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RNE VISAYAS GSM
DATE : MARCH 09-11,2004
IT GT PLAZA LAHUG , CEBU CITY
TABLE OF CONTENTS
1. LOW AVAILABILITY ON TCH and SDCCH
2. LOW TCH ASSIGNMENT SUCCESS
3. TCH and SDCCH CONGESTION
4. TCH and SDCCH DROP CALL RATE
6. UNSUCCESSFUL HANDOVER
7. HANDOVER RETURN TO OLD CHANNEL
8. PING PONG
9. INTRA CELL HANDOVER
12. RNO TOOL – MRR , NCS , FAS
Probable reasons to various performance problems in a mobile network and
instructions on how to solve and identify the problem are given.
A proposed analysis flow is given below:
1 Analyse the characteristics of the problem
2 Identify Base Line Requirements
3 Calculate Overall System Performance
4 Do a Benchmarking - Compare with other Operators
5 Identify Problem Areas
6 Retrieve Detailed Statistics on the Problem Areas
7 Initiate and Perform further Measurements
8 Perform a Careful and Detailed Analysis
9 Take Actions to Solve the Problems
10 Verify improvements
11 Give Recommendations
- Short term (e.g. parameter changes, etc)
A low availability may lead to congestion and should thus be monitored regularly. Faulty equipment should as
soon as possible be removed and replaced.
Check the amount of available control channels. If the percentage of available channels
is not sufficient then check if any channels are blocked or not installed.
See flow chart SDCCH & TCH Availability in the next Slide
Available (not blocked) SDCCH’s of Total Number of Defined SDCCH’s – See CDD Channel Admin
Total Number of Defined SDCCH’s See kpi S_AV_NR , CNUCHCNT and S_AVAIL at MS ACCESS DB
Available (not blocked) TCH’s of Total Number of Defined TCH’s - See CDD Channel Admin worksheet
Total Number of Defined TCH’s Check T_AV_NR , TCH_AVG , TNUCHCNT
1. Probable Reasons- Hardware Fault
Check BTS error log
Replace and repair faulty hardware.
3.Using of Synthesizer Hopping
When using of synthesizer hopping, one dedicated TRU will be used for BCCH, this is one reason for
low TCH availability. Another reason is when using more frequencies than actual TRUs, the system will
count more defined channels.
. Check using of Synthesizer Hopping.
. Check the number of required basic physical channel in a channel
Define 2 channel groups, one for BCCH and one for TCH
Change NUMREQBPC to the actual value.
2. Probable Reasons- Transmission Problem
Check transmission performance.
Add transmission capacity. Change transmission equipment. Change transmission lines.
BSC MML COMMANDS
Note: RLCRP The Last letter of this MML command denotes PRINT and
not critical to invoke!!!
Step by Step procedure to
check BTS alarm logs !!!
Step by Step procedure to check digital path quality high
RLCDP:CELL=CSTABAR;(Cell Multiple Channel Alloc
RLDEP:CELL=CSTABAR;(Cell Design Data)
RLLCP:CELL=CSTABAR;(Cell Load Sharing)
RLSLP:CELL=CSTABAR;(# of SDCCH resources)
RRTPP:TRAPOOL=ALL;(per BSC command)
EXRPP:RP=ALL;(check number of rpps1 in the bsc)
RLGSP:CELL=CSTABAR;(GPRS Config Data)
STDEP: DEV=RTGPHDV-0&&-127;(Check RPP)
First RPP is from 0 - 63 RTPGPHDV,
Second RPP is from 64 - 127 RTPGHDV
Per RPP should have 37 "BUSY" V STATE
For two RPPs there must have 75 "BUSY" V STATE
Common Cell Resources MML Commands
TO RESET the DIP Q
Traffic congestion is one of the major network problems in a mobile system. A high congestion deteriorates the
overall performance of the network and should be minimized.
∀ Short term growth
If the high traffic related to an occasional event, like sport event, fairs, conference, a temporary solution might be
∀. Long term growth
If there is a long term growth the network capacity has to grow according to the demand.
Cellular System Capacity depends on a number of different factors namely:
1. The number of channels available for voice and data
2. The GOS Grade of Service the subscribers are encountering in the system.
What is TRAFFIC ?
It refers to to the usage of channels and is usually thought of as the holding time per
time unit or the number of “call hours” per hour
How MUCH Traffic Can One Cell Carry?
It depends on the number of traffic channels available and the amount of congestion which
is acceptable ( by both the subscriber and the operator)
GOS = Grade of Service - is the percent probability that a call is blocked due to congestion.
Low TCH Assignment Success Rate
Successful assignments show the number of successful TCH allocations at call setup.At unsuccessful
assignment, the Assignment Complete message, sent by the MS, was never received by the BTS.
Probable Reasons The reason for a low number of successful assignments might be the following
. No Dominant Serving Cell
Action: Check coverage plots, perform drive tests.
Solution:Add new base station.
. Severe Congestion on the TCH.
Action: Check failed TCH allocation for assignment due to congestion. Check failed TCH allocation for
handover due to congestion Check TCH congestion.
Low Signal Strength for Call Access.
The signal strength might be higher on the BCCH than on the TCH.
Action: Check signal strength of BCCH and TCH. Perform MTR recordings.
