OMD6010 Traffic statistics analysis ISSUE1.131033203-BSS
Troubleshooting Manual
Upon completion of this course, you will be able to:
Know the traffic statistics system structure
Understand some often-used traffic measurement items
Locate some problems through the traffic statistics system
HUAWEI TECHNOLOGIES CO., LTD.
Chapter 2 Often-used traffic statistics items analysis
Chapter 3 Locate problem through traffic analysis
Chapter 4 Case study for traffic statistics
HUAWEI TECHNOLOGIES CO., LTD.
*
Traffic statistic is registered on OMC through background WS, runs
at the host (MPU of BSC) and is read through the background WS. Due
to the limitation of the host’s memory, the number of
simultaneously running traffic measurement tasks are restricted.
Similarly, due to the limitation of the communication bandwidth
between foreground and background, it is suggested to get the
traffic statistics result 10 minutes after the traffic statistics
cycle.
HUAWEI TECHNOLOGIES CO., LTD.
Task list management
BSC traffic statistics platform provides the function of
registering a new task, deleting a registered task and refreshing
the task list.
Task management
*
Template management
*
Limitations
The traffic measurement task is registered using the BSC traffic
statistics console, the maximum number of tasks of each BM module
is 200.
The maximum number of subtasks in each BM module is 3000.
One task can only include 60 original items.
Items regarding to maximum/minimum value can not be repeatedly
registered in all tasks.
HUAWEI TECHNOLOGIES CO., LTD.
TCH Congestion Rate
*
TCH Blocking Rate is one of the key items. In the formula, the
numerator is the TCH seizure failures. There are two cases:
Including and excluding handover.
TCH Seizure Failures are all of the seizure failures due to no
available TCH channel and queue timeout after the SDCCH is
allocated. It equals to “Assignment Failure (no radio resource
available) Clear Request (no radio resource available) ”.
HUAWEI TECHNOLOGIES CO., LTD.
TCH Congestion Rate
*
TCH Congestion Rate
TCH congestion rate (TCH overflow) = Attempted TCH seizures meeting
a TCH blocked state / Attempted TCH seizures (all) * 100%
HUAWEI TECHNOLOGIES CO., LTD.
SDCCH congestion rate (SDCCH overflow)
*
the measurement point of SDCCH congestion is attempted SDCCH
seizure meeting a SDCCH blocked state, not simply SDCCH seizure
failure.
HUAWEI TECHNOLOGIES CO., LTD.
TCH Call Drop Rate
TCH Call Drop Rate = TCH Call Drops / Successful TCH Seizures (all)
*100%
SDCCH Call Drop Rate
*
TCH call drop can be divided into three categories in more details.
Understanding the difference of each category will be of great help
for the further analysis of different TCH call drop reasons.
Main reason for TCH call drop:
TCH lost radio connections (connection failure)
TCH lost radio connections (error indication)
Ground link disconnection when TCH seized (Abis problem)
The definition of SDCCH call drop rate is almost the same as TCH
call drop rate
Main reason for SDCCH call drop:
SDCCH lost radio connections (connection failure)
SDCCH lost radio connections (error indication)
Ground link disconnected when SDCCH seized (Abis problem)
HUAWEI TECHNOLOGIES CO., LTD.
Handover Success Rate
*
Handover Success Rate
*
Write down the key items of the network
Answer: The most important items for the network are: call drop
rate, TCH congestion rate, SDCCH congestion rate, outgoing and
incoming handover success rate, traffic volume etc.
HUAWEI TECHNOLOGIES CO., LTD.
MTP Measurement Function
Cell Measurement Function
Power control Measurement
Call Drop Measurement
BSC Measurement Function
SCCP Measurement Function
A-interface Equipment Maintenance statistic
CPU Measurement Function
Huawei BSC traffic statistics consists of all the following
measurement function.
BSC measurement function contains BSC overall running counters.
This measurement result reflects the BSC running performance
roughly.
Cell measurement function provides comprehensive traffic
measurements about the cell including SDCCH measurement function,
TCH measurement function, random access measurement function,
inter-cell handover measurement function, intra-cell handover
measurement function and other measurement functions. This
measurement contraposes cells.
