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Table of Contents

1 INTRODUCTION ................................................................................. 4

2 RANDOM ACCESS............................................................................. 5 2.1 REASONS FOR POOR RANDOM ACCESS

PERFORMANCE 5 2.2 USED FORMULAS................................ ............................................ 5 2.3 ANALYSIS ...................................................................................... 6

3 P AGING AND LOCATION U PDATE ................................ .................... 8 3.1 REASONS FOR POOR PAGING AN LU

PERFORMANCE 9 3.2 USED FORMULAS................................ ............................................ 9 3.3 ANALYSIS .................................................................................... 11

3.3.1 Paging ............................................................................. 12 3.3.2 Location Update................................................................ 14

3.4 TROUBLESHOOTING ................................................................ ...... 15 3.4.1 General ............................................................................ 15 3.4.2 Unsuccessful Location Updating ........................................ 18

4 CALL SET-UP ................................................................ .................. 19 4.1 REASONS FOR POOR CALL SET-UP

PERFORMANCE 19 4.2 USED FORMULAS................................ .......................................... 20 4.3 ANALYSIS .................................................................................... 20

4.3.1 Random Access problems ................................................. 21 4.3.2 Cell parameter settings and RN

features 21 4.4 TROUBLESHOOTING ................................................................ ...... 22

4.4.1 General problems ............................................................. 22 4.4.2 Low signal strength ........................................................... 23 4.4.3 SDCCH and TCH congestion............................................. 23 4.4.4 HW faults and other problems ............................................ 23

5 DROPPED CALLS ............................................................................ 24 5.1 REASONS FOR DROPPED CALLS ...................................................... 24 5.2 USED FORMULAS................................ .......................................... 25 5.3 ANALYSIS .................................................................................... 26

5.3.1 SDCCH Results ................................................................ 26 5.3.2 TCH Results ..................................................................... 27

5.4 TROUBLESHOOTING ................................................................ ...... 29 5.4.1 Dropped Calls Due To Bad Quality..................................... 29 5.4.2 Dropped Calls Due To Low Signal

Strength 30 5.4.3 Dropped Calls Due To Other

Reasons 31

6 SDCCH & TCH ................................................................................. 32 6.1 REASONS FOR TRAFFIC CAPACITY PROBLEMS .................................... 32 6.2 USED FORMULAS................................ .......................................... 32 6.3 ANALYSIS .................................................................................... 33

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6.3.1 SDCCH/TCH availability .................................................... 33 6.3.2 Cell size and location analysis ........................................... 34 6.3.3 Feature activation ............................................................. 34

6.4 TROUBLESHOOTING ...................................................................... 35 6.4.1 Congestion, general .......................................................... 35 6.4.2 SDCCH Congestion .......................................................... 35 6.4.3 TCH Congestion ............................................................... 38

7 INTERFERENCE .............................................................................. 40 7.1 REASONS FOR HIGH INTERFERENCE LEVELS ..................................... 41 7.2 USED FORMULAS ......................................................................... 41 7.3 ANALYSIS.................................................................................... 42

7.3.1 Bad frequency plan ........................................................... 43 7.3.2 External interference ................................ ......................... 44 7.3.3 Congestion ....................................................................... 44 7.3.4 Missing neighbour cell relations ......................................... 44 7.3.5 Wrong antenna type or bad

antenna positions 45 7.3.6 HW/SW Problems and site outages ................................... 45 7.3.7 Cell parameter settings and RN

features 45 7.4 TROUBLESHOOTING ...................................................................... 46

7.4.1 Uplink Interference............................................................ 46 7.4.2 Downlink Interference ....................................................... 47 7.4.3 External Interference................................ ......................... 48

8 HANDOVER ..................................................................................... 49 8.1 REASONS FOR POOR HANDOVER

PER FORMANCE 49 8.2 USED FORMULAS ......................................................................... 50 8.3 ANALYSIS.................................................................................... 51

8.3.1 Neighbouring cell relation problems ................................... 51 8.3.2 Cell parameters settings and RN

features 51 8.3.3 Hardware problems. .......................................................... 52 8.3.4 Too many measurement

frequencies in the active BA list 52 8.3.5 Poor coverage and coverage holes .................................... 52 8.3.6 Congestion problems ........................................................ 52 8.3.7 High interference .............................................................. 53 8.3.8 Poor inter-MSC handover

performance 53 8.4 TROUBLESHOOTING ...................................................................... 53

8.4.1 Too few Handover attempts or no handovers 53

8.4.2 Unsuccessful (lost) handovers ........................................... 54 8.4.3 Handover reversions ................................ ......................... 55 8.4.4 Ping-Pong Handovers ....................................................... 57

9 REFERENCES ................................ ................................................. 57

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Introduction

The purpose of this document is to provide an overview on general

GSM radio network performance areas.

