DROPPED CALLS IN GSM

by

Chika AlbertGil

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.

SolutionCheck 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.

SolutionCheck 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.

SolutionCheck 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 CapacityInvestigate 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 siteCorrect power-up problemCarry-out SSV on the site

Overshooting CellsSome 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 siteVisit site within close range of the event

Carry-out SSV on the sites within the area of the event.

Other ReasonsTCH 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.

SolutionCheck 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 errorSome 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 behaviorPoorly 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 flawWhen 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