TCH ASSIGNMENT SUCCESS RATE.docx

TCH ASSIGNMENT SUCCESS RATE Probable Reason: 1) No dominant serving cell: The serving cell cannot cope with the TCH traffic. 2) Severe congestion on TCH: Failing TCH allocation for assignment or handover due to congestion 3) Low signal strength for call access: The signal strength might be higher on the BCCH than on the TCH. 4) Interference: Disturbance on SDCCH or target TCH 5) Faulty transceiver: Faulty equipment The following procedure should be performed for TCH Assignment analysis:

For TCH assignment success rate, the first thing, check the TCH Time Congestion. If there is congestion on TCH, it is recommend doing the dimensioning and adding TRU based on carried TCH traffic demand. If there is no congestion on TCH, check the output power of the BTS. If the output power is low, increase the output power. If the output power is ok, check the faulty BTS by extracting BTS error log. If hardware fault found, swap or repair HW. Perform drivetests to check the coverage and received RxLEV. If no dominant cell or similar signal strengths of a few cells found during drivetests, it is recommended to add BTS. If there is no problem on the dominant cell, check the interference whether co-channel or adjacent channel. 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. KPI Introduction 1. CSSR (CALL SETUP SUCCESS RATE) Definition: Rate of calls going until TCH successful assignment 2. SCR (SUCCESSFULL CALL RATE) Definition: Rate of calls going until normal release that is not interrupted by SDCCH DROP, neither by assignment failures, and neither by CALL DROP.

Visit Our Forum For more Optimization Tips 3. CALL DROP RATE (CDR) Definition: Rate of all losses of TCH connections during a call in relation to the number of successful Call Setups Visit Our Forum For more Optimization Tips 4. HOSR (HAND OVER SUCCESS RATE) Definition: Successful internal and external outgoing handovers of total number of internal and external outgoing handover attempts

5. PSR (PAGING SUCCESS RATE) Definition: Rate of successful paging attempts of total number of paging attempts.The formula is based on NSS point of view (based on MSC or LAC) 6. LOCATION UPDATE SUCCESS RATE Definition: Successful location update attempts of total number of location update attempts. The formula is based on NSS point of view. 7. SDCCH BLOCK RATE Definition: SDCCH congestion of total number of SDCCH seizure attempts 8. SDCCH DROP RATE Definition: Dropped SDCCH connections of total number of SDCCH connections without TCH congestion. 9. TCH ASSIGNMENT BLOCK RATE Definition: Rate of TCH unsuccessful

seizures

during

assignment

procedure

due

to

congestion

Visit Our Forum For more Optimization Tips 10. TCH Assignment Failure Rate (exclude blocking) Definition: Rate of RTCH seizure failed (system + radio) during normal assignment procedure over the total amount of RTCH request for normal assignment procedure 11. EMD (Erlang Minute per Drop) Definition: Total of Erlang minutes (TCH occupation) in one period measurement per drop call (after TCH Assignment). 12. TCH Availability Definition: Available TCH of total number of defined TCH 13. RACH Success Rate Definition : Rate of Successful RACH over the total number of channel required message received

Handover type and Handover Decision Handover type: Handover is a basic functionality of cellular networks. Handovers can be distinguished as either intracell , intercell or inter-BSC handovers. Handovers within a single cell (i.e. changing timeslots and / or carrier frequencies) can be handled autonomously by the controlling BSC. Handovers between cells of the same BSC can also be handled by the BSC . Handovers between cells of different BSC’s must be handled by the initiating MSC. Intracell same cell, other carrier or timeslot Intercell between cells (normal case) Inter-BSC between BSC area Inter-MSC between MSC area Inter-PLMN (only when roaming) Handover Decision The HO threshold comparison includes the evaluation of these following criteria: 1. Level (uplink / downlink) 2. Quality (uplink / downlink) 3. Interference (uplink / downlink) 4. Power budget 5. Umbrella 6. MS-BS distance (maximum or minimum) 7. Turn-around-corner MS 8. Rapid field drop 9. Fast/Slow-moving MS 10. Better cell i.e. periodic check (Power budget or Umbrella) 11.PC due to Lower quality thresholds (uplink and downlink) 12.PC due to Lower level thresholds (uplink and downlink) 13.PC due to Upper quality thresholds (uplink and downlink) 14.PC due to Upper level thresholds (uplink and downlink). Ping Pong Handover As the cellular network growing, it requires addition of new sites to expand capacity and or coverage services. New mobile operators were expecting to grow their network in exponential order to gain market share against mature

competitors. Without any intensive network performance maintenance, common problem such ping pong handover would be degrade user experience like bad voice quality ( SQI speech quality index, MOS mean opinion score ) or even dropped call.