Power control measurement shows how BSC controls the power level
and signal quality of the MS and BTS, which includes numbers of
power control messages that BSC sends to BTS and MS, and the
average signal strength and quality of uplink and downlink.
Call drop measurement function is base on the last valid
measurement report of the SDCCH or TCH before call drop due to
radio link problems, so it is helpful for the network operators to
analyze the causes of call drop. This measurement contraposes
cell.
HUAWEI TECHNOLOGIES CO., LTD.
Defined Adjacent Cell Measurement Function
Receiving Quality Measurement
LAPD Link Measurement Function
Undefined Adjacent Cell Measurement Function
GPRS Related Measurement Function
*
Outgoing inter cell handover measurement function can be registered
successfully only when the adjacent cells are defined. It is used
to measure the details of intra-BSC and inter-BSC handovers,n
including handover requests, successful handovers and unsuccessful
handovers due to various handover cause..
Incoming inter cell handover measurement function is used to
measure the detail of incoming handover from other adjacent cell to
target cell, including handover requests, successful handovers and
unsuccessful handovers due to congestion.
Undefined adjacent cell measurement function is used to measure the
cells reported by MS which have not been defined in the cell
adjacent relation table. The measurement includes BCCH, BSIC,
average signal strength of undefined adjacent cells.
Up-Downlink balance measurement function is used to measure the TCH
channel and the MR that BSC receives contains uplink receiving
level and downlink receiving level.
HUAWEI TECHNOLOGIES CO., LTD.
Chapter 2 Often-used traffic statistics items analysis
Chapter 3 Locate problem through traffic analysis
Chapter 4 Case study for traffic statistics
HUAWEI TECHNOLOGIES CO., LTD.
Systematical logic
Integrality
Observe the change trend of the items for more than one week and
the changing trend of each day.
Relativity
*
From whole to specific, the problem cell can be located quickly.
Observing items’ changing trend and reviewing the network operation
records can be of great help for analyzing the causes of problems.
Further more, items are closely related and mutually
affected.
HUAWEI TECHNOLOGIES CO., LTD.
Analysis process
First we shall analyze and compare the item of BSC measurement
function to have a rough idea for the whole network.
*
Analysis process
*
Those cells in which there are some abnormal items but small
absolute times can be ignored. For example, one handover successful
times among three handovers, then the handover successful rate is
33%. On the other hand, more attention shall be paid to the cells
that have low index rate but high absolute times. The problems of
such cells shall be solved quickly, because they affect the
performance of whole BSC more seriously. For example, during 600
handovers, there are 540 successful handovers, then the handover
successful rate is 90%. It is not very low. But 60 times of
handover failure will bring much affect to the whole network.
HUAWEI TECHNOLOGIES CO., LTD.
Immediate assignment successful rate
TCH call drop rate
Handover success rate
Relate to , coverage area and random access performance.
*
When the MS receives a paging message, it will access the network.
When BSC receives the paging response message including the IMSI,
it will take that the paging is successful. So, if the paging
re-send mechanism is implemented at BSC, the paging successful rate
will be better.
HUAWEI TECHNOLOGIES CO., LTD.
Immediate assignment success rate
When BTS receives a Random Access message from MS, BTS will apply a
channel from BSC. If there is no channel available, BSC will send
immediate assignment rejected message which indicates the failure
of immediate assignment. At the same time, MS’s access will be
bared for some time.
*
Excellent access performance is related to CCCH combination and
Tx-integer setting. In addition, AGCH may also be the bottleneck of
the system.
HUAWEI TECHNOLOGIES CO., LTD.
TCH call drop rate
Possible causes can be
Ground link disconnection when TCH seized (Abis).
Call drop during handover
*
Analyze the main reason of call drop then do further analysis
according to different causes.
HUAWEI TECHNOLOGIES CO., LTD.
TCH call drop formula and measurement point
*
The current seized channel is TCH.
1, When ERR_IND message is received and it’s not in a handover
process. Besides, the connection to the MS has been set up.
2, A call is cleared because the occupied resources are no longer
unavailable ((BLOCKED or DISABLED).
3, A call is cleared because resource check fails.
4, The contents of the HO_DETECT message of incoming inter BSC
handover is invalid.
5, The contents of the HO_CMP_UM message of incoming inter BSC
handover is invalid.
6, Failed to send the HO_CMP message.
