TCH Assignment Analysis: Flow Chart

 

TCH Assignment Analysis Flow chart

Successful assignments assignments show the number of successful TCH allocations at call setup. At unsuccessful assignment, the Assignment Complete message, sent by the MS, was never received by the BTS.  The formula is defined as: % TCH ASSIGNMENT SUCCESS RATE

= TFCASSALL  TASSATT

X 10 100 %

 

Reasons Probable Reason No dominant  The serving cell cannot cope with the serving cell  TCH traffic. Severe congestion Failing TCH allocation for assignment assignment on TCH or handover due to congestion Low signal strength The signal strength might be higher on for call access the BCCH than on the TCH. Interference Disturbance on on SD SDCCH or or tta arget TC TCH Fault Fa ulty y transc transceiv eiver er Faulty Faulty equipm equipment ent Troubleshooting The following procedure should be performed for TCH Assignment analysis: 1: For TCH assignment success success rate, the first thing, check the TCH Time Congestion. 2: If there is congestion on TCH, it is recommend doing the dimensioning dimensioning and adding TRU based on carried TCH traffic demand. 3: If there is no congestion on TCH, check the output power of the BTS. If the output power is low, increase the output power. 4: If the output power is ok, check the faulty BTS by extracting BTS error log. 5: If hardware fault found, swap or repair HW. Perform drive tests to check the coverage and received Rx LEV. 6: If no dominant cell or similar signal strengths of a few cells found during drive tests, it is recommended to add BTS. 7: If there is no problem on the dominant cell, check the interference whether cochannel or adjacent channel. 8: Check the disturbance whether it is on SDCCH or target TCH. If disturbance found, improve the frequency plan. Mostly, the problems of low TCH assignment are TCH availability and interference.

 

Interference Analysis Flow chart

Troubleshooting The following procedure should be performed for interference analysis: 1. Check the drop call performa performance nce for that particular particular cell. cell. When the interference interference problem occurs in the cell, the drop call will be higher than usual. This might depends on the severity of the interference whether it is co-channel or adjacent channel. 2. Check the handove handoverr performanc performance e of the cell. The HO performanc performance e will also look bad especially when you look into the neighbor relation that has interference. For e.g. adjacent channel. (CNA consistency checking can detect this)

 

3. Check the antenna direction, direction, position position etc. etc. This is to see whether whether the direction covers the right area, open space area (this can be seen by having good and updated map) 4. Check if the site is a dragon site. If the site is a dragon site, it might might be possible possible to get interfered by co-channel from far away. 5. Check the co-channel sites, if found, change change frequency frequency and see the result. Mostly, changing the frequency will solve the interference problem. 6. 75% Check the Intracell ha ndover (normally (normally for Intracell for Intra cell handover 25%handover Uplink and Uplink Downlink) and handover if the variation is different from this. Intracell usually indicates bad quality and high signal strength. Too high number of intracell handover show a bad quality cell and if  possible, you can reduce the number of intracell handover of MAXIHO to a smaller value based on the channel group. 7. Locate the interference interference from from statistics statistics based on MS reversion reversion to old old channel of  total attempt. High number of reversion will show that the target frequency might be interfered. 8. Check the statistics from Outgoing Outgoing Handover Handover decision decision due to bad quality Uplink or Downlink from handover decision. High decision of handover due to quality will show the direction of interference. 9. Check if the interference interference is uplink interference interference (this might might be an interference interference from other MSs) by analyze the ICM band for other band (not include band 1). If  found on ICM > 3, change frequency. 10.Check the MS power regulation setting. If any poor setting found, correct the parameter. Improper setting of MS power regulation might cause interference.  The feature used to reduce the MS power when the MS is near to the BTS and hoping that it might not interfere the uplink. 11.Check if the frequency hopping on or off. If more than 1 TRU, turn on the frequency hopping. Turning on the frequency will help to reduce interference by interference averaging. 12.Check if DTXU feature is on or off. If off, turn the DTX feature on. This will save the battery in the MS and reduce the interference. 13.If the interference is downlink (causing (causing by other BTS interference), Check BTS power regulation. If any poor setting found, correct parameter setting. 14.Check if DTXD feature is on or off. If off, turn the DTX feature on. This is used to reduce interference and decrease BTS power consumption 15.If changing frequency or parameter cannot solve the interference for both uplink & downlink, it might be external interference. 16.Check antenna installation, ensure that the antenna is correctly installed 17.Check if another mobile network like AMPS is near to the location. 18.Check from the statistics if there is any pattern of bad quality reason. For example, for surveillance purpose, the CCTV or wireless alarm system might be turn on during nighttime only. 19.If 19. If external interference problem occurs, do drive test and report the usage of the frequency to authorities .

