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RF Dropped Calls (GSM) by Chika Albert
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Transcript of RF Dropped Calls (GSM) by Chika Albert
DROPPED CALLS IN GSM
by
Chika Albert
Gil
From RF DT Basis, we defined dropped calls as an
event that shows the number of abnormal
disconnections during call setup or during
conversation. 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. Long calls are used to test or check for
dropped calls. The network is tested to see how the
cells of the network can handover to themselves for
the entire duration of the call. This indirectly also
defines retainability.
Retainability is the ability of a service once
obtained, continues to render service until the
required duration of the call. A higher
retainability value indicates less number of
dropped calls as it could also be used to
acertain HO performance.
%Dropped calls = Total number of dropped calls x 100 Total number of calls set-up
%Retainability = Total number of good calls x 100
Total number of calls set-up
Causes of Drop Calls
Excessive Timing Advance
The TCH Drop counters due to Excessive
Timing Advance will pegged when the
during the time of disconnection, the last
Timing Advance value recorded was higher
than the timing advance Parameter. This
drop reason is commonly apparent to
isolated or island sites with a wide coverage
area.
Solution
Check if the cell parameter timing
advance limit is < "63"
Set timing advance limit to a value
close to 63.
Tilt antenna/reduce antenna
height/output power, etc. for co-
channel cells.
Low Signal Strength
The drops counters due to Low Signal
Strength will be pegged when the Signal
Strength during the last Measurement
Report before the call dropped is below the
low signal strength downlink and/or low
signal strength uplink. Thresholds LSSDL
and LSSUL are BSC Exchange Property
parameters which is used only for statistics
purposes and does not affect the behavior
of calls.
If both UL and DL Signal Strength are
below the thresholds, only Drop due to
Low SS BL will pegged. 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. Bad indoor coverage will result in
dropped calls. Building shadowing could
be another reason.
Solution
Check coverage plots of cell OD2419E.
Check output power of the cell (OD2419E) on
which the call dropped.
Check if Omni site.
Check antenna configuration & type on the cell.
Check antenna installation.
Perform DT & SSV.
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.
Bad Quality
The drops counters due to Bad Quality will be
pegged when the Signal Strength during the last
measurement Report before the call dropped is
above the BADQDL and/or BADQUL thresholds.
BADQDL and BADQUL (expressed in DTQU) are BSC
Exchange Property parameters which is used only for
statistics purposes and does not affect the behavior
of calls. If both UL and DL Quality are above the
thresholds, only Drop due to BADQBL will pegged.
Problem on Bad Quality is usually associated with
Co-channel Interference on BCCH or TCH.
Faulty Mobile Allocation Index Offset
assignment can cause frequency
collisions on co-sited cells especially on
1x1 Reuse. External interference is also
one possible cause of problem on
quality.
Solution
Check C/I and C/A plots.
Check Frequency Plan (Co-BCCH or Co-BSIC Problem).
Check MAIO, HOP, HSN parameters.
Check frequency hopping if correctly configured (BB
or SY).
Check for External Interference.
Perform drive tests.
Change BCCH frequency.
Change BSIC.
Change MAIO, HOP, HSN.
Change frequency hopping.
Use available radio features.
Interference
When a cell that a call is camped on has
carried or held a call up to its set HO
threshold, it seeks for a pre-defined neighbor
to handover the call. It hands over the call to
the neighbor cell having the best RxLev. But if
the perceived best neighbors all have the
same or very close ARFCN, it drops the call out
of confusion. A case of the same ARFCN is
reported as co-channel interference while
very close ARFCN is the adjacent interference.
Solution:
Check the ARFCN of neighbor site
around the area.
Frequency planning method should be
properly defined
Frequency-reuse should not be used in
a LA.
Solution
Check BTS Error Logs, Alarms and Fault
Codes.
Check connection per TRX and TS.
Check Transmission Link (A-bis).
Check Link Access Protocol D-channel
(LAPD) Congestion.
Correlate Handover Lost to Drops due
to Sudden Loss
Fix Hardware Faults and Alarms.
Reset TRX with high/abnormally
terminated connection counter set-ups
attempts (CONERRCNT ).
Ensure a constant Synchronization and
A-bis Link .
Increase Transmission Capacity
Investigate HO lost problem
Sleeping Cells
During RF DT, some existing cells may not
be up and running. When a cell a call is
camped on has reached its HO threshold
and very close to the supposed sleeping
cell, it drops the call. This so because the
sleeping cell at that very close distance to
the point of drop is expected to receive the
call and continue carrying traffic. A cell is can be detected sleeping if a call at a distance < 0.5km drops.
Solution:
Visit site
Correct power-up problem
Carry-out SSV on the site
Overshooting Cells
Some TCH drops occur due to
overshooting. When a cell shoots >
10km in an urban area, expectedly the
TRx drops without a repeater.
Overshooting cells sometimes reveals
dead or sleeping cells.
Solution:
Down-tilt overshooting site
Visit site within close range of the event
Carry-out SSV on the sites within the
area of the event.
Other Reasons
TCH drops due to Other Reasons are
computed by subtracting the sum of drops
due to Excessive TA, Low SS, Bad Quality
and Sudden Loss from the Total TCH Drop
Counts. Drops due to Other Reasons are
generally associated with hardware
problems, transmission link problems on A-
bis, A-ter or A-interfaces, and sometimes HO
lost.
Solution
Check BTS Error Logs.
Check Alarms and Fault Codes.
Check CONERRCNT per TRX and TS.
Check Transmission Link (A-bis).
Correlate Handover Lost to Drops due
to Other Reasons
Fix Hardware Faults and Alarms.
Reset TRX with high CONERRCNT.
Investigate HO Lost Problem
MS error
Some old mobiles may cause dropped
calls if certain radio network features
are used. Another reason is that the MS
is damaged and not working properly.
Solution
Check MS fleet.
Inform operator.
Subscriber’s behavior
Poorly educated subscribers could use
their handsets incorrectly by not raising
antennas, choosing ill-advised locations
to attempt calls, etc. this will surely lead
to drop calls.
Solution
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
signal strength or others.
Solution
Check if MS power regulation is used.
Check if DTX uplink is used.
THANK YOUFailure establishes one thing; that your zeal to succeed was not strong
enough.
Gil