7, Timer waiting for HO_DETECT or HO_CMP message in incoming inter
BSC handover times out.
8, Intra cell handover failed, or the timer waiting for HO_CMP
message in outgoing intra BSC handover timeouts.
9, Timer waiting for HO_SUCCESS message in outgoing inter BSC
handover times out.
HUAWEI TECHNOLOGIES CO., LTD.
RF lost rate
TCH RF Lost Rate = (times of radio link disconnection when TCH
seized (connection failure) + times of radio link disconnection
when TCH seized (error indication)) / successful TCH seizures (all)
* 100%
*
The RF lost rate reflects the call drop rate due to the Um
interface. Compared with call drop rate, the factor of ground link
disconnection during assignment is ignored in the definition. The
call drop rate minus the RF loss rate is the call drop rate caused
by the ground link.
HUAWEI TECHNOLOGIES CO., LTD.
Often-used Traffic Statistics Items Analysis
TCH congestion rate (TCH overflow)
It is a key item used to estimate the cell load.
*
Compare the traffic load of each cell to see whether the traffic
load is much more different with each other .
HUAWEI TECHNOLOGIES CO., LTD.
Causes of TCH congestion ( TCH seizure failure )
Assignment failure
Equipment fault
No channel available
Detailed causes of seizure failure:
Ground resources are unavailable, such as circuit fault and CIC
error.
Invalid message contents: some content in the assignment command is
invalid.
Assignment failure: assignment failure due to some causes, such as
interference or serious unbalanced link.
HUAWEI TECHNOLOGIES CO., LTD.
TCH Congestion Rate
*
TCH seizure failure:
1. CH_ACT_NACK has been received and the channel type is TCH.
2. When the timer waiting for CH_ACT_ACK message times out.
3. When channel allocation failed after the CH_REQ message is
received.
4. TCH seizure failure for very early assignment.
HUAWEI TECHNOLOGIES CO., LTD.
Often-used Traffic Statistics Items Analysis
SDCCH congestion rate (SDCCH overflow)
*
Too many location updates is a main cause of SDCCH congestion.
Modify parameters and add more SDCCH are reasonable
solutions.
HUAWEI TECHNOLOGIES CO., LTD.
SDCCH congestion measurement point
Attempted SDCCH seizure meeting a SDCCH blocked state is counted
when there is a SDCCH seizure but no SDCCH available.
Attempted SDCCH seizures (all)
Receive CH_REQ and the channel type is SDCCH.
*
Handover measurement point
For different objects such as BSC, band (900/1800),
incoming/outgoing, inter-cell/intra-cell handover and so on, there
are different items.
*
Handover Success Rate
*
Successful incoming internal inter cell handovers measurement
point:
1) Measured when the target cell (current cell) sends ASSIGNMENT
COMPLETE to MSC after the MS has been successfully handed over to
the target cell in the internal intercell handover (handover cause:
directed retry).
2) Measured when the target cell (current cell) sends a HANDOVER
PERFORMED message to MSC after the MS has been successfully handed
over to the target cell in the internal intercell handover
(handover cause: non-directed retry, e.g. signal quality, signal
strength, TA, traffic load, rapid rxlevel drop, better cell,
Response to MSC invocation, and O&M intervention).
Successful incoming interBSC inter cell handovers measurement
point:
During the procedure of interBSC handover, the target (current)
cell receives HANDOVER COMPLETE, and sends it to MSC.
Successful outgoing internal inter cell handovers measurement
point:
During the procedure of internal intercell handover, the source
cell (current cell) receives Inter Clear Request (cause: internal
intercell handover succeeded) from the target cell.
Successful outgoing interBSC inter cell handovers measurement
point:
During the procedure of interBSC handover, the source cell (current
cell) receives the CLEAR COMMAND message (cause: handover success)
from MSC.
HUAWEI TECHNOLOGIES CO., LTD.
Causes of handover
Other reasons
*
In a normal network, more than 50% of the handovers shall be PBGT
handover. If the proportion of a certain kind of handover (exclude
PBGT handover) is too large, the network maybe has some problems
such as equipment fault, interference, incomplete adjacent cells or
improper parameter settings, etc..
PBGT handover: Power Budget handover, based on path loss.