General GSM radio network performance areas may be summarized

as:

• Random Access

• Paging and Location Update

• Call set-up

• Dropped Calls

• SDCCH & TCH

• Interference

• Handover

Following chapter will review each of above areas with focus on

possible reasons for poor performance, formulas for STS monitoring,

performance analysis and troubleshooting.

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1 Random Access

A Random Access burst is the first thing that will be sent when an MS

tries to access the network. The Random Access (RA) performance is

important for the accessibility performance and is linked to the BSIC

planning.

Reasons for poor random access performance

Areas with possible problems with BSIC planning, too low ACCMIN,

wrong MAXTA, interference or bad link budgets. A very high number of

not approved Random Accesses on BSC level might also indicate

problems with software file congestion in the BSC or MSC.

1.1 Used Formulas

RAACCFA: Total Number of Failed Random Access Attempts.

RA_TOT: Total Number of Random Access Attempts.

CNROCNT: Total Number of Accepted Random Accesses.

RA_FAIL: Failed Random Accesses of Total RA Attempts.

RA_ANSWPAG: Answer to Paging of Total Random Accesses.

RA_SERVICE: Other Services Requested of Total Random Accesses.

RA_EMERG: Emergency Calls of Total Random Accesses.

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RA_CALLREE: Random Accesses with Cause “Call

Reestablishments” of Total Number of Accepted Random Accesses

RA_OTHER: All Other Cases of Total Random Accesses.

S_EST: Number of SDCCH Establishments of Total Number of

SDCCH Seizure Attempts when No SDCCH Congestion.

1.2 Analysis A cell can interpret a handover burst (supposed for another cell) as a

Random Access burst, which causes the counter RAACCFA to be

stepped. A necessary condition for this to happen is that the cells have

BSIC and an ARFCN in common. The handover burst is sent by an MS

to the target cell on the new TCH and contains the BSIC for the cell. If

another cell in the vicinity uses the frequency as BCCH and have the

same BSIC, the problem can occur. The general system performance

will not be affected unless any congestion occurs due to this

unnecessary use of RACCH and AGCH (Access Granted channel).

Anyway, a lot of RA failures (RAACCFA) always mean co-channel

interference. A problematic cell has to be checked for neighbours with

identical BSIC and where BCCH for the problem cell is used as ARFCN.

If this neighbour is far away, the co-channel interference will usually not

cause any performance problems (although there are a lot of RAACCFA

detected).

High timing advance can also be a reason for RAACCFA to be stepped.

The parameter MAXTA should be checked in that case.

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employees if AMPS located close to the problem area. These can cause interference within distance of at least 100 m. Sometimes filters can solve the interference AMPS sites.

• MRR, CTR, MTR used to point out areas of problems?

• Frequency change tried?

• Antenna down tilt tried? Before doing down tilt it should be verified TEMS that the interference occurs in the border area of the cell, coverage from the cell that should be tilted is unreasonably large.

• Check that antenna directions are according to the plan.

• Poor coverage?

3.4.2 Low signal strength • Is the cell situated in a poor coverage area, for example on the countryside?

Correlate with the analysis of dropped calls and look especially for drops due to low signal strength. Highlight in the report how many percentages (approximately) of the call setup failures that are due to poor coverage and suggest areas for new sites.

3.4.3 SDCCH and TCH congestion

• Check the SDCCH time congestion. Especially cells close to a location area border can be heavily loaded and need additional SDCCH capacity to be able to set up calls. It does not matter how many idle TCHs there are in a cell if there at the same time is congestion on the SDCCH.

