omo333010 bsc6900 gsm call drop problem analysis issue 1.01
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call dropTRANSCRIPT
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
Statistics of DT call drops: Usually, a normal call release is counted when either of the two
messages, Disconnect or Channel Release appears during a call. A call drop is counted only
when neither of these two messages appears and the MS converts from dedicated mode
to idle mode.
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GSM Call Drop Problem Analysis
According to the reason for call drops, the BSC classifies call drops into various categories.
This helps to identify the type of a call drop and to locate problems.
CM33:Call Drops on Traffic Channel
CM33C:Call Drops on Radio Interface (Traffic Channel)
CM330:Call Drops on Radio Interface in Stable State (Traffic Channel)
CM331:Call Drops on Radio Interface in Handover State (Traffic Channel)
CM332:Call Drops Due to No MR from MS for a Long Time (Traffic Channel)
CM333:Call Drops due to Abis Terrestrial Link Failure (Traffic Channel)
CM334:Call Drops due to Equipment Failure (Traffic Channel)
CM335:Call Drops due to Forced Handover (Traffic Channel)
CM33:Call Drops on Traffic Channel
CM33C:Call Drops on Radio Interface (Traffic Channel)
CM330:Call Drops on Radio Interface in Stable State (Traffic Channel)
CM331:Call Drops on Radio Interface in Handover State (Traffic Channel)
CM332:Call Drops Due to No MR from MS for a Long Time (Traffic Channel)
CM333:Call Drops due to Abis Terrestrial Link Failure (Traffic Channel)
CM334:Call Drops due to Equipment Failure (Traffic Channel)
CM335:Call Drops due to Forced Handover (Traffic Channel)
CM397:Call Drops due to local switching Start Failure
CM385:Call Drops due to Failures to Return to Normal Call from local switching
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
Call drops that occur over Um interface are classified into call drops that occur in the
stable state and call drops that occur during handover.
Among call drops that occur in the stable state, pay attention to CM3300 and CM3301. In
a normal network, CM3301 occupies the largest proportion of call drops.
Usually, most call drops that occur during handover in a network are caused by inter-cell
handover. Therefore, H3127Ca and H3128Ca is in the large proportion of call drops.
According to the proportion of various types of call drops, we can preliminarily determine
whether a certain problem result in the increase of a type of call drop. For example, if a
type of call drop that originally accounts for a small proportion increases suddenly, we
should pay special attention to this type of call drop.
CM33C:Call Drops on Radio Interface (Traffic Channel)
CM330:Call Drops on Radio Interface in Stable State (Traffic Channel)
CM3300:Call Drops on Traffic Channel in Stable State (Error Indication)
CM3301:Call Drops on Traffic Channel in Stable State (Connection Failure)
CM3302:Call Drops on Traffic Channel in Stable State (Release Indication)
CM331:Call Drops on Radio Interface in Handover State (Traffic Channel)
H3027Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHF) (Traffic
Channel)
H3028Ca:Failed Internal Intra-Cell Handovers (Timer Expired) (TCHH)
(Traffic Channel)
H3127Ca:Number of Unsuccessful Outgoing Internal Inter-Cell Handovers
(Timer Expired) (TCHF) (Traffic Channel)
In one practical network, the distribution of various types of call drops is counted as
follows:
Call drops that occur over Um interface occupies 98.21%. Other types of call drops,
however, is in a small proportion. It is general for most of networks. If other type of call
drop, for example, CM334 call drops (call drops due to equipment failure), occupies a
large proportion, we need to check the hardware and alarms.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
“M3030A: Call Drops on TCH(TA)” depends on the parameter “MAXTA” via command
”SET GCELLBASICPARA”.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
Data sources refer to the measure result of Call Drop Measurement per Cell in
performance management in M2000.
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GSM Call Drop Problem Analysis
Rule 1: DCR(Dropped Call Rate)
Put cells in DCR order. Select cells with the DCR greater than the overall DCR,
which rate is calculated basing on the calculation range (for example, the overall
DCR of a BSC).
Rule 2: Number of call drops
Put cells in DCR order. Select the cells with the number of call drops greater than
the average number of call, which is the result of the total number of call drops
divided by the total number of cells in a calculation range (for example, a BSC) .
Filter cells by both DCR and the number of call drops. Then, calculate the proportions of
the number of filtered cells to the total number of cells respectively. After that, take 10%
of the total number of cells from the filtered cells as TOP cells. Then, compare the DCR of
the entire network after removing the TOP cells and the DCR of the entire network with
the TOP cells to determine whether the problem is result from the TOP cells.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
DCR: Dropped Call Rate
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GSM Call Drop Problem Analysis
Signaling analysis: generally, Abis Trace is used for identify the specific reasons basing the
signaling message of call drop and the receiving level and quality in measurement report.
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GSM Call Drop Problem Analysis
Signaling analysis: generally, Abis Trace is used for identify the specific reasons basing the
signaling message of call drop and the receiving level and quality in measurement report.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
If the call drop rate is high in a network, we need to analyze the problem to find out
what’s the problem and what is the scope of the problem. Then, it’s easy to find out the
analysis solution.
First, we should analyze the call drop traffic, determine whether the high call drop rate
exists in the entire network or only in some cells, analyze the proportions of various types
of call drops, and finally determine the problem that results in a type of call drop.
If call drops over Um interface increase, we need to pay attention to factors such as
network parameters, interference, and coverage and solve the problem by improving the
quality of the air interface.
If call drops due to equipment failure and transport failure (CM334 and CM333) that occur
over none radio interface increase, we need to pay attention to hardware failure and
transport failure.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
Currently, local switch of BSC/BTS is not enabled, therefore, do not need to pay attention
to CM397 and CM385.