Poor uplink or downlink signal quality: handover caused by that the
uplink or downlink signal quality is lower than the threshold. It
is emergency handover.
Low uplink or downlink signal strength: handover caused by that the
uplink or downlink signal strength is lower than the edge handover
threshold. It is edge handover.
Too large TA value: handover caused by that TA value is larger than
the threshold.
HUAWEI TECHNOLOGIES CO., LTD.
Handover failure reasons
No available channel
*
Those items show the causes of handover failure so that the problem
can be found quickly.
No available channel: there is no channel available for the
incoming handover.
Illegal frequency: the frequency include in the handover command is
incorrect or erroneously coded.
HUAWEI TECHNOLOGIES CO., LTD.
TCH interference band measurement
The result in each TCH interference band shows the average number
of idle TCH within this interference band in the statistic period,
which reflects the average interference level.
*
BTS measures, calculates and reports the interference of the idle
TCH channels. It is used by BSC as a criterion to allocate the
channel. Interference is divided into six bands artificially
according to the strength. The level used to classify the
interference band is called interference band threshold. BTS uses
this interference band threshold to know the level of the current
signal’s interference strength and reports it to BSC through radio
resources indication message. BSC will decide whether handover is
necessary or not by comparing busy channels’ and idle channels’
resources included in radio resources indication message and
measurement report. The interference measurement result can be used
as a criterion to set the interference band threshold and analyze
the interference. For more details please refer to protocols 0808,
0858, and 1221.
Default values:
HUAWEI TECHNOLOGIES CO., LTD.
Cell frequency scan
Shows the signal strength received by main antenna and diversity
antenna.
Reflects the interference level for specific frequency.
*
By comparing the receiving strength of the main antenna and
diversity antenna, we know whether the direction, downtilt and
receiving path of the two antennas are the same or not. Frequency
scanning is an effective way for judging whether there is
interference on specific frequency or not. This function can only
be used for idle channels.
HUAWEI TECHNOLOGIES CO., LTD.
Receiving level measurement
The receiving level is divided into 6 bands
Band 0 : -110~-100dBm
*
If there are excessive measurement reports with high level but poor
quality, it indicates that there is interference existing in the
system. This shall be confirmed through the analysis of
interference band in TCH measurement function, drive test or
frequency scan. This measurement type is implemented on TRX
basis.
HUAWEI TECHNOLOGIES CO., LTD.
Page *
Exercise
List the often-used traffic statistics tasks that we use to locate
and analyze the problem.
Answer: the often-used traffic statistics tasks are
BSC measurement function
Inter-cell and intra-cell handover measurement function
Outgoing and incoming inter cell handover measurement
function
Up-down link balance measurement function
Call drop measurement function
Cell frequency scan, etc.
HUAWEI TECHNOLOGIES CO., LTD.
Chapter 2 Often-used traffic statistics items analysis
Chapter 3 Locate problem through traffic analysis
Chapter 4 Case study for traffic statistics
HUAWEI TECHNOLOGIES CO., LTD.
*
In practice, the above graph can be used flexibly. With other
features of the network, problems can be located quickly.
HUAWEI TECHNOLOGIES CO., LTD.
Drive test: simulate common subscriber’s behavior.
Analyze objects
TCH call drop analysis
Call drop measurement
TCH call drop
Average downlink quality
Call drop types
Edge call drop: low receiving signaling strength, large TA.
Short distance call drop: low receiving signal strength and small
TA.
BQ call drop: high receiving signal strength and poor
quality.
*
Respectively check relevant traffic statistics results according to
the character of the four types of call drop.
For edge call drop, consider whether the adjacent cell is added and
the adjacent cell is too busy.
For short distance call drop, check whether the coverage is
insufficient.
For BQ call drop, emphasize on locating and solving interference
problem.
The sudden call drop occurs accidentally and maybe caused by
hardware or transmission.
HUAWEI TECHNOLOGIES CO., LTD.
Edge call drop
Isolated site.
Add the necessary neighbor.
*
The core solution is to have a good coverage and eliminate island
and make the handover executed quickly.
HUAWEI TECHNOLOGIES CO., LTD.
Short distance call drop
Poor coverage caused by complicated terrain or high dense
building.
Solutions
Increase EIRP.
Adjust antenna direction and downtilt, make the main lobe point to
high traffic area.
