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