Solution: Adjust and increase TCH output power.
Interference, disturbance on SDCCH or target TCH frequency.
Action: Check Interference.
Solution:Improve frequency plan and decrease interference.
. Faulty Transceiver
Action: Check BTS Error Log.
Solution:Replace or repair faulty equipment.
. Low Output Power
Action: Check set output power parameters and the actual transceiver output power.
Solution: Increase output power or replace faulty transceiver.
Type of Congestion
•Identify if there is only SDCCH or TCH congestion or both. Congestion on both SDCCH and TCH may mean
that the only way to get rid of the congestion is to add more physical capacity in terms of transceivers or sites.
•Consider how many channels that are allocated in the cell. If possible, expand the capacity with new
transceivers, otherwise a new site must be implemented.
•Frequency planning schemes such as Multiple Reuse Pattern could be used to relieve congestion. Micro cells
could be used to take traffic in severe congested areas.
The time congestion should be used instead of congestion based on access attempts as there is no way to
estimate the number of access attempts a single mobile does.
See also flow chart ‘SDCCH Congestion in the next slide.
SDCCH Time Congestion - See kpi counter S_CNGT
100 * CTCONGS / ( PERLEN * 60 ) -SDCCH Time Congestion of Total Measurement Interval
SDCCH Congestion in Total Number of SDCCH Seizure – See counter CMSESTAB
CTCONGS-Total SDCCH Time Congestion
CMSESTAB -Total SDCCH Connections
As a WAY Forward :
Compute 0.5% GOS for SDCCH resources using PEAK traffic.
NOTE : The most accurate SDCCH Dimensioning is achieved by looking at the Congestion on TCH and SDCCH
for the specific cell . The optimum configuration is achieved by selecting a configuration with as many as
TCHs as possible without letting SDCCH GOS exceed 0.5% GOS or ¼ of TCH GOS if TCH GOS is 2%.
Probable Reasons - Increasing Traffic Demand
The high traffic could be related to an occasional event or due to a long term growth.
Action - Check if short term traffic growth. Make trend comparisons.
Check if combined SDCCH is used.
Check SDCCH dimensioning.
Solution: Increase the number of SDCCH channels. Note, that an increase may lead to the need for new
transceivers.If combined SDCCH is used, non-combined channel configuration should be introduced.
Probable Reasons - Bad use of Adaptive configuration of logical channels
By using the Adaptive configuration of logical channels feature, the basic SDCCH configuration in a cell
will be under-dimensioned. If this feature is not used correctly, it will cause SDCCH congestion.
Action: Check if ACSTATE is on. Check parameters related to Adaptive configuration of logical
Solution: If ACSTATE is off, it is suggested to switch on.
Probable Reasons - Long Mean Holding time
If the mean holding time is long, this generates a higher traffic load.
Action: Check SDCCH Mean Holding Time
SDCCH Mean Holding Time
Probable Reasons- False Accesses
No response from MS after Channel Request. The system
waits about 5 seconds before performing a disconnection and
the channel is available again.
Action:Check frequency plan.Check interference.
Solution:Improve frequency plan and reduce interference.
Probable Reasons- Faulty Transceiver
Action: Check BTS error log.
Solution:Change & repair faulty equipment
Probable Reasons- Congestion on
Action: Check signalling
performance & tranmission capacity.
Solution: Add more transmission
capacity or re-route traffic if possible.
Probable Reasons- Many SMS
Action:Check SMS Performance.
Solution:Reconsider the SDCCH
dimensioning and the amount of SMS
messages if congestion.
Probable Reasons-Long mean holding
A long mean holding time may indicate:
. Congestion on TCH
Action: Check TCH Congestion
Solution:Increase the TCH capacity.
See also flow chart ‘SDCCH Mean Holding Time in the next slide.
Probable Reasons - TCH Congestion
TCH congestion causes the mobiles to stay extra long time on the SDCCH. Check if there exists TCH congestion
and if the SDCCH mean holding time
is high. For immediate assignment the time is 2-2,5 seconds.
Action- Check TCH Congestion Check SDCCH Mean Holding Time Check if Assignment to Worse cell is
used and existing parameter setting.
Check if Cell Load Sharing is used.
Solution:Increase TCH capacity.Use the features for traffic distribition such as Cell Load Sharing and
Assignment to Worse Cell.
Probable Reasons - Low Availability
Action: Check SDCCH Availability Check if the channels are manual, control or automatic blocked.
Solution:Change and repair faulty equipment.Review the O&M procedures.
Probable Reasons - Too Frequent Periodic Registration
Check Random Access Distribution Check the timer T3212 in the BSC and the parameters
BTDM and GTDM in the MSC
Solution: Decrease the periodic registration.
Probable Reasons -Wrong SDCCH Dimensioning
Action: Check SDCCH dimensioning. Location Area Border Cell. If the cell is situated on a misplaced
Location Area border, this means that unnecessary many normal LUs are performed.
Action: Check site position and location area border.
Check Location Update Performance Check parameter CRH etc.
Solution: If the site is located close to major road or railway, consider to move the Location Area border.
Increase the hysteresis CRH. The CRH is the hysteresis value used when the MS in idle mode crosses a LA
border. The default value for this parameter is 4. If a high number of Location Updatings occurs in a
Location Area border cell, a higher CRH can be set in order to reduce the number of LUs.