Adjust parameters related to call drop.
*
Note: Short distance call drops seldom occurs. Call drops in urban
areas may be caused by decreasing BTS transmitting power or
increasing downtilt for reducing interference. When increasing BTS
power, we shall consider the interference brought to other
cells.
HUAWEI TECHNOLOGIES CO., LTD.
BQ call drop( high signal strength)
Causes
Uplink or downlink interference.
Interference caused by repeater.
Interference caused by improper frequency planning.
Self-interference caused by BTS.
Optimize frequency planning.
Solve the problem of transmission quality.
*
According to different causes, adjust the frequency plan to
eliminate transmission BER or eliminate the external interference
source. Repeater and over shooting are the most common causes for
interference.
HUAWEI TECHNOLOGIES CO., LTD.
Overall process for call drop analysis
Find out cells with high call drop rate.
Classify the call drop according to the character.
Analyze the cells’ traffic load and total call drop times.
Analyze the call drop measurement function.
Check the interference band.
*
The main causes for call drop
Interference (internal and external).
Improper handover (neighbor planning and handover parameter
setting).
Unbalanced up-down link (TMA, power amplifier, antenna
direction).
Improper parameter settings ( RLT and SACCH multi-frames).
Equipment problem (TRX, power amplifier, and TMA).
*
The above list are the main causes for call drop. According to each
causes, check the relevant traffic measurement items and solve them
one by one.
HUAWEI TECHNOLOGIES CO., LTD.
Interference analysis process
Block TRX in turn and monitor the interference measurement
results.
Calculate the handover caused by poor quality and check the average
receiving quality level for each TRX.
In call drop measurement function, check the average signal
strength and quality of call drops.
Through drive test, check the interference and signal
quality.
Use spectrum analyzer to find the interference source.
Dispose equipment fault (such as: TRX self-oscillation, antenna
inter-modulation).
*
The interference band is measured by idle TCH and sent to BSC via
RF resources indication message. If the number of TCHs in
interference band 3, 4 and 5 is not equal to 0, it means there’s
some interference in the network. The internal interference will
increase in accordance with the traffic load but external
interference does not have relationship with traffic load.
During the cell handover measurement, the outgoing handover
attempts caused by various causes are calculated. If there are
excessive handovers due to poor quality, it indicates there is some
interference. If the uplink quality is bad, it indicates there is
some uplink interference; if the downlink quality is bad, it
indicates there is some downlink interference.
Interference can also caused by overshooting. According to the
actual situation, eliminate the interference by adjusting the BTS
transmitting power, antenna downtilt, adjacent cell relationship
and handover parameters or adjusting the frequency plan.
HUAWEI TECHNOLOGIES CO., LTD.
Coverage problem analysis
Traffic items
In power control measurement function, the average uplink or
downlink signal strength is too low.
In receiving level measurement function, a lot of low signal
strength records can be found.
In inter-cell handover measurement function, the average receiving
signal strength is too low when handover is triggered.
In call drop measurement, the signal strength is too low when call
drop happens, or the TA value is abnormal.
*
Coverage problem analysis
In Power Control measurement
Check whether the average distance between MS and BTS comply with
design.
Check whether the maximum distance between MS and BTS is abnormal
in several continuous periods.
In outgoing inter-cell handover measurement function
*
Coverage problem analysis
Adjust the following parameters based on the drive test
results
BTS transmitting power
*
Improper handover (neighbor planning and handover parameters
Disposing method
Check the handover parameters to see whether there are improper
parameter settings.
In inter-cell handover measurement function, check whether there
are many unsuccessful outgoing cell handovers with unsuccessful
reversions.
*
When the handover cannot be executed normally, call drop may
happen. Causes for handover failure include data mistake, handover
command sent to a wrong cell (co-BCCH and co-BSIC), no channel
available in the target cell, interference or the small overlapping
with adjacent cells. Using traffic statistics, the causes for
handover failure can be located quickly.
HUAWEI TECHNOLOGIES CO., LTD.
Imbalanced up-down link (tower amplifier, power amplifier, and
antenna directions)
Disposing method
Analyze “up-down link balance measurement function” statistics
result and confirm whether the uplink and downlink are
balanced.
