kpi_2g_troubleshooting telecommunication

telecom

1. 2G TROUBLESHOOTING Accessibility/Retainability
DCR (Retainability)
There are two types of call drops given below:
>Call drop over SDCCH: Indicating the call drop occurs in the
course during which BSC assigns a SDCCH to an MS but a TCH
has not been successfully assigned yet .
>Call drop over TCH: Indicating the call drop occurs after BSC
assigns a TCH to MS successfully.
The Channel currently occupied is of TCH type when BSC
sends a clear Request message to MSC.
The cause values for sending clear Request are as follows:
> Radio Interface Message Failure
>O&M Intervention
> Equipment Failure
>Protocol Error Between BSS and MSC
>Preemption

Causes of Drop Call:
>Coverage
>HO
>UPLINK /DOWNLINK CAUSED BY ANTENNA & FEEDER
SYSTEM
>Interference
>Transmission Failure

Coverage:
Discontinuous coverage (Blind area):
>Call drop is caused by isolated BTS. As the signal is of weak
strength and poor quality at the edge of an isolated BTS,
handover to other cells cannot be implemented, and thus call
drop occurs.
>If BTS lies in the place where the landform is intricate and
radio propagation environment is complicated (e.g., a
mountainous area), it may cause call drop owing to
discontinuous coverage.

Poor Indoor Coverage :
>In the place where many buildings are located, call drop
easily occurs due to high transmission attenuation, low
indoor level and great penetrate loss.

Beyond Coverage :
>Owing to some reasons, the coverage of a serving cell is
beyond the defined coverage. For example, the power in cell
A is so high that a MS still occupies the signals of cell A after it
moves out of the coverage of the adjacent cell B that has
been defined by cell A and reaches cell C.
>However, cell A has not defined cell C as an adjacent cell
yet, so at this time the MS cannot find a proper cell when it
tries to perform a handover according to the adjacent cell B
provided by cell A, thus call drop occurs.

Shortage of coverage :
It may be caused by some equipment failure in a cell. For
example, the antenna is obstructed or the carrier taking
BCCH (power amplifier) gets faulty.

Handover:
Unreasonable Parameters :
>For example, if the level of the handover candidate cell is
set to be too low and the handover threshold is set to be too
little, some MSs will be handed over to the adjacent cell
when the level of the adjacent is a little stronger than that of
the serving cell for a time.
>But after a while, if the signal of the adjacent cell faint, and
it happens no proper cell is available for handover, call drop
could occur. See example 6 for call drop resulting from
improper settings of handover parameters.

Adjacent cell undefined:
>If an adjacent cell has not been defined yet, MS will keep
communicating in the serving cell until it goes out of its
coverage. At this time, call drop shall occur since MS cannot
be handed over to a cell with stronger signals.
>Existence od adjacent cells with same BSIC and BCCH
Frequency .

Traffic Congestion :
>Unbalance of traffic may cause handover failure due to lack
of handover channel available for the destination BTS. When
reestablishment of handover channel fails too, call drop
occurs.
>BTS Clock out of synchronization and frequency offset
beyond limits ,which can cause handover failure and call drop
.

Interference
>There are co-channel interference, adjacent-channel
interference and inter-modulation interference. When MS
receives signals in the serving cell with strong co-channel or
adjacent-channel interference, it may aggravate BER and
make MS cannot accurately demodulate BSIC of the adjacent
cell or BTS cannot correctly receive measurement reports of
MS.
>The interference threshold is set as co-channel carrier-to-
interference ratio C/Iƒ9dB and adjacent-channel carrier-to-
interference ratio C/Aƒ9dB. When the interference index is
so bad that it exceeds the threshold, conversations in
network shall be interfered, thus conversation of poor quality
and call drop might occur.

