High SD Failure Analysis

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Technical Document File NameHigh SDCCH Assignment Failure Analysis & Troubleshooting GuideUpdate RecordFile No.Version No.Drafter /

ModifierUpdate DateReasonMajor Points Modified

AHongwei Gao & Chun Chen2007.12.19nullnull

remarkThe first time the file is edited, Reason and Major Points Modified shall be vacant.

Catalog31SDCCH Assignment Failure Troubleshooting Guide

31.1Procedure

41.2Causes of Several SDCCH Assignment Failure

41.2.1Inappropriate Tx-Integer Setting Parameter

61.2.2Lapd delay leads to high SDCCH assignment failure rate

61.2.3Co-BCCH & Co-BSIC interference

71.2.4Over coverage

81.2.5Uplink Noise Interference

91.2.6MS frequent location update due to poor downlink quality

92Cases of high SDCCH assignment failure rate

92.1LAPD delay caused by too many paging

92.1.1Too much paging causes LAPD delay

102.1.2Satellite transmission delay

132.1.3Transmission equipment failure causes LAPD delay

142.2High SDCCH assignment failure rate caused by other signals with the same BCCH and the same BSIC

142.2.1Case 1

152.2.2Case 2

162.3Noise signal access

162.3.1Noise signal is represented as TA oversteps practical coverage

182.3.2Noise signal is represented as receiving level is lower than BTS receiving sensitivity

192.4High SDCCH assignment failure rate because frequency of target channel to hand over is the same with that of BCCH of the problem cell, and target cell has same BSIC with that of problem cell

192.4.1Case 1

202.4.2Case 2

212.5SDCCH assignment failure caused by bad network coverage

212.5.1Case 1

222.5.2Case 2

232.5.3Case 3

242.6SDCCH assignment failure caused by continuous access request of location update

252.7SDCCH assignment failure caused by improper-set Tx-Integer

1 SDCCH Assignment Failure Troubleshooting Guide1.1 ProcedureUpon receiving the complaint of high SDCCH assignment failure, create performance analysis report in OMC-R Client, pick out trouble cells based on SDCCH assignment failure rate. 1 Adjust TxInteger of trouble cell to be 14

2 Check whether BCCH Trx of the trouble cell share the same Lapd with that of other cell. If its the case, make a BCCH shift to avoid the multiplexing.3 Check whether therere cells that share the same BSIC and same BCCH; If its the case, modify it;4 Observe one day. If the problem still exists, take the next step;5 Collect measuring data of the trouble cells and get to know the time span of high SDCCH assignment failure;6 Analyze basic data. If counter 11687(other access request attempt times) is high while counter 11639(other access request success times) is zero, its probable that TCH handover request of other cell is mistaken for access request of this cell. If there are Error Report: Channel Number=Ox88 in mplog file which indicates the existence of the same BCCH handover interference, check whether there is the cell with the same BCCH and BSIC; 7 Analyze basic data, check whether counter 11643(SDCCH assignment failure times including handover) has value, if so, transmission problem may exist. Check whether transmission alarm exists or whether there is lapd error printing by analyzing mplog;8 Analyze basic data to see whether high SDCCH assignment failure still exists. If its the case, check whether TCH assignment failure rate is high too. Generally speaking, TCH assignment failure rate is lower than SDCCH assignment failure rate. If TCH assignment failure rate is high, it suggests cell interference or poor coverage must exist, so improve BCCH interference and the coverage. If TCH assignment failure rate is fairly low, make signaling tracing of the cell for 10 minutes, convert it to be MA10 format and filter Channel Request messages. If there is great Imm Assign failure of Channel Request with great TA, it suggests there is overlapping or false signaling access (big difference between TA of channel required and actual coverage range). For overlapping, adjust BCCH transmission power or coverage range of the cell; For false signaling access, adjust TA Allowed to filter false access; 9 If SDCCH assignment failure is busty, check whether it is periodic (High SDCCH in fixed time of each day), if its the case, the coverage area is blind spot. 10 When SDCCH assignment failure is busty, but not periodic, confirm whether its accompanied by TCH assignment failure. If its the case, there is busty strong interference. 11 If SDCCH assignment failure is busty instead of periodic and without accompanied busty high TCH assignment failure, check counter 11686 (LOC access attempt times), counter 11638(LOC access success times) and counter 11645(SDCCH assignment failure times). If the difference between counter 11686 and counter 11638 is close to counterits most probably caused by disconnection due to MP downlink poor quality, which is common in GSM network.1.2 Causes of Several SDCCH Assignment Failure1.2.1 Inappropriate Tx-Integer Setting Parameter