In call drop measurement function, analyze the average receiving
signal strength and quality for both the uplink and downlink.
*
The basic reason for call drop due to imbalanced links is that MS
makes a call in a worse cell or handover to a worse cell. Only the
downlink signals of the neighbor cells are measured during the cell
reselection and handover. MS does not know the details about
uplink. So, if the handover occurs towards a neighbor cell with
high downlink signal strength but low uplink signal strength, the
call drop or handover failure caused by bad uplink will
occur.
HUAWEI TECHNOLOGIES CO., LTD.
Traffic Statistics Analysis-TCH Call Drop
Balance between uplink and downlink
Let D= (downlink receiving level − uplink receiving level) − (MS
sensitivity − BTS sensitivity).
Usually the MS sensitivity is -102 dBm and the BTS sensitivity is
-108dBm. The formula can be simplified as
D= downlink receiving level − uplink receiving level – 6dB
If D=0, it means uplink and downlink are balanced
If D>0, it means downlink is better than uplink
If D<0, it means uplink is better than downlink
Link balance rank
Range of D
*
The absolute value of D shows how serious the uplink and downlink
are imbalanced. According to the range of D, the MRs can be sorted
into different ranks, showing the up-down link balance situation.
Eleven ranks are defined as shown above. The result of each rank
means the number of MRs. The sum of the results of all ranks of a
TRX is the total number of MRs, which can show the traffic volume
of the TRX.
HUAWEI TECHNOLOGIES CO., LTD.
Improper radio parameter setting (Radio Link timeout, SACCH
multi-frames)
Judgment method
In system information data, check the radio link timeout
*
Increasing the radio link timeout and SACCH multi-frames can reduce
the call drops . And increasing the value of radio link connection
timer can also reduce the call drops.
HUAWEI TECHNOLOGIES CO., LTD.
Equipment problem (TRX, power amplifier, tower amplifier,
etc.)
Judgment method
In TCH measure function, many TCH seizure failures due to A
interface problem.
In call drop measurement function, there are many call drops due to
ground links.
In TCH measurement function, there are many TCH seizure failures
due to equipment failure.
Disposing method
*
If the availability of TCH is abnormal, it indicates some timeslots
have problem. Excessive call drops due to ground link disconnection
indicates transmission or circuits have some problems. In this
case, it is required to check the alarms and the equipments.
HUAWEI TECHNOLOGIES CO., LTD.
SDCCH call drop
Refer to TCH call drop analysis.
The cause and mechanism of SDCCH call drop are almost the same as
TCH.
HUAWEI TECHNOLOGIES CO., LTD.
1.Illegal Channels
2.Illegal Carrier
3.Illegal TA
4.Timer out
Analyze the incoming and outgoing handover first,
According to the first step, analyze handover of all of the
adjacent cells or of a particular adjacent cell,
Associate with TCH measurement, alarm and other factor to find out
the reason.
HUAWEI TECHNOLOGIES CO., LTD.
Congestion
Interference
Coverage
*
Improper handover parameters: high inter cell handover hysteresis,
very low load receiving threshold, very long P/N time, etc..
Clock problem is one of the main cause for handover failure.
HUAWEI TECHNOLOGIES CO., LTD.
Find out the cells with low handover success rate.
Find the out the cells with high handover failures.
Compare the incoming cell handover failures and outgoing cell
handover failures.
Register the task to measure the incoming cell handover and
outgoing cell handover.
Find out handover failure relation (failure to all the neighbor
cells or part of the neighbor cells).
HUAWEI TECHNOLOGIES CO., LTD.
Disposing method
Check whether the handover threshold such as TA, BQ and handover
function switch are suitable or not.
Check whether the successful TCH seizures for handover are much
more than successful TCH seizures for call. If handover times
divided by call times is larger than 3, then it indicates that
there may be ping-pong handover. Check the parameter settings and
adjust them (layer setting, layer handover hysteresis, inter cell
handover hysteresis, PBGT threshold, etc.).
*
Hardware fault
Problem description
The target cell has idle channels but when applying for the
channels CH_ACT_NACK or TIMEOUT message appears.
TCH availability is abnormal.
*
Hardware fault
Disposing process
Monitor transmission and board alarms (FTC board failure, A
interface PCM sync alarm, LAPD link disconnected, TRX alarm).