Causes UL /DL caused by Antenna & Feeder System
>Improper installation of antenna and/or feeder. For
example, the Tx antenna between two cells is installed just
reversedly, which shall make the uplink signal level is much
poorer than the downlink one, thus cause call drop, single
pass or difficult connection occurring far from the BTS.
>If single polarization antenna is adopted, a cell has two sets
of such antennas. If their azimuths are different, call drop
might occur.
• A directional cell has a main antenna and a diversity
antenna, so it is possible that BCCH and SDCCH of this
cell come from the two different antennas. Different
azimuths will cause different coverage, consequently,
although the user can receive BCCH signal, it cannot
occupy SDCCH sent by another antenna when
originating a call, thus call drop occurs.
>As there are Abis interface and A interface link, poor quality
transmission and unstable transmission link also may cause
call drop.
• Observe transmission and board alarms (e.g., FTC
failure alarm, A interface PCM out of sync alarm, LAPD
link break alarm, power amplifier alarm, HPA alarm,
TRX alarm, CUI/FPU alarm). Based on alarm data,
analyze whether transmission is intermittent or
whether there are faulty boards (e.g., the carrier board
is faulty or in poor contact).
• Check transmission paths, test BER and check whether
E1 connector or grounding of equipment is reasonable,
thus decrease call drops by ensuring stable
transmission quality.
• Observe whether there are too many call drops caused
by transmission problem via traffic measurement.
a) in TCH performance measurement of traffic
measurement observe whether there are too many A
interface failures when TCH is occupied.
b) In TCH performance measurement observe whether
the TCH availability rate is abnormal.
c) In TCH performance measurement observe
whether there are too many call drops caused by
interruption of terrestrial link.

2. 2G TROUBLESHOOTING Accessibility/Retainability
Drop Call Rate está relacionado con los sigs KPIs:
>TCH Drop Rate
>HO Success Rate

TCH Drop Call (Retainability):

The formula for measuring TCH Drop Rate (Optimi, Movistar,
Ericsson <Ericsson uses more parameters in case of
denominator who make reference to HO>) is:

2G TRTOU
=
yprtsf c yprtsfldg c hprtsf c hprtsfldg
hTillivv c hTillivvldg c yTillivv c yTillivvldg
n b:>>

TFNDROP The total number of dropped full-rate TCH in UL
subcell.
TFNDROPSUB The total number of dropped full-rate TCH in
OL subcell.
THNDROP The total number of dropped half-rate TCH in
UL subcell.

TCHNDROPSUB The total number of dropped half-rate in
OL subcell.
THCASSALL Number of assignement complete messages
for all MS power classes in UL subcell, half-rate.
THCASSALLSUB Number of assignement complete
messages for all MS power classes in OL subcell, half-rate.
TFCASSALL Number of assignement complete messages
for all MS power classes in UL subcell, full-rate.
TFCASSALLSUB Number of assignement complete
messages for all MS power classes in OL subcell, full-rate.


The TCH Drop Call reasons has the following priority order:
>Excessive TA
– TA > TALIM cell parameter
>Low signal strength in downlink and/or uplink
– SSUL < LOWSSUL (BSC parameter)
– SSDL < LOWSSDL
>Bad quality in downlink and/or uplink
– RxQualUL> BADQUL (BSC parameter)
– RxQualDL> BADQDL
>Sudden loss of connection
– None of the above
>Other
– Non radio-related
TCH Drop Call Optimization
Analyze the following issues that could be possible reasons
for poor TCH Drop performance:
– Low Signal strength on Uplink/Downlink
– Interference on Uplink/Downlink
– Excessive TA
– Incorrect/Not optimum Parameter settings for example
power regulation
– Missing Ncells or Congestion in Ncells
– MS or MS Battery problems
– Subscriber Behavior
– Antenna/Hardware or Transmission faults
– Incorrect Installations























































































Dropped Calls on TCH
Check radio network features
Power regulation
used properly?
DTX
used?
Check output power
Check locating parameters
Bad parameter
setting?
Power
balance?
Adjust output power
Introduce
DTX
Correct parameter
setting
Frequency
hopping
used?
Activate frequency
hopping
NO
Correct power regulation
parameters
YES
NO
YES
YES
NO
NO
YES
NO
Missing
neighbours?
YES
Run NCS
YES
Check dropped call
reason
Bad
quality?
Low signal
strength
Perform interference
analysis
Perform low signal
strength analysis
A
NO
NO
YES
YES
NO
Most dropped
Call during
HO?
Check lost
handovers
Perform handover
analysis
Swap & repair
HW
Check site
position
Dragoon
site?
Check MS fleet
Best server
exists?
Check antenna
installation
Perform site survey
Check BTS Error Log
HW
fault?
Transmission
fault?
Check link quality and
Synchronization
Remove site or change
frequency
Perform link
investigation
Add site
Perform MTR/CTR
recordings
Perform drive tests
High timing
advance
END
A
Check site location
& TALIM
YES
NO
YES
NO
NO
YES
NO
YES
YES
NO
NO
YES
Check Dropped Calls per TS basis

