omo333010 bsc6900 gsm call drop problem analysis issue 1.01

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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.



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




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





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.



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)


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.



“M3030A: Call Drops on TCH(TA)” depends on the parameter “MAXTA” via command

”SET GCELLBASICPARA”.



Data sources refer to the measure result of Call Drop Measurement per Cell in

performance management in M2000.



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.



DCR: Dropped Call Rate



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.



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.



Currently, local switch of BSC/BTS is not enabled, therefore, do not need to pay attention

to CM397 and CM385.



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.



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.


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.



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.




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.



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.



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.



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.



omo333010 bsc6900 gsm call drop problem analysis issue 1.01

Documents

traffic channel cm33c

handover traffic channel

handover traffic channel

channel release

permission gsm

um interface

certain problem result

failure cm385