tch call drop & solutions
DESCRIPTION
Cell interference and solutionTRANSCRIPT
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)
Indicator definition
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)
Indicator definition
Traffic call drop rate Indicator definition: traffic volume/number of TCH call dr
ops*60 Counter formula:
V3:((C900060129+C900060127)/3600)*60/(C900060054+C900060055)
Indicator definition
Call drop counter
C900060054
Number of TCH/F call drops
C900060055
Number of TCH/H drops
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
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
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
Call drop due to radio link failure
UL failure
???DL failure
???
Call drop due to radio link failure
Relevant counters
C900060244 Number of radio link failures( On TCH/F speech )
C901070037
Number of radio link failures (On TCH/H speech )
Call drop due to radio link failure
MS BTS BSC
Connection Failure Indication
F1
MSC
Clear request
Radio link timeout
Measurement report(SACCH)
Measurement report(SACCH)
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).
Call drop due to handover failure
Relevant counters
C900060220 Number of handover failures (
On TCH/F speech ) C900060151
Number of handover failures( On TCH/H speech )
Call drop due to handover failure
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
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 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
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
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.