3. 2G TROUBLESHOOTING Accessibility/Retainability
DCR (RETAINABILITY)
Causas:
>Low SS UL
>Low SS DL
>Sudden
>RxQual UL
>RxQual DL
>Others

RxQual UL:
Causas:
Una celda A radia e interfiere el área de cobertura de una
celda B. Los MS conectados a la celda B están interfiriendo a
los MS conectados en la celda A.
Checar:
TA - Time in advance

RxQual DL:
Causas:
Una BTS A radia e interfiere el área de cobertura de una celda
B y la BTS A interfiere a los MS conectados a la BTS B.
Checar:
Cobertura de BTS A y B
TA – Timing Advance
Asignación de frecuencias
HCN
Asinación de MAIO
HW – Posible radio dañado

Low SS UL:
Checar:
TA relacionado con problema en TMA. Observar gráfica de
SS_DL y SS_UL si se mueve un valor de 12dB por lo tanto si es
TMA.

Low SS DL:
Checar:
HW - Antena dañada
HW - VSWR con valor alto
HW - TRX dañado

Sudden:
Checar: Problema relacionado directamente con HW

Others:
Checar:
Corrimientos en enlace
Transcoders de la BSC
HOFR (RETAINABILITY)
The formula for measuring HOFR (Optimi, Movistar) is:

HEU B L
hsCStTp D hsCStldT
hsCStTp
n b:>>

ATT= HOVERCNT 2 sum of HO attempts
SUCC=HOVERSUC 2 sum of HO Successful

Handover in Ericsson BSS system is controlled by what is
called as the locating algorithm in the BSC. Locating algorithm
operates on the basis of Measurement Reports (MR)
i
sent in
by the MS on SACCH.
The inputs that the BSC uses for making a handover decision,
from the received MRs from the MS is the DL signal strength,
DL quality, and the signal strength of the six best reported
neighbours. From the serving BTS, for the same MS the BSC
will use UL signal strength, UL quality and TA.

Possible reasons for poor HO Performance:
>Neighbor Cells impact the following radio network areas:
- Coverage
- Interference
- Incorrect parameter settings
- Unnecessary or Missing NCELL relations
- Hardware or Link/Transmission Faults
- Missing Frequencies in the Measurement list
- Poor Inter MSC/BSC handover performance
- TCH Congestion
- Frecuency plan, specifically for BCCH carriers
- The values of the hysteresis
- The filter lengths
- The BSIC planning
- The length of the active mode BA list
- The defined nieghbours
- The settings of parameters controlling TA and BQ
urgency HO
- The setting of parameters controlling Assignment to
other cell
- The values of the layer change thresholds when more
than one Hierarchical layer si used
>Handovers for Downlink Quality
-The HO is performed because DL conditions on the serving
cell are becoming too poor, there the BSC selects the best
neighboring cell as the target for the HO
Investigation
-Too strict configuration of the Handover parameters
-Hardware problems : Especially if the rates are high (around
100%) it can be due to the radio parts of the BTS on the
downlink (TRX, transmission antenna, feeders ...)
-GSM interference : co-channel or adjacent channel
-Radio Coverage : HO for UL Quality as well as HO for Level
reasons (UL and/or DL) shall be observed on the cell.

>Handovers for Uplink Quality- The HO is performed
because UL conditions on the serving cell are becoming too
poor, there the BSC selects the best neighboring cell as the
target for the HO
>Investigation
-Bad parameters settings
-If rates are high, probably hardware problem on the radio
uplink transmission part (TRX, antenna, diversity, feeders)
-Coverage problems(if also associated to HO for uplink level)
-Jamming on the uplink (if confirmed by bad RACH efficiency)
-GSM interference : if there’s an equivalent rate of handovers
for downlink quality

>Handovers for Downlink Level
-The HO is performed because DL conditions on the serving
cell are becoming too poor, there the BSC selects the best
neighboring cell as the target for the HO
>Investigation
-In normal cases, this should be very rare so if the rates are
high it is probably: Bad parameters settings or Hardware
problems on the downlink

>Handovers for Uplink Level
-The HO is performed because UL conditions on the serving
cell are becoming too poor, there the BSC selects the best
neighboring cell as the target for the HO
>Investigation
-Insufficient coverage (if associated also with an equivalent
rate of uplink quality)
-Bad handover parameters settings for the cell
-Hardware problems (high rates) if the downlink and the
uplink are not balanced
-Could come from ping pong handover between two cells
(one leaves one of the cells for quality, then comes back for
level and so on, for example)

4. 2G TROUBLESHOOTING Accessibility/Retainability









































































ACCESSIBILITY

Ability for mobiles to set up calls.