3.4.4 HW faults and other problems If a frequency change did not have the expected effect or if the cov erage is far less than the frequency planning tool shows it can depend on one of the faults listed below. Some possible faults are:

• If a cell is not covering the area that it is supposed to cover according to the frequency-

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planning tool it can depend on that the antenna is connected to the wrong feeder.

• The site can in reality be lower than in the predictions in the frequency planning tool, giving less coverage than planned.

• There can be alarms indicating HW faults.

• Software file congestion

4 Dropped Calls The retainability performance evaluates the systems ability to handle established connections. Dropped calls are probably the single most important quality item to control in the system. The level of dropped calls in the system is in high extent depending on the initial RF planning, optimization and also the system growth.

4.1 Reasons for dropped calls Possible reasons for a high rate of dropped calls could be:

• TCH Congestion

• Parameter Settings

• HW problems

• Interference

• Poor signal strength

• Missing cell relations and/or missing measurement frequencies

The reasons for dropped calls can, according to STS, be:

- Low signal strength

- Bad quality

- Sudden loss of connection (only TCH)

- Excessive timing advance

- Other

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4.2 Used Formulas S_DR-C: Dropped SDCCH Connections of Total Number of SDCCH Connections.

S_DR_ERLM: Erlang Minutes per Dropped SDCCH Connection.

S_DR_SS: Dropped SDCCH Connections due to Low Signal Strength of Total Number of Dropped SDCCH Connections.

S_DR_BQ: Dropped SDCCH Connections due to Bad Quality of Total Number of Dropped SDCCH Connections.

S_DR_TA: Dropped SDCCH Connections due to Excessive Timing Advance of Total Number of Dropped SDCCH Connections.

S_DR_OTH: Dropped SDCCH Connections due to Other Reasons than Low Signal Strength, Bad Quality or Excessive Timing Advance of Total Number of Dropped SDCCH Connections.

T_TRAF: Average TCH Traffic Level.

T_CONGT: TCH Time Congestion of Total Measurement Interval.

T_AVAIL: Available TCHs (not blocked) of Total Number of Defined TCHs.

T_DWN: Average Cell downtime for active cells

H_SUC: Successful Handovers of Total Number of Handover Attempts.

T_DR-S: Dropped TCH Connections of Total Number of Calls Terminated in the Cell.

T_DR_ERLM: Erlang Minutes per Dropped TCH Connection.

T_DR_SS_DL: Dropped TCH Connections due to Low Signal Strength on Downlink of Total Number of Dropped TCH Connections.

T_DR_SS_UL: Dropped TCH Connections due to Low Signal Strength on Uplink of Total Number of Dropped TCH Connections.

T_DR_SS_BL: Dropped TCH Connections due to Low Signal Strength on both links of Total Number of Dropped TCH Connections.

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T_DR_SUD: Suddenly lost connections of Total Number of Dropped TCH Connections.

T_DR_BQ_DL: Dropped TCH Connections at Bad Quality on Downlink of Total Number of Dropped TCH Connections.

T_DR_BQ_UL: Dropped TCH Connections at Bad Quality on Uplink of Total Number of Dropped TCH Connections.

T_DR_BQ_BL: Dropped TCH Connections at Bad Quality on both links of Total Number of Dropped TCH Connections.

T_DR_TA: Dropped TCH Connections due to Excessive Timing Advance of Total Number of Dropped TCH Connections.

T_DR_OTH: Dropped Calls due to Other Reasons than Low Signal Strength, Bad Quality or Excessive Timing Advance of Total Number of Dropped TCH Connections.

4.3 Analysis

4.3.1 SDCCH Results

If a high drop rate on SDCCH has been noticed the following actions is recommended in order to proceed and solve the problems.

• Improvements for dropped calls on TCH will improve the drop call rate on SDCCH. I.e. recommend to trouble shoot the TCH drop calls first if there is poor performance on both TCH and SDCCH.

• The drop call rate on SDCCH can be improved if the congestion on TCH is decreased. Recommend to use the feature assignment to worse cell or increase the capacity on TCH.

• The reasons for low SS drops could be too few sites, wrong output power, shadowing, no indoor coverage or network equipment failure.

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• The reasons for dropped calls due to bad quality on the uplink or downlink are related to internal or external interference and trouble shooting is needed to find the interferers. The situation could be temporary improved by means of applicable features. Recommend features that are not activated or recommend alternative parameter setting.

