target cell to wait for HANDOVER COMPLETE from MS expires, in this case, this item is measured in the source cell

Measurement Point of TCH Call Drop

 In Intra-BSC handover procedure, the source cell, excluding the source cell for directed retry, measures the item when the timer for the source cell to wa

it for Inter Clear Request with cause value HANDOVER COMPLETE from th

e target cell expires

 In Intra-BSC handover procedure, when the target cell AM/CM net-drive fail

s (due to timeout or negative acknowledgement), the target cell sends Inter Clear Request to source cell, in this case, this item is measured in the sourc

e cell

 In the case that MS reverses to the original channel after intra BSC handove

r fails, the source cell first releases the terrestrial connection but the AM/CM re-net-drive fails (due to timeout or negative acknowledgement) In this case this item is measured in the source cell

 The resource of the lower_priority call will be preempted by the higher_priori

ty call if MSC and BSC both supports preemption, which will cause call drop

 This item is measured when the RSL link of the TRX that the call is using di

TCH Call Drop - Assignment

A1

ERROR INDICATION CONNECTION FAILURE INDICATION

A2

TCH Call Drop – Intra-BSC Handover

TCH Call Drop - Incoming BSC Handover

SABM

HANDOVER REQUEST

Measurement Point of Successful TCH Seizures

 Measurement point of Successful TCH seizures

 Upon BSC’s reception of CHANNEL ACTIVATION ACKNOWLEDGE message from BTS in very early assignment TCH process

 In the case the target cell of directed retry is located in other BSC and directed retry succeeds, MSC sends CLEAR COMMAND message to the originating BSC to release the original connection This item is measure

d when the originating BSC receives this message

 In the case the directed retry target cell is located in the same BSC and the directed retry succeeds, target cell sends Inter Clear Request message to the source cell to request to release the resource and the original connection This item is measured when the source cell receives this message

Measurement Point of Successful TCH seizures

 Measurement point of Successful TCH seizures

 This item is measured when BSC sends ASSIGNMENT COMPLETE message to MSC after the assignment procedure is successfully implemented

 In incoming BSC handover procedure, MS sends HANDOVER ACCESS message to the BSC This item is measured when BSC receives HANDOVER DETECT message from BTS

 In the process of incoming internal inter cell handover and intracell handover in BSC, MS sends HANDOVER ACCESS message to BSC BSC measures this item in the target cell when receiving HANDOVER DETECTION message from BTS

 Very early assignment process

Channel Request (RACH) Channel Required

Channel Activation (TCH or SDCCH)Channel Activation AcknowledgeImmediate Assignment CommandImmediate Assignment (AGCH)

Successful TCH Seizure – Very Early Assignment

Successful TCH Seizure – Assignment

 Assignment process

ASS_REQ CH_ACT

CH_ACT_ACK

ASS_CMD ASS_CMD

EST_IND

Successful TCH Seizure – Intra-BSC Handover

 Intra-BSC handover process

HO_Detect

PHY INFO

First SABM

Establish_IND PHY INFO

Handover Complete HO_PerformedUA

MSC

Successful TCH Seizure – Inter-BSC Handover

 Inter BSC handover process

Handover Complete HO_CMP

Clear_CMD Clear_CMP

Chapter 1 Principle of call drop

Section 1 Definition of call drop Section 2 TCH call drop

Section 3 SDCCH call drop

Formula of SDCCH Call Drop Rate

 Formula of SDCCH call drop rate:

 SDCCH call drop rate = SDCCH call drops/ successful SDCCH seizures*100%

Measurement Point of SDCCH Call Drop

 This item is measured when BSC receives ERROR INDICATION message from BTS due to an abnormal case for a radio link layer connection

 This item is measured when BSC receives CONNECTION FAILURE INDICATION message from BTS because an active connection has been broken for some reason such as SDCCH link failure or hardware failure (see GSM 0508 for details)

 In incoming BSC handover procedure on SDCCH, this item is measured in the target cell in the case of the failure of decoding HANDOVER DETECTION and HANDOVER COMPLETE message

 In the process of incoming BSC handover on SDCCH, this item is measured

in the target cell when the timer for the target cell to wait for the HANDOVER COMPLETE message expires

Measurement Point of SDCCH Call Drop

 In the process outgoing BSC handover on SDCCH, this item is measured when the timer for the source cell to wait for CLEAR COMMAND message from MSC expires (T8 expires)

 In the process of intra BSC handover on SDCCH, this item is measured in the source cell when the timer for the source cell or the target cell to wait for HANDOVER COMPLETE message expires

 This item is measured when the RSL link of the TRX that the call is running

on disconnect, which will cause call drop, this item measures call drop on SDCCH for RSL disconnection

Establish Indication (L3 Info)

Connection Failure Error Indication Abis Failure

Measurement Point of SDCCH Call Drop

Measurement Point of Successful SDCCH Seizures

 In immediate SDCCH assignment procedure, this item is measured when B

SC receives CHANNEL ACTIVATION ACKNOWLEDGE message from BTS

 In the process of incoming BSC handover on SDCCH This item is measure

d when BSC receives HANDOVER DETECTION from BTS

 In the process of incoming internal inter cell handover and intracell handover

in BSC.BSC measures this item in the target cell when receiving HANDOVE

R DETECTION from BTS

Chapter 1 Principle of call drop

Chapter 2 Analysis of call drop

Chapter 3 Call drop cases

Analysis of Call Drop

 content

 main causes of high call drop rate

 troubleshooting of high call drop rate

Analysis of Call Drop

 According to the definition of call drop measurement point, call drop is usuall

y caused by the following:

 Radio link fault During the communication, messages can not be receiv

ed correctly

 Abis link broken during conversation

 Call drop during handover

 Other system faults

 Timers that may cause call drops (BSC timer):