Call attempts
Start from: Successful random accesses
To: successful immediate SDCCH assignements
Finally: Successful assignement on TCH

Probable causes related to accessibility:
- Congestion due to amount of traffic
- Interference (Quality of network)
- Hardware or Transmission issues
- Software file congestion
- Coverage (no coverage)

Accessibility includes de following KPIs:
- Paging Success Rate
- Location Update Success Rate
- Random Access Failure
- SDCCH Time Congestion
- TCH Assignment Failure

Paging Success Rate (ACCESSIBILITY)
The formula for measuring Paging Success Rate (Optimi,
Movistar) is:

iir
=
fiHfThTspH D fiHS sssvr
hTillivv c hTillivvldg c yTillivv c yTillivvldg
n b:>>

PAGPCHCONG p Number of paging messages discarded due
to full cell paging queue
PAGETOOOLD p Number of paging messages discarded due
to being too long in the paging queue. At the point when a
page is taken from the paging queue, its age is calculated and
compared to the BTS parameter AGE-OF-PAGING (the
parameter is set to 5seg in Ericsson BSS). If it is too old, it is
discarded and PAGETOOLD is incremented.
THCASSALL p Number of assignement complete messages
for all MS power classes in UL subcell, half-rate.
THCASSALLSUB p Number of assignement complete
messages for all MS power classes in OL subcell, half-rate.
TFCASSALL p Number of assignement complete messages
for all MS power classes in UL subcell, full-rate.
TFCASSALLSUB p Number of assignement complete
messages for all MS power classes in OL subcell, full-rate.

Paging process:
> Step-1
In response to an incoming call, the MSC initiates the paging
process by broadcasting a “paging request” message on the
paging sub channel (IMSI or TMSI of the MS and its Paging
Group) and starts timer T3113. A “paging message” consists
of the mobile identity (IMSI or TMSI) of the MS being paged
and its “paging group number”.
>Step-2
The BSC receives this page and processes the paging request
and schedules it for transmission on the PCH at appropriate
time.
>Step-3
The MS on its part will analyse the paging messages sent on
the paging sub channel corresponding to its paging group.
>Step-4
Upon receipt of a page at the MS, the MS responds by
transmitting a channel request on the RACH.
>Step-5
BSS in response to the received “channel request”, will
process it and immediately assign the MS a SDCCH
(immediate assignment / assignment reject; done over
AGCH).
>Step-6
Check use of radio
features
C
NO
Are all radio
features used?
YES
Check coverage
NO
Low SS on cell
border?
Perform drive test
YESImprove coverage
Check interference
YES
Bad
quality?
Reduce interference for
potential candidate
YES
Delayed handover
decision?
Check handover
parameters
YES
Introduce unused
features
· Timer Expiry After
MS is Lost
NO
D
NO
Unsuccessful Handovers
Check congestion
performance
TCH
congestion?
Check SAE setting
NO
SW
congestion?
Check Locating parameters
Check Neighbouring cell definitions
Wrong cells
defined?
Missing
neighbours?
Too many
neighbours?
Too many
measurement
channels?
Strange or corrupt
parameter setting?
NO
NO
NO
NO
NO
Add TCH
capacity
YES
Increase SAEYES
Remove incorrect
definitions
YES
Add missing
relation
YES
Remove unnecessary
relations
YES
Review and correct the
defined MBCCHNO
YES
C
NO
Correct parametersYES
D
Check BTS Error Log
HW
fault?
Check link quality
Transmission
fault?
Swap & repair HW
YE
S
NO
Improve
transmission
YE
S
Perform site visit
NO
Check antenna
installation
Antenna connected to
wrong feeder?
Incorrect
downtilt?
Hided
antenna?
Bad antenna
installation?
NO
NO
NO
Change feeder
YE
S
Correct tilting
YE
S
Change antenna
position
YE
S
Correct installation
YE
S
END
NO
Is the cell at inter-MSC
border?
YE
S
Check inter-MSC HO
performance
NO

