TCH Call Drop & Solutions

GSM TCH Call Drop & Solutions

ZTE university

Training goals

To master definitions of call drop indicators and relevant counters

To master causes of the three types of call drop and the corresponding solutions

To know analysis of call drop and location of call drop problem with traffic statistics

Contents

Indicator definition Handling of call drop problem Call drop due to radio link fault Call drop due to handover failure Call drop due to LAPD link failure Typical cases

Indicator definition

TCH call drop rate (excl. HO) TCH call drop rate (excl. HO) = total of TCH call

drops/total of TCH occupancy times (excl. HO) ×100% Counter formula:

V3:(C900060054+C900060055)*100%/(C900060028+C900060036+C900060199+C900060210)


TCH call drop rate (incl. HO) TCH call drop rate (incl. HO)=total of TCH call

drops/total of TCH occupancy times (incl. HO)×100% Counter formula:

V3:(C900060054+C900060055)*100%/(C900060028+C900060036+C900060199+C900060210+C900060098+C900060102+C900060120)


Traffic call drop rate Indicator definition: traffic volume/number of TCH call dr

ops*60 Counter formula:

V3:((C900060129+C900060127)/3600)*60/(C900060054+C900060055)


Call drop counter

C900060054

Number of TCH/F call drops

C900060055

Number of TCH/H drops

Contents


Flow of handling call drops A cell suffers from high TCH call drop

rate

Analysis of call drop types

Call drop of RF loss

Complete

Call drop of handover failure Call drop of LAPD

If radio parameters set reasonably?

If interference exists?

Any equipment hardware fault?

Any antenna system fault?

Eliminate antenna

fault

Eliminate equipment

fault

Check& eliminate

interference

Adjust radio

parameters

If radio parameters set reasonably?

If interference exists?

An equipment hardware fault?

Any coverage problem?

Improve coverage

Any hardware fault at BSC?

Any fault with BTS

transmission?

Any hardware fault at BTS?

Eliminate hardware

fault

Eliminate transmission

fault

Eliminate hardware

fault

No

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

Yes

Locate & analyze call drop with traffic statistics

performance report of BSC level

performance report of cell level

BTS measurement

Cell radio measurement

Performance report of cell level CS basic measurement

Warning statistics

Analysis of common problem

Analysis of specific problem

Warning analysis

Analysis of Traffic statistics

Signaling analysis

Other network optimization methods

Analytical framework 1 2 3 4 5

Problem solvedLocate cell with high call drop rate Analyze casueConfirm call drop type Confirm call drops on carrier

From macro to micro view

Count call drops on carrier

Convenient for problem location and analysis

form common to specific Execution & problem solution

Locate & analyze call drop with traffic statistics

Common causes for call dropsUnreasonable setting

of handover

parameters

Coverage problem

Equipment hardware

fault

Interference

UL/DL unbalance

Unreasonable settings of

other parameters

Common problem is

analyzed

result in call drop

Contents


Call drop due to radio link failure

UL failure

？？？DL failure

？？？


Relevant counters

C900060244 Number of radio link failures（ On TCH/F speech ）

C901070037

Number of radio link failures （On TCH/H speech ）


MS BTS BSC

Connection Failure Indication

F1

MSC

Clear request

Radio link timeout

Measurement report(SACCH)



Start T3109

Stop T3109Clear Command

RF Channel Release

RF Channel Release ACK

Clear Complete

Call drop due to radio link failure Causes for call drop due to radio link failure & handling proced

uresWeak coverage, poor radio signal; Interference;Unreasonable settings of radio para

meter; Incomplete or wrong adjacent cell da

ta ;Unreasonable settings of handover

parameters;Congestion in adjacent cell;Equipment hardware fault;Antenna system fault;Subscriber’s fault.

Main causes

Check radio parameters. Adjust unreasonable settings of radio parameters;

Check indicators like BER and level of idle interference band, reduce or eliminate radio interference ;

Check if coverage problem exists through DT;

Check and eliminate equipment fault ; Check antenna system.

Handling procedures

Contents

Indicator definitions Handling of call drop problem Call drop due to radio link fault Call drop due to handover failure Call drop due to LAPD link failure Typical cases

Call drop due to handover failure

Classification of Handover control timer in BSC T8 expires (inter-BSC handover) T3103 expires (inter-cell handover under one BSC) T3107 expires (intra-cell handover).


Relevant counters

C900060220 Number of handover failures （

On TCH/F speech ） C900060151

Number of handover failures（ On TCH/H speech ）


Call drop due to intra-cell handover failure

BSCBTS:TRXMS

ASSIGNMENT COMMAND

CHANNEL ACTIVATE

G1

CHANNEL ACTIVATE ACK

SET T3107

T3107 Timeout

MSC

Clear Request

Call drop due to handover failure Call drop due to intra-BSC handover failure

BSCOld BTS:MS

CHANNEL ACT

H1

CHANNEL ACT ACK

New BTS

SET T3103

T3103 Timeout

MSC

HO_CommandHO_Command

Clear request

Call drop due to handover failure Call drop due to inter-BSC handover failure

MS New BTSNew BSC

CHL_ACT

MSC

CHL_ACT_ACKHO_REQ_ACK

Old BSCOld BTS

HANDOVER REQUIRED

HANDOVER COMMANDHANDOVER COMMANDHANDOVER COMMAND

SET T8

T8 Timeout

I1

Clear request

Call drop due to handover failure Main causes for call drop due to handover failure & handling procedures

Interference;

Hardware fault;

Unreasonable settings of radio

parameters ;

Inappropriate adjacent cell relati

on or wrong adjacent cell data;

Unreasonable settings of hando

ver parameters ,which results in

Ping-Pong handover.