Probable Reasons -SMS Usage. Extensive SMS usage increases the SDCCH traffic and could cause
congestion if badly dimensioned SDCCH channels.
Action: Check SMS activity.
Solution: Redimension the SDCCH channels with consideration taken to SMS usage.
Probable Reasons -Cell Broadcast Used
Action: Check if Cell Broadcast is active. If active, check if it is used by the operator.
Solution: Remove Cell Broadcast if not used.
Probable Reasons - IMSI Attach/Detach in Use. An introduction of IMSI attach/detach will increase the
traffic on SDCCH.
However, the benefits are that the paging success rate will increase. The recommendation is to use Attach/Detach.
Probable Reasons -Software File Congestion
Action: Check SAE setting. High Ratio of Random Accesses
Action: Check Random Access performance
TCH Mean Holding Time
See also flow chart ‘TCH Mean Holding Time’
TCH Mean Holding Time
TCH Mean Holding Time in Underlaid Subcell
Probable Reasons-Short mean holding time
Short TCH mean holding time might be the case for micro cells or cells close to
A short mean holding time may indicate:
•Abnormal Amount of Disconnections
Action: Check Dropped Calls
Solution:Investigate further the reason for the short mean holding time. Many Handovers out
from the Cell
Action: Check Handover Performance.
Solution:The handover related cell parameters may need to be changed.
∀ Many Intra-cell Handovers
Action: Check Intra Cell Handover parameters.
Solution:Check and change parameter setting. The reason may be interference.
∀ Faulty Transceiver
Action: Check BTS error log.
Solution:Change & repair faulty equipment.
Long mean holding time
A long mean holding time may indicate:
Action: Check channel release time. Check if any devices are busy more than 1 hour.
Solution:Remove hanging subscribers.
Inbuilding Cells (repeater or micro cells)
Action: Check site location. No further action.
The time congestion should be used instead of congestion based on access attempts as there is no way to estimate
the number of access attempts a single mobile does.
The following formulas should be used to estimate the congestion.
TCH Time Congestion of Total measurement Interval
Number of dropped calls due to TCH Congestion or transcoder congestion of Total Number of TCH Assignment
Probable Reasons . Increasing Traffic Demand
The high traffic could be related to an occasional event or due to a long term growth.
Action: Check if short term traffic growth. Make trend comparisons. Check TCH dimensioning Check the
use of congestion relieving features such as Assignment to Worse cell, Cell Load Sharing and HCS.
Solution: Increase the number of transceivers. This may lead to problems with floor space, antenna
installations, CDU type, expansion cabinets and combiner type. If not used, introduce Assignment to Worse cell
and Cell Load Sharing. Note that the interference level will increase if Assignment to Worse cell is used as some
mobiles will be closer to a co-channel cell than was intended in the frequency plan. The feature will be more
effective if the neighbours are not congested. In a tight network with a
high reuse and congestion in a larger area, the feature might only make the situation worse.
Bad allocation of TCH in a system may cause unnecessary congestion. Investigate if possible to move
transceivers from non-congested areas to congested areas. Of course, the base station type, CDU-type, current
number of transceivers, floor space, combiner type, etc., should be considered before a recommendation to
move transceiver could be made.
Action: Check TCH traffic and dimensioning.
Solution: Redimension the TCHs.
. Hardware Fault & Installation Fault
Faulty equipment will lead to that all time slots could not be used for handling traffic causing congestion. Low
availability can happen if the channels have been manually or automatically blocked and taken out of service.
Action: Check TCH Availability.Check TCH blocking.
Solution:Change and repair faulty equipment.Review the O&M procedures.
. High Antenna Position
A high antenna position could mean a too large service area. Also antennas placed on hilltops will cover
large areas. A large coverage area might mean that the cell takes a lot of traffic.
Action: Check antenna height. Check antenna type Check antenna tilt
Lower antenna if there is no risk for loss of coverage (no coverage at all). Tilting of the antenna or
changing antenna type may also decrease the coverage area.
Wrong Use of Hierarchical Cell Structure, HCS
If the cell is incorrectly defined as higher priority level of Hierarchical Cell Structure or HCS parameters are not
being used properly, it will draw in more traffic than other cells.
Action: Check HCS related parameters.
Solution:Change to normal value.
. Low Handover Activity
A low handover activity may lead to congestion if the MS is forced to stay on a cell longer than necessary.
Action: Check if congestion in neighbouring cell.Check handover performance Check neighbouring cell
definitions. Missing relations
could cause handover problems.
Correct handover parameters such as too high or too low hysteresis
values, missing neighbour relations, one-way handovers.
Congestion in neighbouring cell need to be decreased.
. Congestion in Surrounding Cells
Action: Check congestion in neighbouring cells.Review neighbour cell list. New relation could relieve the
congestion.Check if Assignment to Worse cell is used. If assignment handover to worse cell is used (directed
retry). Check the setting of the parameter
Add new neighbour cells if appropriate.
There is a wide range of factors can result in that a subscriber fails to complete a call satisfactorily. The only problem
many subscribers will tolerate in a public network is a busy tone from the called party. Unfortunately, reality does not
always match expectations when it comes to mobile network, which results in customers complaining about poor
performance of the service. They will eventually change operator if the perceived bad performance
continues.A subscriber should be able to place calls satisfactorily and hang up in its own time, regardless of the
technical reasons for call-completion problems.