5. 2G TROUBLESHOOTING Accessibility/Retainability
MS Paging response- After receiving the immediate
assignment command, MS switches to the assigned “SDCCH”
and transmits a “Paging Response”.
>Step-7
The establishment of the main signalling link is then initiated
(E1) with information field containing the “PAGING
RESPONSE” message and the “paging response” is sent to the
MSC.
>Step-8
Upon receipt of the “Paging Response” MSC stops the timer
T3113. If the timer T3113 expires and a “Paging Response”
message has not been received, the MSC may repeat the
“Paging Request” message and start T3113 all over again. The
number of successive paging attempt is a network dependent
choice
>Paging Capacity for BTS:
MFRMS*(CCCH blocks - AGBLK) * X (X=4 when TMSI based
paging is used & X=2 when IMSI based paging is used)
When the rate of “paging load” at the RBS becomes higher
than what the RBS is able to handle (paging capacity of RBS),
RBS will start discarding pages (check for high “page discard”
stats at the cell level).
Paging Queue Length = 14 – (Number of Paging Groups/10)
Which means higher the number of paging groups in a cell
(that is higher the MFRMS settings), lower will be the Paging
Queue length, this will compensate for lower number of
Paging Queue available at lower MFRMS. One “Paging
Queue” per “Paging Group” is available at the cell level (that
means when we decrease the number of paging groups for
the cell by reducing MFRMS to lower numbers, we actually
reduce the number of available paging queue).
>Paging capacity in BSC:
Paging Bottleneck at the BSC usually is the number of RP
signals that can be sent from the CP to the RPDs in the TRHs.
Calculations for number of pages per second:

(1)

(2)

RPSIG  Maximum number of RP signals per second
TRXpTRH  Average number of TRXs per TRH
NO_LA  Number of Location Areas
NO_TRH  Number of TRHs belonging to the BSC
NO_TRX  Number of TRXs belonging to the BSC
NO_CELLS  Number of cells
RPp  Probability that an RP signal is sent to an TRH
NO_PAGE  Number of pages per second
With “more” TRHs the paging capacity for the BSC decreases.
With “more” LACs within a BSC the paging capacity for the
BSC increases.

Analyze the following issues that could be possible reasons
for poor Paging Performance:
– Insufficient coverage
– High interference
– Non-optimum Paging Strategy
– Non-optimum Paging parameter settings
– Paging Congestion in MSc, BSC or BTS
– De-activated or incorrect use of Paging features
The following actions can improve Paging Congestion:
MSC Paging Congestion
- Increase SAE
- BSC/BTS Paging Congestion
- Check Paging Strategy
- Increase number of Location Areas (This will increase
SDCCH load)
- Set BCCHTYPE to NCOMB
- Use TMSI paging requests
- Ensure IMSI attach/detach is activated (ATT = yes)
- Decrease T3212/BTDM (This will increase SDCCH load)

























Location Update (ACCESSIBILITY)

Location Area (LA) dimensioning strategies:
>Size of a LA must not exceed the maximum paging capacity
for the BTS / BSC
>In a rural area, it is easy to find LA border cells. But there’s
no reason to have a smaller LA than required
>General rule of thumb: 1 LAC per BSC
>If a BSC covers a large area with high traffic, consider
splitting the LA. This will reduce the paging load in BTS and
BSC
>In larger cities, SDCCH load will be higher for LA border cells.
If difficult is experienced to find LA border cells and the BSC
coverage area is small, several BSCs can share one LA
>Define LA border at cells with low subscriber density

































RPpTRHNO
RPSIG
PAGENO
⋅
=
_
_






⋅
−−=
LANOTRXNO
CELLSNO
RPp
TRXpTRH
__
_
11
Low Paging Success
No TMSI Attach
/ Detach?
Activate
Wrong periodic location
updating setting?
Correct T3212 in BSC,
BTDM & GTDM in MSC
SDCCH
Congestion?
Increase SDCCH channel
Adaptive configuration feature
Paging
congestion?
Plan more LA
Use TMSI paging
TMSI paging
not use?
Activate
after not periodic
registration not use?
Activate
END
Yes
No
Yes
Yes
Yes
Yes
Yes
No
No
No
No
No
Implicit detach
Low Location Updating
Success
border
cell?
Reallocate to another LA or MSC
Increase CRH
Low CRH
Hysterisis?
Increase CRH
Bad LA
Demensioning?
Re-dimensioning LA
Short perodic
registration?
Check T3212 in BSC &
BTDM in MSC
Interference?
Check frequency plan
Activate radio feature
Software File
Congestion?
Correct settings for SAE 500 Block
MLUAP, MLCAP,MLVAP,MMMLR
SDCCH
Congestion?
Increase SDCCH channel