Ping Pong handover is shown from the successful handover back to old cell within pre-defined time of total handover, e.g. less than 10 seconds. Since not all BSS vendors provide such performance counters it might be identified by simple metric that shall be expressed as total successful handover over number of call or connection, e.g. more than 200% indicates ping pong handover. The actual ping pong handover is easily identified through drive test with sense of optimization engineer without require any calculation. Coverage holes might lead to Ping-Pong handover especially for slow moving mobiles. It may be cause by shadowing by high building. Several possible equal signals from two or more base stations might lead to Ping-Pong handover as well which is so called not optimized cell dominance. The following action items shall be used for optimization solutions: 

Check the parameter setting such as hysteresis, offset, priority layer in dual-band case, etc. If poor parameters setting found, then correct the related parameters. Compare to default parameters design.

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Check the output power BSPWR and BSPWRT which normally put on maximum value or compare to design.

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Check path balance on transceiver link, e.g. loss in uplink and downlink chain. Defective RF modules might be the reason of imbalanced link.

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Check the site location such as possible coverage holes or no dominant cells.

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Perform drive test on that particular area to check the signal strength, if the average of signal strength of the neighbors are the same, no dominant cell is found.

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For Long term action if no dominant cell shall be adding new site, while short term action shall be uptilt the most dominant cell as the serving cell for that area and reduce coverage for others.

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Probable Reasons of Bad Handover Performance

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---Neighboring Cell Relation Action:Add neighbor cell relation. ---Missed measurement frequencies in BA-list Action:Check measurement frequencies list. ---Permitted Network Color Code problem Action:Check NCC Permitted ---HW faults. Action: Check BTS error log. ---Blocking on Target Cell Action:Remove Blocking on Tager Cell ---Congestion A high congestion might lead to dragged calls (handover performed at a not intended location) and a lot of unsuccessful handovers. Action: Check TCH congestion. ---Timer Expire After MS is Lost The MS never answers the base station. Action: Check coverage. Check interference.

---Link Connection or HW Failure Action: Check BTS error log. Perform site visit. Perform link performance measurements. ---Bad Antenna Installation Action: Perform site survey and check antenna installation. Check antenna cabling. ---Many Neighbors Defined Many defined measurement frequencies defined (>16) will decrease the accuracy of the mobile measurements to locate the best six servers. Many measurement frequencies mean few samples per frequency and problem for mobiles to decode the BSIC. 

Action: Check number of definitions.

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---Delayed Handover Decision

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A delayed handover decision can be due to congestion in the target cell. Action: Check handover parameters. ---Wrong Locating Parameter Setting Action: Check locating parameters. ---Bad Radio Coverage Action: Check coverage plots. ---High Interference, Co-Channel or Adjacent The potential handover candidate is disturbed by interference. Outgoing handover due to bad uplink quality may indicate interference from co-channel another MS. On the border, the quality may be rather bad and the signal strength low. Bad downlink quality may indicate interference from another co-channel base station. Action: Check interference. Check if many handovers are performed due to downlink or uplink bad quality. ---Receiver Antenna Problem or RBS HW problems (in candidate cell) Action: Check antenna installation. Check RBS HW and Error log of the target cell ---Poor Inter-MSC/BSC Handover Performance For outer or external cell, wrong definitions in either MSC or BSC may be reason for the problem. Action: Check inter-MSC/BSC handover performance. ---Incorrect Down Tilt Action: Perform site survey and check antenna installation. Solution: Correct antenna tilting.

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

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Step to check TCH Drop Analysis. 1. Radio Link Time-Out

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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.

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Visit Out Forum For more Optimization Tips 2. Layer 2 Time-Out

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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

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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.

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4.MSCTime-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).

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5. Assignment to TCH Before sending an Assignment Command from the BSC at TCH assignment, the following two criterion have to be fulfilled:a.Theremustbe a TCH channel available, i.e. no congestion b. The locating algorithm must have received at least one valid measurement report.

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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.

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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

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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.

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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

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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

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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.

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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.

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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.

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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.

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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

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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 

Probable Reasons of SDCCH Congestion

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---Low Availability Action: Check SDCCH Availability. Check if the channels are manual, control or automatic blocked. ---Increasing Traffic Demand The high traffic could be related to an occasional event or due to a long term growth. Action: Check if short term traffic growth. Make trend comparisons. Check if combined SDCCH is used.