Analyze whether the transmission is disconnected or the boards have
some fault (for example: the TRX is damaged).
Check whether there is clock alarm.
*
Cells with low incoming handover success rate.
Neighbor of the cell with low incoming handover success rate.
Locating the problem
In incoming inter cell handover measurement function, check whether
many handover failures are caused by congestion.
For low incoming handover success rate, check the cell’s traffic
.
*
Handover failure (no channel available) in the traffic measurement
indicates the handover failure due to congestion of the target
cell.
HUAWEI TECHNOLOGIES CO., LTD.
Adjust the cell’s coverage (adjust BTS transmitting power,
RXLEVEL_ACCESS_MIN, RACH access threshold, and the antenna downtilt
and height).
Adjust parameters (CRO, load handover parameters, cell priority and
handover parameters).
*
Page *
Question
The item “radio handover success rate” is the ratio of successful
handovers to handovers. The handovers are counted when sending or
receiving HO_CMD or HO_REQ_RSP in the handover process. Please
write down the possible reasons that can cause the low radio
handover success rate between BSCs.
HUAWEI TECHNOLOGIES CO., LTD.
Page *
Answer
There are two kinds of outgoing handovers. One is intra-BSC
handover, and the other is inter-BSC handover.
The possible reasons are list as following. For inter-BSC
handover:
If the uplink signal strength of the target cell is low, the MS
cannot access the target cell. Thus the handover fails
If the target cell is a wrong cell which has the same BCCH and BSIC
as the expected target cell, MS will send access request to a wrong
cell. Then MS cannot access. Thus the handover fails.
If the CGI is wrong, MSC will send the handover request with wrong
CGI. Then MS will send access request to a wrong cell. It has the
same effect as the above one.
HUAWEI TECHNOLOGIES CO., LTD.
*
Judgment method
There are many channel request rejections due to channel
busy.
Incoming handover measurement shows that there are too many
unsuccessful incoming cell handovers (congestion).
Disposing process
Expand or adjust the configuration between high and low traffic
cells
Adjust coverage (adjust BTS transmitting power, antenna direction,
downtilt, height, etc.).
Adjust parameters (CRO, Rx_Lev_Access_Min, load handover
parameters, cell priorities, handover parameters).
*
Problem description
Interference brings unacceptable BER which affects the assignment
process.
Downlink Interference makes MS’s DSC decrease to 0, then MS
reselects to another cell with low signal strength, and this is a
potential reason for TCH seizure failure.
If TCH seizure failures (including handover) minus attempted TCH
seizures meeting TCH overflow is large, then there may be some
interference.
Disposing process
*
Disposing process
In cell frequency scan measurement function, check the measurement
results got from main receiving antenna and diversity receiving
antenna.
In Up-down link balance measurement function, check the measurement
report numbers in each rank.
Check antenna direction, downtilt and connection.
*
*
MS shall not reside in the cell with low signal strength, so set
the RXLEV_ACCESS_MIN to balance the traffic carefully.
HUAWEI TECHNOLOGIES CO., LTD.
*
Insufficient system capacity
Improper LAC planning
*
Too many location updates are the common cause for SDCCH
congestion. Insufficiency of system capacity can be indicated by
TCH congestion. Interference and low random access threshold will
cause illusory random access request.
HUAWEI TECHNOLOGIES CO., LTD.
Successful immediate assignments / immediate assignment
transmissions >85%.
The above formula shows the ratio between number of EST_IND
messages that MS sends to BSC and the immediate assignment commands
that BSC sends to BTS. It indicate whether there are some improper
parameters in the system information.
Disposing process
Adjust the access parameters (Random access error threshold, RACH
minimum access level, MS Max Retrans, Tx-integer).
*
Appropriately increasing T and S (maximum number of extended
transmission timeslots) values can be of help to reduce the
collisions on the random access channel, and appropriately
increasing RACH minimum access level can improve the immediate
assignment success rate. In urban and countryside, RACH minimum
access level should be set to difference value according to
frequency reuse pattern.
HUAWEI TECHNOLOGIES CO., LTD.
Problem description
Many location updates happen at the border of different location
areas.
Massive location updates happen simultaneously.