Adaptive configuration feature
Automatic
deregistration?
Activate automatic
de-registration
END
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
No
No
No
No
No

6. 2G TROUBLESHOOTING Accessibility/Retainability
SDCCH Congestion(Accessibility)
SDCCH Activities:
>Mobility Management
– Normal Location update
– Periodic Registration
– IMSI Attach / Detach
– Connection Management
– Call setup
– SMS Point to Point
– Fax Setup

SDCCH Channel Allocation:
>Channel Configuration can be done as follow
– Combined BCCH/SDCCH on 1 TS (SDCCH/4)
– Non-Combined BCCH and SDCCH on 2 TS (SDCCH/8)
– Cell Broadcast Channel (CBCH)
– The Cell Broadcast service provides the transmission of
an SMS from a message-handling centre to all MSs in
the serving area of the BTS.
– If the Cell Broadcast service is active in a cell, one
signaling sub-channel is replaced by one CBCH resulting
in a SDCCH/7

SDCCH Channel Allocation Profile (CHAP)
>SDCCH Channel Allocation Profile (CHAP)
– Channel Allocation is the feature that selects and
allocates suitable channels when one or more channels
are required
– Channel Allocation Profile (CHAP) is the parameter that
provides different channel allocation strategies
– For SDCCH assignments, CHAP 8 prioritize new
assignments as follows:
1) OL/SDCCH
2) UL/SDCCH (if OL/SDCCH is congested)

SDCCH Dimensioning Strategy:
>GOS Approach:
– SDCCH should be dimensioned for better GOS compare
to TCH. Typical range is 0.5% - 1%.
– Immediate Assignment of TCH with “TCH as last option
strategy” is recommended.
– It is recommended to add SDCCH when 0.5 Erlang of
signaling traffic is carried by TCH.
– Half Rate penetration, usage of Extended Range cell
and HSCSD should be taken into consideration.
>STS Approach:
Accurate dimensioning is achieved by using STS cell statistics

SDCCH Congestion Optimization:

>Analyze the following issues that could be possible reasons
for SDCCH Congestion:
– SDCCH Dimensioning
– Incorrect use of SDCCH capacity features such as
Adaptive Configuration of Logical channels
– TCH Congestion
– Location Areas not optimized
– SMS usage and Cell Broadcast channel
– SDCCH Availability

>The following recommendations will have a positive impact
on SDCCH congestion
– Try not to use combined BCCH/SDCCH (SDCCH/4)
– Use the optional SDCCH capacity features such as the
Adaptive Configuration of Logical Channel feature when
available.
– Use Immediate Assignment on TCH, SDCCH first.
– For manual dimensioning, use STS cell statistics
– Configure one SDCCH on the BCCH carrier and the
others on the hopping layer (Non-BCCH)
– Use the Ericsson SDCCH Dimensioning guideline.

>The following issues are worth taking into account when
analyzing SDCCH Congestion:
– Use LAPD STS statistics to evaluate LAPD congestion
and optimize the LAPD Concentration factor
– Analyze border cells with a large amount of random
access attempts resulting in SDCCH congestion.

>Use Random Access optimization to solve the SDCCH
congestion
– Take into account Timeslot priority when deciding
where to configure SDC





























































SDCCH Congestion
Check site position
Location area
border?
Change Location
Area Border
Check and increase
CRH
Check TCH traffic
TCH
Congestion?
Check SMS activity
Many SMS
messages?
E
Check periodic
registration
Too frequent
registration?
Check HW availabilty
Low
availability?
See TCH & SDCCH
Availability
Add TCH
capacity
Redimension
SDCCH
Change registration
interval timers
NO
YES
NO
NO
NO
YES
YES
YES
NO
YES
YES
Check channel configuration
Combined
SDCCH?
Check traffic trend
Short term
traffic growth?
Check SDCCH dimensioning
E
Underdimensioned
SDCCH?
Check SDCCH mean holding time
Long mean holding
time?
Is cell broadcast
used?
Avoid cell broadcast if
possible
Make channel
non-combined
No activity
Check HW and number
of False Accesses
Redimension
SDCCH
END
NO
NO
YES
YES
YES
YES
YES
NO
NO
NO
Check Adaptive configuration of
logical channel and parameters
ACSTATE off? Switch onYES

7. 2G TROUBLESHOOTING Accessibility/Retainability
SDCCH Drops(Accessibility):

The formula for measuring SDCCH Drop Rate(Ericsson,
Optimi) is:

leret w o
Tprtsf D TptSvTspH D TptSvTspHldg
TIlSl ig
n b:>>

CNDROP The total number of dropped SDCCH channels in
a cell
CNRELCONG Total number dropped (released connections
on SDCCH due to TCH or Transcoder congestion in UL subcell.
CNRELCONGSUB The subset d for OL subcell

CMSESTAB Total number of successful MS channel
establishment on SDCCH.