Dropped calls show the number of abnormal disconnections during call setup or during conversation.Use the different
counters for dropped calls, i.e. dropped calls due to low signal strength, bad quality, too high timing advance and
miscellaneous to get an indication of the reason for possible bad performance.The mechanisms behind dropped calls on
TCH and SDCCH are described below:
1. Radio Link Time-Out
Every time a SACCH message can not be decoded the radio link time-out counter is decreased by 1. If the
message can be decoded the counter is incremented by 2. However, the value can not exceed the initial
value. The initial value is set by the parameter RLINKT for radio link time-out in the mobile station and by
RLINKUP for time-out in the BSC.If the mobile moves out of coverage and no measurement reports are
received in the BSC, there will be a radio link time-out and the message Channel Release (cause: abnormal
release, unspecified) is sent to the mobile station and the SACCH is deactivated in the BTS. A Clear Request
message is sent to the MSC. To be sure that the mobile has stopped transmitting the BSC now waits
RLINKT SACCH periods before the timeslot is released and a new call can be established on the channel.
2. Layer 2 Time-Out
If the BTS never get an acknowledge on a Layer 2 message after the time T200XN200, the BTS will send
Error Indication (cause: T200 expired) to the BSC, which will send Channel Release (cause: abnormal
release, timer expired) to the mobile station and a Clear Request to the MSC. The SACCH is deactivated
and the BSC waits RLINKT SACCH periods before the timeslot is released and a new call can use the
channel. This is only valid if the call is in steady state, i.e. not during handover or assignment.
3. Release Indication
When the BTS received a layer 2 DISC frame from the mobile it replies with a Layer 2 UA frame to the
mobile station and a Release Indication to the BSC.The system does only react on Release Indication if it is
received during a normal disconnection situation. If such a message is received unexpectedly this will
usually cause radio link time-out or timer T200 expiration as the mobile station stops the transmitting of
measurement reports. It is also possible that the release will be normal depending on when the Release
Indication is received.
4. MSC Time-Out
If the MSC never received a response on a message (e.g. Identity Request) and there is no radio link time-out or layer 2 time-out, the
MSC will send a Clear Command to the BSC. The time-out is depending on the message. When receiving Clear Command, the BSC
will send a Channel Release (cause: normal release) and then deactivates the SACCH.
Reject (only SDCCH):
If the MSC never receives a response on the first message after Establish Indication, the MSC will send a reject message. If the
connection was a Location Update it will be a Location Update Reject (cause: network failure) and if the connection was a mobile
originating call (CM Service Request) a CM Service Reject (cause: network failure) will be sent. The MSC will then send a Clear
Command to the BSC and the call is cleared by Channel Release (cause: normal release).
5. Assignment to TCH
Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled:
a. There must be a TCH channel available, i.e. no congestion
b. b. The locating algorithm must have received at least one valid measurement report If either of the criterion is not fulfilled,
Assignment Command will not be sent and a Channel Release (cause: abnormal release, unspecified) will be
sent to the mobile station and a Clear Request to the MSC. From a subscriber point of view, the most serious dropped calls are
those that interrupts an ongoing conversation, i.e. a call dropped on the TCH. If the call is dropped on the SDCCH the user
simply re-dials again and hopefully succeeds with the new call setup.
From a system point of view, the dropped calls on the SDCCH are more serious. A radio link time-out on the SDCCH will
occupy an SDCCH sub-channel for approximately (RLINKUP+RLINKT)/2 seconds and increase the risk for SDCCH
CONERRCNT is one counter of object type MOTS,
which indicates abnormally terminated connections on
one time slot and is incremented for TCH and
when a connection is dropped. The counter
CONERRCNT should be used to identify
problem device and interference on a certain channel.
The counter will identify a TS that have more dropped
calls than the average TS.
When a call is abnormally disconnected, that is CLEAR REQUEST is sent to
the MSC, a check is made in the function Assignment or Handover if any of
the following type of urgency state existed. If more than one type of urgency
state are indicated by the locating procedure, the following priority (highest
priority first) is used to determine the type of urgency state:
1 Excessive TA
2 Low signal strength in downlink and/or uplink
3 Bad quality in downlink and/or uplink
4 Sudden loss of connection
When the type of urgency state is determined, one of the counters for this
urgency state is stepped. It should be noticed that at most one counter is incremented
for a dropped connection (except for low signal strength where at
most two counters are incremented).