• The reasons for drops on to high timing advance is related to the site location i.e. close to open water, desert or hilly terrain and the setting of MAXTA and TALIM. Setting MAXTA to 63 and TALIM to 62 could solve the problem and/or tilt the antennas, reduce antenna height, change antenna or reduce output power.

• Miscellaneous problems could for example be mobile errors which can occur when old mobiles may cause dropped calls if certain radio network features are used. Another reasons could be that the MS is damaged and not working properly.

4.3.2 TCH Results

If a high drop rate on TCH has been noticed the following actions is recommended in order to proceed and solve the problems.

• High drop rate due to high outage time or low availability. Inform the operation and maintenance department about the problems or check the reasons for the downtime. Check also the alarm list or BTS error log.

Another way is also to check the resolution time for the different alarm categories. Many problems with dropped calls are often related to insufficient O&M routines and not to radio problems.

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• Dropped TCH due to bad quality are often due to interference problems and/or low signal strength in areas where there is no dominant server. The reason could also be missing neighbour relations so that the mobile is not connected to the strongest server and therefore perceives bad quality. The interfered cell should be investigated in order to find the source for the interference. In most cases the interference is internal but can also originate from external sources such as other networks, radio stations, microwave links etc. The interference could also be reduced by means of applicable features such as frequency hopping, assignment to better cell, DTX, MS/BTS power control etc. Recommend applicable features or alternative parameter setting if founded incorrect.

• Dropped TCH due to low signal strength are in most cases related to coverage gaps but can also be a result of missing neighbour relations, hidden antennas, wrong antenna type (to low gain), antenna or feeder problems, incorrect power settings, etc. The reason can also be unforeseen subscriber behaviour i.e. the subscribers use their mobiles indoor, in elevators, parking lots etc. This can also be indicated if there is a high percentage of suddenly lost connections.

• Dropped TCH due to excessive timing advance should in normal cases not occur in the network. The reasons for timing advance drops are site location (close to open water, desert or hilly terrain) and the setting of MAXTA and TALIM. Setting MAXTA to 63 and TALIM to 62 could solve the problem. Or reduce the coverage by down tilting the antennas, reduce antenna height, change antenna or reduce output power. If the site is located close to open water etc. the extended range feature could be considered.

• TCH drops due to other reasons than low SS, bad quality, excessive timing advance could for example be because of BTS HW problems, transmission problems, service affecting maintenance work, uplink interference problems (external or internal), mobile station problems etc.

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4.4 Troubleshooting The trouble shooting of dropped calls is divided into three areas, dropped calls due to bad quality, low SS and other reasons.

4.4.1 Dropped Calls Due To Bad Quality

If the cell suffers from dropped calls due to bad quality the first step is to check that the parameters QLIMDL and QLIMUL are set to correct values. SAUDI 55

Additional data should be collected in order to make correct conclusions.

Check if there are any normal disconnections at bad quality in the cell, this could give indications on that there really is a bad quality problem in the cell.

Run MRR on the cell and check the average RXQUAL value in the cell.

Display C/I and C/A predictions in planning tool. Check if the cell is located in any interference area. Remember that there might be interference in the cell even if it is not displayed in the planning tool. Check idle channel measurements (ICM) for the cell to see if there exists uplink interference in the cell.

Check the handover statistics on a neighbour relation level for the cell. See if there is any neighbour relation with a high number of bad quality handovers. This information might give you the location for the bad quality area in the cell.

Frequency Allocation Support (FAS) can be used in order to find the interfered frequency. This can be useful especially for frequency hopping systems. FAS gives however only the uplink information.

Perform TEMS drive tests in the suspected area. Try to locate the interferer, one way for C/I problems is to halt the affected cell and perform a frequency scanning in order to locate the interfering cell.

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See if it is possible to change frequency or reduce the signal strength of the interferer. For example down tilting the antenna. Check the antenna diagram in order to see the effect of different tilt angels. A second alternative is to change the frequency or increase the signal strength in the interfered cell. Check if there is any missing neighbour relations causing low SS and by that bad quality drops.