SDCCH Drops are classified into one of the following
categories:
>SDCCH Drops because of Timing Advance
>SDCCH Drops because of Low Signal Strength
>SDCCH Drops because of Bad Quality
>SDCCH Drops due to Other Reasons

SDCCH Drop Optimization
>Analyze the following issues that could be possible reasons
for SDCCH Drops:
– Low signal strength
– Interference
– Pathloss Imbalance between UL/DL
– High Timing Advance
– MS error or Subscriber behavior
– TCH Congestion
– Transmission Congestion (LAPD Concentration)
– Hardware or Transmission failures

>The following issues are worth taking into account when
analyzing SDCCH Drops:
SDCCH Drops and TCH Drops often drop because of the same
RF reasons, such as for example insufficient coverage
– Only use SDCCH power regulation and SDCCH HO when
the network has good coverage
– BSC and transmission problems can impact SDCCH
drops so investigate Transcoders, A-interface and LAPD
– SDCCH Drops on the hopping layer are normally
showing worse values than SDCCH on the BCCH




















































RANDOM ACCESS SUCCESS RATE (Accessibility):

The formula for measuring SDCCH Drop Rate(Ericsson) is:

tildT w o: D
tiiTTyi
TptsTp c frtiT c tiiTTyi
n b:>>

RAACCFA Number of Failed Random Accesses. This
counter is incremented for a Random access received with
too high TA, values that are not used or in case of "software
file congestion" (i.e. when the internal storage area in the
BSC is full which is a very rare case only occurring at very high
loads
CNROCNT Number of Failed Random Accesses. This
counter is incremented for a Random access received with
too high TA, values that are not used or in case of "software
file congestion" (i.e. when the internal storage area in the
BSC is full which is a very rare case only occurring at very high
loads.
PDRAC The counter value is incremented when a 44.058
CHANNEL REQUIRED containing 44.018 CHANNEL REQUEST
with establishment cause "One Phase Packet Access" or
"Single Block Packet Access" is received on RACH.

The measurements are done per cell for random access
channels.
For every cell there are counters registering the number of
random access attempts received in the BSC.
Random Accesses can be distributed into the following types:
>MS Originated RA
>Call Re-establishment
>Emergency Calls
>Answer to Paging
>All Other Cases
– IMSI Attach/Detach
– Location Updates
– Period Registration

>Analyze the following issues that could be possible reasons
for high Random Access failure rate:
-High interference
-Lack of Coverage
-High Timing Advance
-Handover access burst mistaken as random access burst
-Non-optimized parameter settings
-Software Congestion (SAE)

Check SDCCH Dropped Per Cell
Check drop
reason
Bad
quality?
•Check ICM
•Check frequency Plan
•
Frequency scan•
Run MRR & FAS
Low
SS?
•Refer to Low SS
analysis flowchart.
High
TA?
•Check site location
& TALIM
TCH
Congestion ?
•Check S_DR-S_NTC
•Increase TCH or using
Check BTS error
log
HW
fault?
•
Swap &
Repair HW
B
NO
YES
YES
YES
YES
YES
NO
NO
NO
NO
Radio Features, CLS, HCS
B
Transmission fault?
•Perform link
investigation
Check Link Quality
Check MS fleet
Perform Drive Test
Perform MTR/CTR/MRR
Perform site survey
Check antenna installation
END
YES
NO

8. 2G TROUBLESHOOTING Accessibility/Retainability
























TCH Assignment Failure (Accessibility):