COUNTERS COUNTERS DESCRIPTION
CDIS_OTH CNDROP - CDISSS - CDISQA - CDISTA SDCCH drop due to other reasons
CDISQA CDISQA Total SDCCH Drop due to Bad Quality
CDISSS CDISSS Total SDCCH Drop due to Low Signal Strength
CDISTA CDISTA Total SDCCH Drop due to Excessive Timing Advance
CNDROP CNDROP Total Dropped SDCCH Connections
CNRELCONG CNRELCONG Released SDCCH connections due to TCH Congestion
CNROCNT CNROCNT Total Accepted Random Accesses
CNUCHCNT CNUCHCNT Total Number of Defined SDCCH's
TFDISQABL TFDISQABL Total TCH Drop due to Bad Quality Both link
TFDISQADL TFDISQADL Total TCH Drop due to Bad Quality Down link
TFDISQAUL TFDISQAUL Total TCH Drop due to Bad Quality Up link
TFDISSBL TFDISSBL Total TCH Drop due to Low Signal Strength Both link
TFDISSDL TFDISSDL Total TCH Drop due to Low Signal Strength Down link
TFDISSUL TFDISSUL Total TCH Drop due to Low Signal Strength Up link
TFDISTA TFDISTA Total TCH Drop due to Excessive Timing Advance
TFMSESTB TFMSESTB Total TCH Connections FR
TFNDROP TFNDROP Total Dropped TCH Connections FR
TFSUDLOS TFSUDLOS Total Sudden Loss
TCH and SDCCH DROP CALL COUNTERS
T_DR-C 100 * TFNDROP / TFMSESTB Dropped TCH Connections of Total TCH Connections FR and HR
100 * TFNDROP / ( TFCASSALL + ( SUMIHOVERSUC -
SUMIHOSUCBCL - SUMIHOSUCWCL ) - ( SUMOHOVERSUC -
SUMOHOSUCBCL - SUMOHOSUCWCL ) )
Dropped TCH Connections of Total Conn. Terminated in the Cell FR
T_DR_BQ_BL 100 * TFDISQABL / TFNDROP
Dropped Connections at Bad Quality both links of Total Number of
T_DR_BQ_DL 100 * TFDISQADL / TFNDROP
Dropped Connections at Bad Quality downlink of Total Number of
T_DR_BQ_UL 100 * TFDISQAUL / TFNDROP
Dropped Connections at Bad Quality uplink of Total Number of
T_DR_ERL 60 * TFNDROP / ( PERLEN * ( TFTRALACC / TFNSCAN ) ) Dropped TCH Connection per Erlang FR and HR
T_DR_ERLM PERLEN * ( TFTRALACC / TFNSCAN ) / ( TFNDROP ) Erlang Minutes per Dropped TCH Connection FR and HR [min]
100 * ( SUMOHOVERCNT - SUMOHOVERSUC -
SUMOHORTTOCH + SUMEOHOVERCNT -
SUMEOHOVERSUC - SUMOHORTTOCH ) / ( TFNDROP ) Lost HOs of Dropped TCH Connections FR and HR
100 * ( TFNDROP - ( TFDISSBL + TFDISSDL + TFDISSUL +
TFDISQABL + TFDISQADL + TFDISQAUL + TFDISTA +
TFSUDLOS) ) / TFNDROP Dropped TCH Connections due to Other Reasons FR
T_DR_SS_BL 100 * TFDISSBL / TFNDROP
Dropped Connections at Low SS both links of Total Number of
T_DR_SS_DL 100 * TFDISSDL / TFNDROP
Dropped Connections at Low SS downlink of Total Number of
Dropped TCH C
T_DR_SS_UL 100 * TFDISSUL / TFNDROP
Dropped Connections at Low SS uplink of Total Number of Dropped
T_DR_SUD 100 * TFSUDLOS / TFNDROP
Suddenly lost Connections of Total Number of Dropped TCH
T_DR_TA 100 * TFDISTA / TFNDROP Dropped TCH due to Excessive Timing Adv. FR and HR
S_DR-C 100 * CNDROP / CMSESTAB Dropped SDCCH Connections of Total SDCCH Connections
S_DR_BQ 100 * ( CDISQA + CDISQASUB ) / CNDROP Dropped SDCCH Connections due to Bad Quality
S_DR_ERL 60 * CNDROP * CNSCAN / ( CTRALACC * PERLEN ) Number of Dropped SDCCH Connections per Erlang
S_DR_ERLM ( CTRALACC * PERLEN ) / ( CNDROP * CNSCAN ) Erland Minutes per Dropped SDCCH Connection [min]
S_DR_HO 100 * ( CCHHOCNT - CCHHOSUC - CCHHOTOCH ) / CNDROP Lost Handovers of Total Number of Dropped SDCCH connections
100 * ( CNDROP - ( CDISSS + CDISSSSUB + CDISQA +
CDISQASUB + CDISTA ) ) / CNDROP Dropped SDCCH Connections due to Other Reasons
S_DR_SS 100 * ( CDISSS + CDISSSSUB ) / CNDROP Dropped SDCCH Connections due to Low SS
S_DR_TA 100 * CDISTA / CNDROP Dropped SDCCH Connections due to Excessive Timing Advance
TCH and SDCCH DROP CALL FORMULA
Dropped Call Analysis
1. Check dropped calls per cell. Select cells with high dropped call rate.
2. Check reason for dropped calls for selected cells
3. Check ratio of lost handovers to dropped calls.
The reason is to determine if the high dropped call rate is related to
handover performance. Check also which cell relations that have more
dropped calls than the average neighbour relation.
4. Check dropped calls per TS to find faulty devices or interference. Check
BTS error log
. Run CTR/MTR/CER on the cell with high dropped call rate.
. Perform drive tests and site surveys.
. Check frequency plan, coverage and interference plots
. Check output power and cell parameter settings
Dropped Calls on SDCCH
Radio Link Layer Description
The following cases will step CNDROP.