4.4.2 Dropped Calls Due To Low Signal Strength

If the cell suffers from TCH drop due to low signal strength the first step is to check the power setting, power balance and neighbour relations in the affected cell. Check for example on a map with site positions or in the cell-planning tool for any obviously missing neighbour relations. Check also the amount of normal disconnections at low SS for the cell.

Check the alarms on the RBS to verify that there is no VSWR alarm causing the reduction in output power.

MRR can be used for checking the timing advance and RXLEV distribution in the cell. This can give an indication if the subscribers are close to the base station or in the outskirts of the cell. If most of the subscribers are on low TA values the low SS drops might be lack of indoor coverage or if most of the subscribers are on high TA values the problem might be a missing site or neighbour relation. By checking RXLEV for the cell indication and compare with coverage plots in planning tool indications of obstacles covering the antenna, feeder problems or other reasons for low SS might be found.

Use the handover statistics on a neighbour relation level to get an indication on where in the cell the problem might be. Check also if any of the target neighbours suffers from severe congestion.

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If there are a lot of suddenly lost connections in the cell this could indicate that there is a tunnel, underground parking lot, high indoor usage etc. Try to find the most likely position in the cell where this kind of drops might occur.

Check in planning tool to see if any coverage gaps or areas with low coverage can be found in the cell. Verify that the different clutter values make sense in the planning tool. Is the site position OK? Is the antenna azimuth correct, is it shooting to the correct location, road, building etc.

Perform drive tests in the cell and check for missing neighbours, swapped antennas etc. Perform the drive test close to the site and try to see if it is line of sight or of the antennas are hidden by any obstacles. Make a site visit and check the antennas if necessary.

If the low signal strength is not related to any faults or missing configuration probably a new site or improved indoor coverage is needed and the problem should be passed on to the cell planning department.

4.4.3 Dropped Calls Due To Other Reasons

If the cell suffers from dropped calls besides the reasons low SS, bad quality and excessive timing advance the dropped calls will be counted as other reasons. That is that the counters for SS, quality and timing advance are not incremented and only the CNDROP, TFNDROP or THNDROP are stepped.

This could for example be the case in cells with uplink interference.

Check if ICM is indicating uplink interference in the cell.

Other possible reasons could be problems with the mobile stations of BTS HW problems, transmission problems, and service affecting maintenance work.

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Check with the operation and maintenance department or check the applicable alarm logs if there have been any HW problems, transmission problems, and service affecting maintenance work during the time period. The average cell downtime and TCH and SDCCH availability should also be checked.

If mobile station problems are suspected in the network this needs to be raised with the Customer and his customer care department to investigate the problem further.

5 SDCCH & TCH Congestion on SDCCH makes it impossible to set up a call unless the feature “immediate assignment on TCH” or “adaptive configuration of logical channels” is used. Congestion on TCH makes it impossible to set up a call unless features such as Assignment to Worse Cell or Cell Load Sharing are used. TCH congestion also means that handover from another cell is impossible to perform.

5.1 Reasons for traffic capacity problems Possible reasons for traffic capacity problems are:

• High number of blocked devices

• HW problems.

• Poor dimensioning of SDCCH and TCH

• Features impacting the traffic behaviour

• Traffic distribution between 900 and 1800 cells

• Traffic distribution between micro- and macro cells

5.2 Used Formulas RA_OTHER: Random Accesses with Cause “All Other Cases”, e. g. Location Updating,

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Detach, Attach, etc. of Total Number of Accepted Random Accesses.

S_TRAF: Average SDCCH Traffic Level.

S_CNGT: SDCCH Time Congestion of Total Measurement Interval.

S_MHT: SDCCH Mean Holding Time.

S_AV_NR: Average Number of Available SDCCHs.

S_AVAIL: Available (not blocked) SDCCHs of Total Number of Defined SDCCHs.

S_DR: Dropped SDCCH Connections of Total Number of SDCCH Connections.

T_AS_SUC: Successful TCH Assignments of Total Number of Assignment Attempts.

T_TRAF: Average TCH Traffic Level.

TF_CNGT_U: TCH/F Time Congestion in under laid Sub cell of Total Measurement Interval.

T_MHT: TCH Mean Holding Time.

T_AV_NR: Average Number of Available TCHs.

