ropped Call(TCH Drop-SDCCH Drop)-TCH Drop Analysis

Step to check TCH Drop Analysis.

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 timeout 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

Normal Release:

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. 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. 

TCH Drop reason (1)

The classification of TCH Drop Reasons are arranged in the order of priority:

1.Excessive Timing Advance

2.Low Signal Strength

3.Bad Quality

4.Sudden Loss of Connection

5.Other Reasons

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 TALIM

Parameter. This drop reason is commonly apparent to isolated or island sites with a wide

coverage area.

Action:

Check if the cell parameter TALIM is < "63"

Solution:

Set TALIM to a value close to 63.

Tilt antenna/reduce antenna height/output power, etc. for co-channel cells.

TCH Drop Reasons (2)

Low Signal Strength on Down or Uplink or Both Links

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 LOWSSDL and/or

LOWSSUL Thresholds. LOWSSDL and LOWSSUL 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.

Action:

Check coverage plots.

Check output power.

Check power balance and link budget.

Check if Omni site.

Check antenna configuration & type.

Check antenna installation.

Perform drive tests & site survey.

Check TRX/TS with high CONERRCNT.

Solution:

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.

Block/Deblock TRX

TCH Drop Reasons (3)

Poor Quality on Down or Uplink or Both Links

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 BAD Quality BL

will pegged.

Problem on Bad Quality is usually associated with Co-channel Interference on BCCH or

TCH. Faulty MAIO assignment can cause frequency collisions on co-sited cells especially on

1x1 Reuse. External interference is also one possible cause of problem on quality.

Action:

Check C/I and C/A plots.

Check Frequency Plan (Co-BCCH or Co-BSIC Problem).

Check MAIO, HOP, HSN parameters.

Check FHOP if correctly configured (BB or SY).

Check for External Interference.

Perform drive tests.

Solution:

Change BCCH frequency.

Change BSIC.

Change MAIO, HOP, HSN.

Change FHOP.

Record RIR or on-site Frequency Scanning to identify source of interference.

Use available radio features.

TCH Drop Reasons (4)

Sudden Loss of Connection

Drops due to Sudden Loss are drops that have not been registered as low signal strength,

excessive timing advance, bad quality or hardware (other) reasons, and the locating

procedure indicates missing measurement results from the MS.

There are some common scenarios that could lead to Sudden Loss of connections such as

very sudden and severe drops in signal strength, such as when subscribers enter into

buildings, elevators, parking garages, etc., very sudden and severe occurrence of

interference, MS runs out of battery during conversation, Handover Lost, BTS HW faults,

Synchronization or A-bis link fault (transmission faults), and

MS Faults.

Action:

Check BTS Error Logs, Alarms and Fault Codes.

Check CONERRCNT per TRX and TS.

Check Transmission Link (A-bis).

Check for DIP Slips.

Check LAPD Congestion.

Correlate Handover Lost to Drops due to Sudden Loss

Solution:

Fix Hardware Faults and Alarms.

Reset TRX with high CONERRCNT.

Ensure that Synchronization and A-bis Link are stable.

Change RBLT with high DIP Slips.

Change CONFACT or increase Transmission Capacity

Investigate HO Lost Problem

TCH Drop Reasons (5)

TCH Drops due to 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, Ater or Ainterfaces, and sometimes Handover Lost.

Action:

Check BTS Error Logs.

Check Alarms and Fault Codes.

Check CONERRCNT per TRX and TS.

Check Transmission Link (A-bis).

Check for DIP Slips.

Correlate Handover Lost to Drops due to Other Reasons

Solution:

Fix Hardware Faults and Alarms.

Reset TRX with high CONERRCNT.

Ensure that Synchronization and A-bis Link are stable.

Change RBLT with high DIP Slips.

Investigate HO Lost Problem

Problem reason of drop in SDCCH

Low Signal Strength on Down or Uplink

The reason 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 < style="font-weight: bold;">Solution: Set

TALIM to a value close to 63. Tilt antenna/reduce antenna height/output power, etc. for

cochannel cells.

Mobile 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.

Action: Check MS fleet.

Solution: Inform operator.

Subscriber Behavior

Poorly educated subscribers could use their handsets incorrectly by not raising antennas,

choosing illadvised 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 signal strength or others.

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

Solution: Increase capacity on TCH or using features like Assignment to another cell, Cell

Load Sharing, HCS, Dynamic Half-Rate Allocation and FR-HR Mode Adaptation etc