∀. Error indication (Sequence Error ) and Channel Release, abnormal release:
A fault has occurred on the radio link layer (GSM 08.58). This message is sent if the Layer 2 sequence counters
on the air interface have failed and can not recover. An example when this happens is when the BER is high and
signal strength is low. This is thus caused by bit errors.
∀. Radio Link Time-Out
The probable cause is low signal strength or high interference.
∀. Time-Out on Layer 2: T200 Expiration
Probable reason is low signal strength or high interference.
∀. Unexpected Release Indication
The reason to unexpected release indication could be that the mobile station
gets confused when the user behaves strangely
.Abnormal Release unspecified
•Release Indication SAPI 3, abnormal release unspecified
The system sends an Establish Request SAPI3 to establish a SAPI 3 link for sending an
SMS. If the mobile fails to reply on SABM the BTS sends Release Indication and Error
Indication: Timer T200. The system responds to this by a Channel Release, abnormal
release: unspecified. This case can be caused by low signal strength or high interference.
∀.Release Indication after no response from mobile station,
The mobile does not respond when it should. One case could be when no Setup message is
received from the mobile station. After ten seconds the mobile station is releasing the
connection. This will cause a radio link time-out.
This is probably a mobile station error.
∀.No Assignment Command, abnormal release, unspecified
The probably reason is either that no measurement reports are received from the mobile
station or congestion, and no available channels.
∀.Unsuccessful Assignment on TCH, abnormal release unspecified
The mobile never establishes itself on TCH and failed to return to SDCCH. After a time-out
in the BSC, the call is released by sending Channel Release (cause: abnormal release
unspecified) to the mobile station.
The probable reason is low signal strength or high interference.
The following cases may occur:
1. Assignment Failure is sent, abnormal release unspecified The assignment to TCH failed but the mobile succeeded to return to
the old SDCCH. The system disconnects the call after a time-out and sends Channel Release with cause “abnormal release,
2. Layer 2 time-out on Assignment Command
The mobile station does not send acknowledge on Layer 2, probably because of low signal strength or bad quality. The mobile is
3. Mobile fails to return to the old SDCCH, abnormal release unspecified The mobile received the Assignment Command, fails to
establish contact with TCH and finally fails to return to the old SDCCH.
Probable Reasons -. Low Signal Strength on Down or Uplink
The reaon for poor coverage could be too few sites, wrong output power, shadowing, no indoor coverage or
network equipment failure.
Action: Check coverage plots.Check output power. Perform drive tests. Check BTS error log
Solution: Add new sites. Increase output power. Repair faulty equipment.
Poor Quality on Down or Uplink
Action: Check C/I and C/A plots.Check frequency plan.Perform drive tests.
Solution: Change frequency. Use available radio features.
Too High Timing Advance
Action: Check if the cell parameter TALIM is < 63. Check if the co-channel cells are over-heard.
Solution: Set TALIM to a value close to 63.Tilt antenna/reduce antenna height/output power, etc. for co-channel
. Mobile Error
Some old mobiles may cause dropped calls if certain radio network features are used. Another reaons is that the MS is
damaged and not working properly.
Action: Check MS fleet.
Solution: Inform operator.
. Subscriber Behaviour
Poorly educated subscribers could use their handsets incorrectly by not raising antennas, choosing ill-advised
locations to attempt calls, etc.
Action: Check customer complaints and their MS.
. Battery Flaw
When a subscriber runs out of battery during a conversation, the call will be registered as dropped call due to low
Action: Check if MS power regulation is used. Check if DTX uplink is used.
. Congestion on TCH
The SDCCH is dropped when congestion on TCH.
Action: Check TCH congestion ⇒chapter 3.3.
Solution:Increase capacity on TCH or using features like Assignment to anothercell, Cell Load Sharing, HCS etc.
Dropped Calls on TCH
Probable Reasons Radio Network Related Problem Description
. Low Signal Strength on Down or Uplink
Normally a call is dropped at the border of large rural cell with insufficient coverage. Bad tunnel coverage
cause many dropped calls as well as so called coverage holes. A quick reduction in signal strength can be the
result of moving into a garage, elevator or even behind a street corner. Bad indoor coverage will result in
dropped calls. Building shadowing could be another reason.
Action: Check coverage plots. Perform drive tests & site survey Check output power.
Check power balance and link budget
Check if omni site.
Check antenna configuration & type.
Check antenna installation
Add a repeater to increase coverage in for example a tunnel.
Change to a better antenna (with higher gain) for the base station.
Add a new base station if there are large coverage holes.
Handover is a key function in a GSM network. If the handover performance is poor the subscriber will
perceive the quality of the network as bad.
Handover performance statistics should preferably be measured on 24 hour data or longer.
Total number of internal and external outgoing or incoming Handover Attempts
Total number of Handover Attempts per cell relation
Handover Attempts of total Handover Decisions
The following checks should be performed:
1. Check for handover relations with low traffic (e.g. less than 10% of
average number of handovers per relation).
2. Check for unbalanced handover relations (e.g. relations which have not
approximate the same number of handovers in either direction.
Remark: Unbalanced handovers relation indicate problem if measured on
whole days, assuming that the same number of people go in and out of the
cell coverage area.