Disposing method
Configure more SDCCHs
*
As a basic principle, one SDCCH channel is needed for every two
TRX. If the TRX number is odd, add one more SDCCH channel. Massive
location update caused by the passing of the train can only be
solved by configuring more SDCCHs.
HUAWEI TECHNOLOGIES CO., LTD.
The border of different location area is the street.
*
Avoid setting two location areas in the urban area of a small and
medium-sized city. Location areas overlapping with each other too
much are also not recommended.
HUAWEI TECHNOLOGIES CO., LTD.
RACH minimum access level is low.
Interference in the system, which will bring a lot of illusory
SDCCH channel requests.
Disposing process
Eliminate the interference
Chapter 2 Often-used traffic statistics items analysis
Chapter 3 Locate problem through traffic analysis
Chapter 4 Case study for traffic statistics
HUAWEI TECHNOLOGIES CO., LTD.
Problem description
Handover success rate is always very low because of the congestion
(about 70%)
In the evening of 3rd. Dec, site D located in urban is expanded
from “S2/2/2” to “S3/3/3”. The busy hour handover success rate does
not get improved after expansion. Sometimes handover success rate
is lower than before. At the same time, lots of subscribers
complain about the network quality.
Refer to busy hour traffic statistics of 3rd. Dec.
*
Intra-BSC incoming handover failures (others)
TCH congestion
Intra-BSC incoming handover failures (others)
TCH congestion
Analysis
Analyzing the traffic statistics before expansion and after
expansion, we find that before the expansion the handover failure
is caused by congestion and the radio handover success rate is
normal. In Urban area many cells’ radio handover success rate
decreased after expansion, therefore we conclude that the cause of
handover failure after expansion has been changed.
*
Analysis
After checking the hardware of site D through maintenance console,
we find the state of TMU board is abnormal and clock is unstable.
Finally we affirm the low handover success rate is caused by the
wrong setting of switches in TMU board. For sure, the high call
drop rate is caused by handover failures.
The day after processing, the busy hour handover success rate is
higher than 90%. Thus the problem is solved.
Refer to busy hour traffic statistics of 17th. Dec.
*
Intra-BSC incoming handover failures (others)
TCH congestion
Case Study-One (Handover)
Conclusion and suggestion
Pay attention to the difference between radio handover success rate
and handover success rate because it can help us to locate handover
problem efficiently.
*
Fault description
When we analyze the traffic statistics, we find that a cell’s call
drop rate in busy hour is large than 2%.
In cell call drop measurement function, we find that the average
uplink level of call drop is 1 (-109dBm), while the downlink level
is 26 (-84dbm). High call drop rate is caused by imbalance between
uplink and downlink.
*
Analysis
We can exclude the problem of antenna and feeder because only one
of the two TRXs is abnormal. Therefore we think that the problem
may be caused by the uplink channel of TRX or CDU.
After we change the CDU, the problem is solved.
*
Conclusion and suggestion
To find the cause of call drop, we should register the following
useful traffic statistics:
TCH measurement function
Inter cell handover measurement function
Up-down link balance measurement function
By analyzing the result of the above traffic statistics, we can
locate the cause of the call drop (handover, interference, coverage
etc.) and then register more detailed traffic measurement
tasks.
*
Fault description
In the network, the radio link connection success rate is low.
After analyzing the traffic statistics, we find that it caused by
SDCCH congestion and the congestion only exists in a few
sites.
*
Analysis
Register “SDCCH measurement function” and analyze the result. We
find that most of the SDCCH seizures are used for location update.
After we analyze the site distribution, we find that the congested
BTSs are located at the border of two location areas along the
railway. So we can conclude that SDCCH congestion shall be caused
by massive location updates.
In SDCCH measurement function, we find that most of the location
update happened in a specific 5 minutes. After checking the train
timetable, we find that 4 or 5 trains passed by in this period.
When the trains pass by, a large amount of location updates happen
simultaneously.
*
Conclusion and suggestion
For SDCCH congestion, firstly we should register SDCCH measurement
function, and then analyze the traffic statistics to find the cause
of the problem (Location update, SDCCH handover, call setup
etc.).
Then check the parameter settings, interference, location area
planning etc., to do further analysis.
Adding SDCCH channels or enabling dynamic SDCCH allocation function
can solve the congestion caused by insufficient capacity.
*