 

TCH Drop Analysis 1. Radio Link Time-Out Every time a SACCH message cannot 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 cannot 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 acknowledgement acknowledgement on a Layer 2 message after 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  measurementt reports. It is also possible that the release will be normal depending measuremen 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 timeout, 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, 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 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 criteria is not fulfilled, Assignment Comma Command nd will not be sent and a Channel Release (cause: abnormal release, unspecified) unspecified) will be sent to the mobile station and a Clear Request to the MSC.

Reasons for TCH Drop  The classification of TCH Drop Reasons are arranged in the order of priority: 1.ExcessiveTiming 1.ExcessiveTim ing Advance 2.Low Signal Strength 3.Bad Quality 4.Sudden Loss of Connection 5.Other Reasons

1.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. Troubles Tro ubleshoot hooting ing : Check if the cell parameter TALIM is < "63" 

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Solution: Set TALIM to a value close to 63.  Tilt antenna/reduce antenna height/output power, etc. for co-channel cells.

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. Troubleshooting: Check coverage plots. Check output power. 

Check and link budget. Check power if Omnibalance site. Check antenna configuration & type. Check antenna installation. 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 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 are used only for statistics purposes and do 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 collisio collisions ns on co-sited cells especially on 1x1 Reuse. External interference is also one possible cause of problem on quality. Troubleshooting: 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. 

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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 occurrenc e of interference, MS runs out of battery during conversation, Handover Lost, BTS HW faults, Synchronization or A-bis link fault (transmission (transmission faults), and MS Faults. Troubleshooting: Check BTS Error Logs, Alarms and Fault Codes. Check CONERRCNT per TRX and TS. 

Check Check Transmission for DIP Slips. Link (A-bis). 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 Transmission Capacity Investigate HO Lost Problem 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 transmission link problems on A-bis, Ater or Ainterfaces, Ainterfaces, and sometimes Handover Lost. Troubleshooting: 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. 

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Ensure that Synchronization and A-bis Link are stable. Change RBLT with high DIP Slips. Investigate HO Lost Problem

SDCCH Drop Analysis Reasons for SDCCH Drop 1.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. Troubleshoot: Check coverage plots Check output power. Perform drive tests. 

 

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Check BTS error log Solution: Add new sites. Increase output power Repair faulty equipment.

2. Poor Quality on Down or Uplink  

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Troubleshoot: Check C/I and C/A plots. Check frequency plan. Perform drive tests. Solution: Change frequency. Use available radio features.

3. Too High Timing Advance Troubleshoot: Check if the cell parameter TALIM is < 63 Solution: Set TALIM to a value close to 63. Adjust Tilt antenna for co-channel cells. Reduce antenna height for co-channel cells. Adjust Output power for co-channel cells. 

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4. 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. Troubleshoot: Check MS fleet. Solution: Inform operator.

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5. Subscriber Behavior Poorly educated subscribers could use their handsets incorrectly by not raising antennas, choosing ill-advised locations to attempt calls, etc. Troubleshoot: Check customer complaints and their MS. 

6. Battery Flaw When a subscriber runs out of battery during a conversation, conversation, the call will be registered as dropped call due to low signal strength or others. Troubleshoot: Check if MS power regulation is used. Check if DTX uplink is used.

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7. Congestion on TCH  The SDCCH is dropped when congestion on TCH. Troubleshoot: Check TCH congestion 

 

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Solution: Increase capacity on TCH or using features like Assignment Assignment to another cell Cell Load Sharing HCS Dynamic Half-Rate Allocation FR-HR Mode Adaptation etc.