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GSM Call Drop Problem Analysis
TOP cell/area filtering
Find what are TOP cells and determine whether a TOP area exists according to the
geographical display.
Alarm analysis
Check the related alarm information and solve the problems for which alarms are
reported.
Engineering check
Check whether the engineering quality is eligible: whether the hardware is in good
condition, whether the connecting lines are correctly connected, and whether the
feeders are not inversely connected.
Parameter check
Check the parameters according to the recommended value and the actual
situation of the practical network, and adjust the parameters that are improperly
set.
Interference check
Check whether any repeater interference, external interference, and
intermodulation interference exists.
Coverage check
Check whether any poor coverage area or any blind coverage area exists in the
TOP cells or in the TOP area.
Neighboring Cell Check
Check whether any redundant neighboring cells or missing neighboring cells
configuration.
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GSM Call Drop Problem Analysis
Checking of hardware failure and transmission failure: Usually, an alarm is reported when
these problems occur. Sometimes, however, no alarm is reported when these problems
occur. In this case, analyze the traffic (such as how many resources are available according
to TCH Usage and TRX Usage, whether the links are normal according to Channel Active
NACK and Channel Active Timeout) to determine whether the problem exists and check
the problem on site if necessary.
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SRAN CS Problem Analysis and KPI Overview
Failure Type
Level Function Set Function Subset Counter
Transmission
Failure
BSC LAPD
Measurement LAPD Link
Measurement All Counters
BTS BTSM
Measurement
LAPD Link Measurement at
the BTS All Counters
Cell Call
Measurement
Channel Activation
Measurement per Cell
CR33A:Channel Activation Attempts
CR33B:CHAN ACTIV NACK Messages Sent by BTS
CR33C:Channel Activation Timeouts
Interference checking: whether severe interference exists according to interference bands
4&5 at intervals. If use the average value of DCR, it’s may not easily to find out the
interference via the interference bands . According to the drive test result, analyze whether
co-frequency/adjacent-frequency interference or external interference exists. Then, check
the possible reasons that cause the interference.
Coverage checking: Analyze the traffic and find whether problems such as high proportion
of great TAs, imbalance between uplink and downlink, and high proportion of low levels
exist. Preliminarily determine whether problems of poor coverage and imbalance of uplink
and downlink exist. According to the drive test result, determine whether poor coverage
areas exists or not, if yes, check whether the transmitting power and tower amplifier are
set properly.
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GSM Call Drop Problem Analysis
Objectives
To disable redundant neighboring cells
Disable redundant neighboring cells to improve handover rate.
Reserve the space of the BA2 table to collect MR data.
To add missing neighboring cells
Reduce the interference between neighboring cells during frequency
replanning.
Improve the handover success rate.
Reduce the call drop rate.
Optimizing redundant neighboring cells based on handover traffic statistics
Collect handover measurement data under GSM cell –GSM cell for one week and
check the data as follows:
If the sum of (H370c+H380)is smaller than 10, the related neighboring cell
may be redundant.
If the ratio of H375B/H370c is greater than 10%, the related neighboring cell
may be redundant.
Identify the redundant neighboring cell based on geographical position.
If the neighboring cell is far from the serving cell, or the neighboring cell
is near the serving cell but the azimuths of the two cells are opposite to
each other, then this neighboring cell is redundant and should be
deleted. Otherwise, check whether the handover parameters are set
properly.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
GSM Call Drop Problem Analysis
GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
GSM Call Drop Problem Analysis
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Call Drop Problem Analysis
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Call Drop Problem Analysis
1. Because no matter which TRX of this cell is blocked, the congestion rate is always relatively
high. There can be interference or the terrain in the coverage range of the cell is possibly
complex.
2. It is concluded that, by viewing and analyzing the traffic statistic data, the interference
band of cell 3 basically stays at 4 or 5 in daytime, and it stays at band 1 or band 2
between 23:00 PM and 7:00 AM. In addition, the call drop rate and the interference band
are regular.
3. First take co-channel and adjacent-channel interference into consideration. Change the
frequency. The frequency spacing of cell 3 is increased to 1MHz . But the problem persists.
4. Then consider the equipment problems. Interchange the antenna and feeder of cell 3 with
that of cell 1, but cell 3 interference remains the same. Therefore, it can basically be
concluded that there is no problem with the BTS devices below the antenna and feeder.
After the above possibilities are excluded, the fault can be located as external interference.
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GSM Call Drop Problem Analysis
1. Although there is a 10 MHz distance between this frequency band and that used in this
cell, it is a continuous signal and it can be more possibly to conflict and inter-modulate
with other signals. Some parts of intermodulation components may fall in the receiving
band and form interference.
2. In daytime the traffic is larger than that at night,so the intermodulation signal
(interference) is stronger at daytime than at nighttime.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
It is found that, after multiple on-site dialing tests, there really exist call drops and noise.
However, it can be seen from the test MS that it always stays in a service cell of a remote
BTS A before call drop, and its TA value is about 17, and the receiving signal strength is
about -80dBm.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
When there is an isolated coverage island from a cell in an area, if MS stays in this cell at
the island area and make a call, no matter how the signal changes, handover cannot be
implemented normally and a call drop occurs. To avoid such situation, two means can be
used. The better one is to adjust the antenna of the cell to eliminate the isolated island
phenomenon. However, due to the complexity of radio propagation, usually multiple
experiments are required to eliminate the isolated island effect while the coverage area is
not obviously affected. In addition, it is difficult to completely eliminate the isolated island
phenomenon of high buildings. The another means is to define new adjacent cells for the
cell with isolated island.
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis
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GSM Call Drop Problem Analysis