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Check SDCCH dimensioning. ---Bad use of Adaptive configuration of Logical Channels By using the Adaptive configuration of logical channels feature, the basic SDCCH configuration in a cell will

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be under-dimensioned. If this feature is not used correctly, it will cause SDCCH congestion. Action: Check if ACSTATE is on. Check parameters related to Adaptive configuration of logical channels

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---Long Mean Holding Time If the mean holding time is long, this generates a higher traffic load. Action: Check SDCCH Mean Holding Time ---Too Frequent Periodic Registration Action: Check Random Access Distribution. Check the timer T3212 in the BSC and the parameters ---BTDM and GTDM in the MSC Solution: Decrease the periodic registration. ---Location Area Border Cell If the cell is situated on a misplaced Location Area border, this means that unnecessary many normal LUs

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are performed. Action: Check site position and location area border. Check Location Update Performance. Check

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parameter CRH etc. ---Extensive SMS Usage Extensive SMS usage increases the SDCCH traffic and could cause congestion if badly dimensioned SDCCH

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channels. Action: Check SMS activity. -Cell Broadcast Used Action: Check if Cell Broadcast is active. .If active, check if it is used by the operator. -IMSI Attach/Detach in Use. An introduction of IMSI attach/detach will increase the traffic on SDCCH. However, the benefits are that

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the paging success rate will increase. The recommendation is to use Attach/Detach. -Cell Software File Congestion Action: Check SAE setting. High Ratio of Random Accesses Action: Check Random Access performance

Low Signal Strength Analysis Low Signal strength is one of the reason of drop call. It can be indicated by many calls disconnected at low signal strength by subscriber, drop calls due to excessive TA, poor handover performance and poor call setup performance.

Probable Reason Poor BSC Exchange Property setting

High LOWSSDL & LOWSSUL will give more drop reason due to SS and this might not show the actual drop. It is because drop due to SS is more priority than Quality.

No dominant cell Cell might be isolated or standalone. Antenna tilt & orientation

Too much downtilt sometimes might not cover a larger area and the subscriber might lose the SS.

Output Power

Low output power might cause smaller border cell.

The following procedure should be performed for low signal strength analysis: 1:Identify the baseline requirement of design and BSC exchange property (setting for LOWSSUL/LOWSSDL). 2:Check the value for LOWSSDL & LOWSSUL. If it is higher than ACCMIN, change the parameter to a reasonable value since the drop reason will be more priority to SS compared to Quality. 3:Check the site position, antenna direction, position etc. This is to ensure the possible location is open to interference (open water environment) or isolated. Good map is needed for this. 4:Check if the site is sectorized or Omni. If it is Omni, set the cell into sectorized cell. 5:Check if the signal strength is uplink or downlink limited. Mostly, It is designed to be downlink limited.

6:Check the coverage cover expected area from the planet. If it is not, check the antenna tilt and orientation. Change the direction or tilt if it is too much downtilt or pointing to a wrong direction. 7:Sometime, low output power might cause low SS. Check output power and if it is low, increase the output power. 8:Check cell whether it has hotspots from drivetests. If found, adding new site is recommend. 9:In order to check power distribution, run Cell Traffic Recording (CTR) to that particular cell. 10:Check if the cell has indoor coverage problem. If yes, add micro site instead. Dropped Call due to Sudden Drop On circuit switch service, when a call is abnormally disconnected, a Clear Message with cause code Call Control be treated as normal Disconnection is sent to the MSC – named Clear Request Message. Refer to Ericsson system, the following Urgency condition is checked at that time and the relevant counter is incremented as a consequence: 1.

Excessive TA

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Low Signal Strength

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Bad Quality

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Sudden Drop

As named, stright forward meaning for the dropped call is described for the first three items. However, Sudden Drop is quite not easy to understand.Sudden Loss are drops that have not been registered as bad quality, signal strength, timing advance. The term Sudden Loss is used because if the network cannot establish a connection with the lost MS after a pre-defined period, the sudden loss counter is incremented if the last reported measurement from the MS does not fulfill any of the reasons mentioned.A connection is marked as Sudden loss if none of the three types of urgency states (that is excessive TA, low signal strength or bad quality) are indicated and the locating procedure indicates missing measurement results from the MS.Drops due to ‘Other’ reasons are generally associated with hardware problems and disturbances, number of drops due to ‘Other’ reasons is obtained by subtracting the drops with known reasons from the total number of drops.Main contributors in sudden and other TCH drop: Very sudden and severe drop in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc. Very sudden and severe occurrence of interference or bad quality.;MS running out of battery during conversation. 

Handover Lost.

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BTS HW faults.

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Synchronization or Abis link fault (transmission faults).

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MS Faults.