3. Check if the reason for unbalanced handover is congestion.
∀. Bad Locating Parameter Setting
A bad setting of locating control parameters might result that the locating will seldom rank the cell as a candidate.
Action: Check parameter setting.
Solution:Correct bad parameter setting.
∀. Unnecessary Neighbouring Cell Relation
None or very few handovers might indicate an unnecessary neighbouring cell relation.
Action: Check neighbour cell relations ⇒chapter 11.
Solution:Check if the relations really should be defined. Remove unnecessary cell relations.
∀. The Base Station is Defined But Not in Service.
Action: Check reason for BTS not in service.
Solution:Take action to set the base station in service.
∀. HW faults.
Action: Check BTS error log .
∀. Missed measurement frequencies in BA-list
This will cause no handover decisions to the target cell.
Action: Check measurement frequencies list.
Solution:Add missed frequencies.
∀. Permitted Network Color Code problem
If NCCPERM doesn’t include neighbouring cell’s NCC, there will be no handovers.
Action: Check NCCPERM.
Solution:Add NCC of neighbours to NCCPERM.
∀. Wrong Use of HCS
Action: Check HCS related paremeters
Solution:Change to normal value.
∀. Poor inter-MSC/BSC handover performance
If the cell is at the border of inter-BSC or inter-MSC, poor inter-MSC/BSC handover performance will cause few
or no handover attempts.
Action: Check inter-MSC/BSC handover performance
There can be two reasons why an attempt is counted as unsuccessful: either the mobile station was lost or the call
was reverted to the old cell and channel.Unsuccessful handover may lead to a dropped call.Investigate if there is
any difference between incoming and outgoing handovers. This can give further reasons of the problem area.
The lost handovers are registered at outgoing handover. Note that also dropped call counters are stepped.
Probable Reasons. Congestion
A high congestion might lead to dragged calls (handover performed at a not intended location) and a lot of
Action: Check TCH congestion.
Solution: Add more TCH capacity.
. Timer Expire After MS is Lost
The MS never answers the base station.
Action: Check coverage Check interference.
. Link Connection or HW Failure
Action: Check BTS error log.Perform site visit.Perform link performance measurements.
Solution:Repair faulty equipment.
. Bad Antenna Installation
Action: Perform site survey and check antenna installation .Check antenna cabling.
Solution:Adjust antenna installation, antenna type or cabling.
Antennas Connected to Wrong Feeder
Action: Perform site survey and check antenna installation. Check antenna cabling.
Solution:Correct the antenna to the right sector.
. Incorrect Down tilt
Action: Perform site survey and check antenna installation.
Solution:Correct antenna tilting.
. The MS Measures Signal Strength of Another Co- or Adjacent Cell than Presumed.
Action: Check frequency plan. Perform drive tests.
. Wrong Neighbouring Cell is Defined.
This can happen when a new site is added and old definitions are not undefined and new relations are
Action: Check parameter setting.Run Undefined Neighbouring Cell Recording.
Solution: Define or remove neighbouring cells.
. Wrong Output Power due to Faulty Transceiver.
Action: Check output power setting. Check BTS error log
. Wrong Locating Parameter Setting.
Action: Check locating parameters
Solution:Correct parameter setting.
. Poor inter-MSC/BSC handover performance.
For outer or external cell, wrong definitions in either MSCs or BSC may be reason for the problem.
Action: Check inter-MSC/BSC handover performance.
Missing Handover Relations
Missing handover relations will act as coverage holes. Suddenly the MS can not perform handover to the best
server (i.e. not in the BA-list), instead a handover is performed to the second best candidate.
Action: Check existing handover relations. Check with actual site positions and coverage plots if more
relations need to be added.Run Undefined Neighbouring Cell Recording.
Solution:Add missing handover relations.
. Too Many Neighbours Defined
Many defined measurement frequencies defined (>16) will decrease the accuracy of the mobile measurements to
locate the best six servers. Many measurement frequencies mean few samples per frequency and problem for
mobiles to decode the BSIC.
Action: Check number of definitions.
Solution: Remove unnecessary definitions.
. Too Many Measurement Channels Defined.
Action: Check number of measurement channels
Solution: Remove unnecessary measurement channels.
. Bad Use of Radio Network Features
Incorrect use of radio features such as Dynamic Power Control, Locating, Intra-Cell Handover, Frequency Hopping,
Action: Check Feature parameter setting.
Solution:Correct strange and erroneous parameter setting.
. Delayed Handover Decision
A delayed handover decision can be due to congestion in the target cell.
Action: Check handover parameters.
. Bad Radio Coverage
Action: Check coverage plots.
. High Interference, Co-channel or Adjacent.
The potential handover candidate is disturbed by interference.Outgoing handover due to bad uplink quality may
indicate interference fromco-channel another MS. On the border, the quality may be rather bad and the
signal strength low.Bad downlink quality may indicate interference from another co-channel base station.
Action: Check interference.
Solution:Change frequency plan.
A handover reversion is when the MS is going back to the old channel. This happens when the MS fails to
establish itself on the new traffic channel but succeeds to return to the old traffic channel. If the mobile does not
succeed to return it will be lost.