 T3103: starting from sending HANDOVER CMD and ending at receiving HANDOVER CMP Time out of the timer will cause call drop

 T3109: starting from sending CHAN REL and ending at receiving REL I

ND

Radio Link Fault

 Signaling process chart of radio link fault

Measurement Report

Measurement ResultConnection Failure

Clear_REQ (Radio Interface Failure)

Radio Link Fault

 Diagram of radio link timeout

Handover Command

Handover Access HO_Detect

SABM Physical Information (TA)

UA Handover Complete

Handover Complete EST_IND

Set T3103

Reset T3103

Causes of Radio Link Fault

 The causes of radio link fault can be

 Improper parameter setting

 Radio link timeout, SACCH multi-frames

 Handover parameters

 Power control parameters

 Equipment problem (Antenna, feeder, combiner, TRX)

Radio Link Fault - Interference

Radio Link Fault - Interference

 Solution

 First check equipment problems

 Make an drive test, check the interference area and distribution of signal quality Find the interference frequency

 Further search for the interference source with the spectrum analyzer

 Activate hopping, DTX and power control functions to lower the internal interference of the system

Radio Link Fault - Interference

 Judgment Process

 Analyze the occurrence regularity of interference band in the traffic measurement

 Observe the receiving level performance

 Find the poor quality handover rate

 Observe receiving quality performance

 Observe call drop performance

 Observe whether there are many handover failures and reversion failures

Radio Link Fault - Coverage

Radio Link Fault - Coverage

 Judgment Process

 Power control measurement function

 Receiving level measurement function

 Cell measurement function/inter-cell handover measurement function

 Call drop measurement function

 Defined adjacent cell measurement function

 Undefined adjacent cell measurement function

 Outgoing inter-cell handover measurement function

 Up-down link balance measurement function

Radio Link Fault - Coverage

 Solution

 Adjust network parameters

 Add BTS

Radio Link Fault - Parameters

 Main parameters that may affect the call drop rate:

 Radio link timeout and SACCH multi-frames

 RACH busy threshold and RACH minimum access level

 MS minimum receiving signal level

 Call re-establishment permitted

Radio Link Fault - Parameters

 Main parameters that may affect the call drop rate:

 NCC permitted

 Handover related parameters

 Power control related parameters

 Frequency planning parameters

Radio Link Fault

 Judgment process

 System information data

 Cell data

 Radio link connection timer (T3105)

 Maximum times of physical information

 Call drop performance measurement function

 Judge from the cause of call drop

 error indication

 connection failure

Handover Problem

 Judgment process

 In outgoing inter-cell handover measurement function, the outgoing inter-cell handover success rate is low (for a certain cell) Find the adjacent cell to which the handover success rate is low and find the cause

 Incoming inter-cell handover success rate is low The handover judgment parameter setting of the target cell may be improper

 In TCH measurement function, handover times are not in normal proportion to successful TCH seizures for call (handovers/calls>3)

Chapter 1 Principle of call drop Chapter 2 Analysis of call drop

Chapter 3 Call drop cases

Call Drop Case 1

 Fault Description

 The BTS distribution of an area is as illustrated in the diagram (red numbers stand for BCCH frequencies No hopping, no DTX) Some subscribers complain that call drop in second sector of base station C is serious Hardware fault has been ruled out

please confirm whether the frequency distribution in the cells

Call Drop Case 1

Call Drop Case 1

Call Drop Case 1

 The conclusion is that there is interference from the analysis of comprehensive traffic statistic and drive test

Call Drop Case 1

 Troubleshooting

 A site investigation shows that the operator has a repeater It is a broadband repeater It transmits the signals from a remote TACS site TACS signals are amplified and then there is interference in second sector of base station C

 Problem has been located: interference causes the call drop

 Finally, lower the power of the repeater The interference band reduce

to IB1 Now the high call drop rate problem at site C is solved

Call Drop Case 1

 Common methods of checking and clearing call drop due to interference

 Rule out the internal interference caused by equipment problems and check the separation of BTS transceivers, antenna feeder installation, and so on

 Check the interference band

 Drive test

 Check traffic statistic of handover causes to get judgment

 Clear uplink interference

 Clear downlink interference

 Check whether DTX, frequency hopping technology and power control application are reasonable

 Use PBGT handover algorithm flexibly to avoid co-channel and adjacent-channel interference effectively

Call Drop Case 2

 Fault description

 The call drop rate in cell 3 of a BTS is 10% accompanied with high congestion rate, but call drop rate and congestion rate in cell 1 and cell

2 are normal

Call Drop Case 2

 Analysis

 Check the related traffic statistic

 Check whether there is high interference band in TCH measurement function

 Check the situation of call drop in call drop measurement function

 Check whether handover of the cell is normal

 Check whether there is interference through checking frequency planning, moreover confirm whether there is external interference with spectrum analyzer

 Drive test

 Check the hardware

Call Drop Case 2

 Change frequency The frequency of cell 3 is changed to 1MHz away from the original value But the problem persists

 Judge whether the equipment is faulty

 Locate external interference

Call Drop Case 2

 Troubleshooting

 Make a scanning test with a spectrum analyzer

 A suspect signal with 904.14MHz center frequency, 300K bandwidth

is found It is similar to an analog signal and it exists continuously

 At the distributor output port of cell 3, the signal strength is –27dBm cell 2 is –40dBm, cell 1 is –60dBm It accords with the degree of interference

 Traffic volume is higher in the day time than that at night

 Now the problem is found: 904MHz external interference source

Call Drop Case 2

 Conclusion: solution of interference

 Solve internal interference through checking frequency planning

 After internal interference is excluded, we can locate external interference with spectrum analyzer