Main causes

Check radio parameters, adjust the

unreasonable settings, and add

necessary neighbor relations;

Check indicators like BER and level

of idle interference band, reduce or

eliminate radio interference;

Check equipment hardware,

change panels with problems.

Handling procedures

Contents


Call drop due to LAPD link failure Call drop due to LAPD link failure

Principle:

When LAPD link breaks, calls on carriers will be interrupted. BSC

counts after it receives message showing that LAPD link is broken. Main causes for call drop due to LAPD link failure & handling procedures

BTS hardware fault

BTS transmission

problem

BSC hardware fault

Main causes

Investigate & eliminate BTS hardware fault;

Investigate & eliminate BTS transmission problem;

Investigate & eliminate BSC hardware fault

Handling procedures

Call drop due to LAPD link failure

Relevant counters

C901070017Number of LAPD link failures （

On TCH/F speech ） C901070038

Number of LAPD link failures （ On TCH/F speec

h ）

Contents


cases 1:Severe call drops caused by the illegal user

Description: 2 cells of the GSM network in XX had severe call drop p

roblem, about dozens of times per hour in the day time.

Cause Analysis & Procedure: According to the 24-hour performance statistics, most o

f the call drops were in the daytime. While very few of them were in the night. So the engineer suspected that the problem was related with the user behavior.

cases 1: Severe call drops caused by the illegal user

After tracing the Abis interface signaling, we found: (1) The handsets with the call drop problem all used the same

IMEI number.(2) The dialed numbers were all the emergency number: 112;(3) The call drop occurred about 10s after the call was connect

ed. After the call drop, the user continued to dial 112 again and again.

Based on the above factors, we made the judgment: the call drop was caused by the user himself. For example, the workers in a factrory were testing the batteries of handsets, and they took out the battery while the call was still going on. So if we disable the emergency call function of the cell, the user will try to use another operator's network. After the operation, we found that the amount of call drops in the cell was greatly reduced. After we enabled the emergency call function later, the call drop problem didn't occur any more, becasue the user selected another operator's network.

cases 1: Severe call drops caused by the illegal user

Summary: By analyzing the Abis signaling file, we can make judge

ment about the call drop problem and find out the regularity of the problem. The network performance index and user experience may be harmed when the network resource is occupied by some illeagle user. We can find out the illeagle user by signal tracing or analyzing the CDR from the switching side.

cases 2: Call drops caused by handover failure of the handset Description:

After the equipment been swapped to the GSM network, one subscriber complained that under the mobile environment, his call was automatically hanged up within one minute after connection. The subscriber's handset is HS-D907 and it worked normally under the MOT equipment network before the swap. Another subscriber complained that when he made a call by HS-D907 on the highway, the call was frequently hanged up about dozens of seonds after connection. In addition, the subscriber said the handset never had the above problem in other places.

Cause Analysis & Procedure: The engineer traced the Abis interface MM signaling from the

switching side. When the XX handset is the calling party, it enters the Conversatio

n state after receiving "connect Ack". Several seconds later, the BSSAP entity sends a "cbclearcmdEvent" message to the handset, and the handset automatically hangs up.

When the XX handset is the called party, it enters the Conversation state after receiving "connect Ack". Several seconds later, the BSSAP entity also sends a "cbclearcmdEvent" message to the handset, and the handset automatically hangs up.

According to the signaling tracing analysis, the core network makes the judgement that the connection is actively released by the wireless side. The releasing reason is 1, and the meaning of this value is:

1=Radio interface failure(1)

cases 2: Call drops caused by handover failure of the handset

cases 2: Call drops caused by handover failure of the handset The engineer traced the Abis interface signaling fr

om the OMCR side. After tracing the signaling in the 900/1800 area of Cell 3

and conducting call trace by MA10 software, we found that the handset released the channels after the handover failure, and the handovers were all simultaneous handovers

During the conversation, every time when the handover command was initiated, the handset pointed to "full rate or half rate version 3", then the handover was failed.

cases 2: Call drops caused by handover failure of the handset After comparing the version with the BSC voice ve

rsion, the core network engineer found that the preferred full rate voice version for the wireless side was version 2, while the switching side only supported voice version 1 and 3, voice version 2 was not selected.

Steps: In "Configure the relation between BSC and trunk grou

p", the engineer added the TFRV2 to the property of all trunk groups of the 79 and 80 BSC from the switching side. After that, the automatic hang up never happened again during the dialing test.

cases 2: Call drops caused by handover failure of the handset Under the AMR mode, the HS-D907 handset misunderstands the encry

pted fields in the handover command, so the handover will be failed. Once the encrypted fields contain non-encription information, the handset will report invalid mandatory filed, then the handover is failed, and the call drop occurs.

TCH Call Drop & Solutions

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