The potential handover candidate is disturbed by interference and thus making it impossible to complete the
handover signalling.Outgoing handover due to bad uplink quality may indicate interference from co-channel
another MS. On the border, the quality may be rather bad and the signal strength low. Bad downlink quality may
indicate interference from another co-channel base station
Action: Check if many handovers are performed due to downlink or uplink bad quality.
. Lower Output Power on TCH than on the BCCH in the Target Cell.
Action: Check output power. Check BSTXPWR and BSPWR defination
. Not good neighbouring cell relation defined.
Action: Delete the relation if not necessary.
Receiver Antenna Problem or RBS HW problems (in candidate cell)
Action: Check antenna installation. Check RBS HW and Error log of the target cell
Successful Handovers back to old cell within 10 seconds of Total successful handovers.
Probable Reasons - Wrong Parameter Setting
Action: Check handover hysteresis. Check locating parameters
Solution:Correct unfortunate parameter setting.
Lower Output Power on TCH than on the BCCH in the Target Cell.
Action: Check output power.
Coverage holes might lead to Ping-Pong handovers especially for slow moving mobiles. The problem could be
shadowing by high buildings and straight streets with good coverage from another nearby base station.
Action: Perform drive tests.Check locating parameters.
Solution: Add coverage, i.e. a new micro base station. Change locating parameters such as hysteresis values.
. No Best Server
Several possible equal signals from two or more base stations might lead to a Ping-Pong effect. This is valid
especially for slow moving mobiles for example handheld indoor. Often, this situation also causes bad quality
and finally a high ratio of dropped calls.
Action:Perform drive tests.
Solution: Add new base station to increase coverage.
The percentage of urgency handovers is a good indicator or interference and from which direction the problem occurs.
The wrong setting of Timing Advance Limit will also cause urgency handovers.Always start to check that the setting
of the parameters TALIM, QLIMUL/DL as well as the filter length QLEN is correct.
. Wrong setting of TALIM
TALIM is the timing advance limit for doing urgency handover. A lower setting of TALIM will cause many
handover decisions due to excessive timing advance.
Action: Check TALIM.
Solution: Change TALIM to normal value.
. Bad Coverage
There might be a combination with bad coverage and high interference.
Action: Check coverage plots.
Solution:Improve coverage. A change of frequency might solve the problem but is a more unreliable method as
this change might be forgotten at next new frequency plan introduction.
. Uplink Interference
This might be an interference from other MSs.
Action: Check co-channel sites Check if external interference.Check coverage.
. Downlink Interference
This might be an interference from other base stations.
Action: Check co-channel sites Check if external interference Check coverage.
Intra-Cell Handover Performance
An intra-cell handover is normally performed in cells with high signal strength and bad quality.
Reasons to unsuccessful intra-cell handovers or abnormally high amount of intra-cell handovers.
. Faulty Parameter Setting
Action: Check Intra-Cell Handover parameters.
Solution:Correct strange or faulty parameters.
. Internal Interference
Action: Check if uplink or downlink problem.Check co-channel sites Check Dropped Call Performance
Check Handover Performance
Solution: Change frequency plan.
A cell with only one frequency disturbed but with several tranceivers will ‘get’ interference on all call
connections if frequency hopping is introduced.For each call connection, also the disturbed frequency is used in
sequence.A high number of Intra-Cell Handovers indicates that there is interference.The disturbed link could
easily be identified by the formulas for up- and downlink interference. To be sure that this is an external problem,
a site visit should be performed and RF measurements should be carried out.
Action:Check ratio Intra-Cell Handovers to TCH Connections.Check if uplink or downlink problem.Check if
frequency hopping is used.Perform Idle Channel Measurements.Perform site visit and drive test.Perform RF
measurements in the disturbed cell.
Solution:Identify and remove external interference. A temporary solution might be to change the disturbed
The type of interference should be identified,
interference, co-channel or adjacent channel interference, means that the problem exists within the own network due
to a bad frequency plan, bad site location, congestion or too high antenna location.
External interference means that there is another transmitter or something else acting as a transmitter outside the
network such as TV transmission, repeaters microwave links, or other mobile systems like AMPS. To solve this
problem the operator either has to change affected frequencies to avoid the disturbance or to negotiate with the owner
of the source of the disturbance.
If the interference is intermodulation the signal is reflected in an undesired way. There might be problems with the
combination of different transmitters on the same base station or faulty equipment.
IMtool+ calculates all intermodulation products of up to 20 transmitters, up to the
fifth order. Calcs that result in negative numbers are not valid. The total number of
calculations performed and the total number of valid results within the specified
range are listed at the bottom of the page.
Transmitter A = 952.00000 Transmitter B = 937.40000
Transmitter C = 950.20000 Transmitter D = 952.60000
Transmitter E = 953.00000 Transmitter F = 955.00000
List IM hits between 890.00000 MHz and 960.00000 MHz
a = 952.00000 b = 937.40000
c = 950.20000 d = 952.60000
e = 953.00000 f = 955.00000
-a + 2b = 922.80000 -a + 2c = 948.40000
-a + 2d = 953.20000 -a + 2e = 954.00000
-a + 2f = 958.00000 2a + -c = 953.80000
2a + -d = 951.40000 2a + -e = 951.00000
2a + -f = 949.00000 -2a + 3b = 908.20000