The formula for measuring TCH Assignment Failure (Ericsson)
is:
iilldT
w u
Tilliv c BM.R"ig c im$D O&Eqig c imFF
illivv c BM.qig c imF D O&E"ig c im$$
P b:>>

outg(AB+AW)=SUMEOABSUCC+SUMEOAWSUCC+SUMOABSU
CC+SUMOAWSUCC
inc(AB+AW)=SUMEIABSUCC+SUMIAWSUCC+SUMIABSUCC+S
UMIAWSUCC

SUMEOABSUCC 2 Sum of Successful External Assignment
Handovers to Better Cell (Outgoing Handover)
SUMEOAWSUCC 2 Sum of Successful External Assignment
Handovers to Worse Cell (Outgoing Handover)
SUMOABSUCC 2 Sum of Successful Internal Assignment
Handovers to Better Cell (Outgoing Handover)
SUMOAWSUCC 2 Sum of Successful Internal Assignment
Handovers to Worse
Cell (Outgoing Handover)


SUMEIABSUCC 2 Sum of Successful External Assignment
Handovers to Better Cell (Incoming Handover)
SUMEIAWSUCC 2 Sum of Successful External Assignment
Handovers to Worse Cell (Incoming Handover)
SUMIABSUCC 2 Sum of Successful Internal Assignment
Handovers to Better Cell (Incoming Handover)
SUMIAWSUCC 2 Sum of Successful Internal Assignment
Handovers to Worse Cell (Incoming Handover)

The TCH assignment success rate measures how often a call
set-up, ordered by the MSC, has been successful
>The major issues impacting TCH assignment failures are:
-TCH congestion
-Radio reasons such as coverage and interference
-Hardware or transmission problems
-Congestion can be defined as follow:
-The probability that a call attempt cannot be handled
successfully, because of not enough free circuits and other
switching equipments
-The Congestion (Blocking) Probability represented by B (n, A)
is determined by Offered traffic A and the number of
available circuits n

Congestion Improvement
>Congestion can be improved by short, medium and long
term actions
>Medium and Long term:
– Expansions: Add capacity to existing cells
– Sectorize omni cells
– Indoor and capacity microcells
– Additional macro sites
>Short term:
– Optimization of access parameters
– Activation and optimization of capacity features

Congestion Improvement – Expansions
- Expansions need to be planned in advance – Before
capacity is needed
- Important for operators to have an expansion
procedure in place
- Expansions can be limited by a variety of issues, for
example available spectrum
- The following presentation take a more detailed look at
traffic and capacity management

Congestion Improvement – Short Term
>The following features can be used to ensure efficient use of
installed capacity and to reduce congestion:
- Overlaid/Underlaid subcells
- Cell Load Sharing (CLS)
- AMR Half-rate (AHR)
- Assignment to Worse Cell
- Hierarchical Cell Structures (HCS)

Analyze the following issues that could be possible reasons
for TCH Assignment failures:
- Low Signal strength or no dominant serving cell
- Interference
- Congestion on TCH
- Incorrect use of Capacity features
- Faulty Hardware or transmission
- Incorrect Output power

It is worth taking into account the following issues when
optimizing TCH Assignment failures:
- Make sure that problems with network availability are
not the cause of congestion
- Often congestion problems are caused by network
design issues such as cells covering a large area
- Try to make sure that as far as possible calls are made
on the best serving cell to reduce the negative impact
on Retainability and Voice Quality
- Radio capacity features such as for example CLS should
preferably be only used as short or medium term
solutions for congestion.






















Not Approved Random Accesses
Check BSIC allocation
Check frequency plan
Access burst from
another co-channel
cell
Check cell parameter setting
MAXTA too
low?
Check SAE (COFLP,
DBTSP:TAP=SAACTION;)
Software file
congestion?
Check interference
High noise
floor?
END
Reduce interference
Increase SAE
Increase
MAXTA
Change BSIC or
frequency plan
Check if unknown access code
YES
NO
NO
YES
NO
YES
NO
YES
Check site location
High located
site?
Consider tilting or
lowering site
NO
YES
Check TCH congestion
Low TCH Assignment Success Rate
Congestion
on TCH?
Check output power
Low output
power?
Check output power
parameters
Corrupt parameter
setting?
Check Signal Strength of
BCCH and TCH
Low SS for call
access?
Check BTS Error Log
HW
fault?
NO
YES
END
Correct parameters
Swap & repair
HW
Check coverage plots
Perform drive tests
Dominant
server exists?
YES
NO
YES
Add
BTS
NO
Add tranceivers or BTSYES
NO
YES
YES
NO
Adjust TCH output
power
NO
Check Interference
Disturbance
on SDCCH or
target TCH?
Improve & adjust
frequency plan
NO
YES