T_AVAIL: Available TCHs (not blocked) of Total Nu mber of Defined TCHs.

T_DR_ERLM: Erlang Minutes per Dropped TCH Connection.

5.3 Analysis

5.3.1 SDCCH/TCH availability

From the STS data it can be seen how much of the hardware that is being used. Normally the availability for SDCCH and TCH should be 100%. For the cells showing low availability, check the BTS error log to make sure that there are no problems with the hardware error logs.

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5.3.2 Cell size and location analysis Check where different high traffic cells are located to verify if they are located in the same areas. Look for cells with high traffic loads surrounded by cells with low traffic loads. Check the congestion (together with the number of TRXs) in the small cells. The reason to the low traffic loads could be that the cells have been given too small dimensions. Check if the coverage from the small cells could be improved. A larger coverage area might capture more traffic and off-load the neighbouring cell.

Check if the large cell actually is too big and consequently captures more traffic than it should.

5.3.3 Feature activation

In this chapter it is described how short-term actions can be taken to decrease congestion and improve the capacity of the system.

5.3.3.1 SDCCH Congestion Selecting the number of time slots in a cell that are going to be used for signalling is a critical part of network optimization. Increased use of subscriber services such as Short Message Service can make the demand for SDCCHs more unpredictable.

The feature Immediate Assignment on TCH can be used to lower the load on SDCCH. Note, however, that the SDCCH first strategy is recommended, i.e. an idle SDCCH is always allocated and in case of SDCCH congestion, the signalling is performed on a TCH instead.

If the optional feature "Adaptive Configuration of Logical Channels" is available and activated in the BSC, the system can automatically assign SDCCHs. The way the feature should be used depends on the channel dimensioning strategy and therefore great care should be taken when implementing this feature. No recommended parameter values are given in the User Descriptions.

5.3.3.2 TCH Congestion Make sure that the congestion is not caused by hardware problems or link Make sure that

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the congestion is not caused by hardware problems or link failures. If no problems can be found, check if it is possible to add extra TRXs to the cell or to add (micro) cells in the area.

It may also be possible to activate the feature Cell Load Sharing and/or Assignment to Worst Cell as short-term solutions.

5.4 Troubleshooting

5.4.1 Congestion, general

Check if the congestion can depend on a short-term growth or a long-term growth:

· Short term growth

If the high traffic related to an occasional event, like sport event, fairs, conference, a temporary solution might be considered.

· Long term growth

If there is a long-term growth the network capacity has to grow according to the demand. Check if there is an expansion planned in the near future for the TCH congested cells.

Check if the congestion is on SDCCH, TCH or both.

5.4.2 SDCCH Congestion The time congestion should be used instead of congestion based on access attempts as there is no way to estimate the number of access attempts a single mobile does.

Increasing Traffic Demand

The increase in traffic could be related to an occasional event or due to a long-term growth.

· Check if short term traffic growth. Make trend comparisons.

· Check if combined SDCCH is used.

· Check SDCCH dimensioning.

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Þ Increase the number of SDCCH channels. Note that an increase may lead to the need for new transceivers.

Þ If combined SDCCH is used, non-combined channel configuration should be introduced.

Long Mean Holding Time

If the mean holding time is long, this generates a higher traffic load.

· Check SDCCH Mean Holding Time.

TCH Congestion

TCH congestion may cause the mobiles to stay extra long time on the SDCCH before being allocated TS on a TCH. Check if there exists TCH congestion and if the SDCCH mean holding time is above 7 seconds. For immediate assignment the time is 2-2, 5 seconds.

· Check TCH Congestion.

· Check SDCCH Mean Holding Time.

· Check if Assignment to Worse cell is used and existing parameter setting.

· Check if Cell Load Sharing is used.

Þ Increase TCH capacity.

Þ Use the features for traffic distribution such as Cell Load Sharing and Assignment to Worse Cell.

Low Availability

· Check SDCCH Availability.

· Check if the unavailable channels are manual, control or automatically blocked.

Þ Change & repair faulty equipment.

Þ Review the O&M procedures.

Too Frequent Periodic Registration

· Check Random Access distribution.

· Check the timer T3212 in the BSC and the parameters BTDM and GTDM in the MSC.

Þ Decrease the number of periodic registration.