The interval(TS number) between the two BCCH request messages is a random number in {S,S+1,,S+T-1}. In which, T is the TS number which TxInteger represents(not TxInteger), S value are as followsTS number TxInteger representsSCCCH combines with SDCCHSCCCH combines with SDCCH

3, 8, 14,505541

4, 9, 167652

5,10,2010958

6,11,2516386

7,12,32217115

TxInteger valueTS number represents

03

14

25

36

47

58

69

710

811

912

1014

1116

1220

1325

1432

1550

From the above two grids, we get the relation between TxInteger with the interval between request message of the two channels

TxIntegerIntervalms

CCCH combines with SDCCHIntervalms

CCCH combines with SDCCH

12501ms~593ms109~129 slot267ms~359ms58~78 slot

13750ms~865ms163~188 slot396ms~511ms86~111slot



Generally speaking, Tx-Integer is 4 by default. When transmission link delay is large, TxInteger is little, it will result in many MS access requests. Abis one-way signaling transmission delay is 60ms~100ms. Take for instance, regardless of Um delay, delay of one immediate assignment flow is as follows:

Channel Required uplink 60ms

Channel Activation downlink 60ms

Channel Activation Ack uplink 60ms

Immediate Assign downlink 60ms

Time delay between MS sending Channel Request and receiving Imm Assign shall be around 240ms. But if transmission link delay is large and TxInteger is inappropriately set,for example, 15 corresponds to Channel Request overtime 300ms, its probably that MS resends Channel Request due to overtime before receiving Imm Assign, at the same time, MS receives Channel Request Imm Assign of the first time and complete access procedure, thus Channel Request Imm Assign of the second time fails. 1.2.2 Lapd delay leads to high SDCCH assignment failure rateThere are several causes leading to lapd delay

1 If Lapd14 multiplex is adoptedits probable that multiple BCCH share the same lapd, which will lead to great lapd throughput and finally time delay

2 Large lapd flow results in delay. For example, inappropriate LAC allocation will lead to great paging number which will result in lapd flow control at last

3 Transmission equipment has great delay. For example, ABIS adopts satellite transmission. 4 PS effect. PS is sensitive to network delay. If lapd has delay, PS message will be resend which will add to the throughput and finally results in lapd delay, and so on its vicious circle. If lapd delay reaches certain degree which leads to MS resending Channel Request, SDCCH assignment will fail and SDCCH assignment may fail also. Please refer to the following diagram

1.2.3 Co-BCCH & Co-BSIC interferenceThere are two situations which lead to co-BCCH & co-BSIC interference:1 Two cells sharing BCCH & BSIC: Channel Request sent by MS is received by two cells simultaneously and is assigned by SDCCH. MS can only provide access of one SDCCH, thus SDCCH assignment of another cell will fail;Firstly six-bit color codes are added to the original eight information bits. These six-bit color codes are generated by (BSIC and six-bit parity codes) mode 2. Co-BCCH and co-BSIC may make BTS decode it as MS initial access of other site which may result in SDCCH assignment failure. Due to limited information of random access signaling (8bit and BSIC), although BSIC is different, orthogonality of BSIC of two cells may cause false access. Therefore, we should avoid co-BCCH and co-BSIC. 2 Two cells share the same BSICBCCH of one cell share the same frequency with TCH of another cellTCH TS handover access request of another cell is accepted as Channel Request of another cell and SDCCH is assigned, which will result in great SDCCH assignment failure. When MS initiate Channel Request, RA is random and not consistent. MS access request resend interval is a false random number, therefore FN of MS channel requests are not continuous. According to the protocol, if handover access message sent by MS is the same with random access request message in format(AB frame) but MS handover access message content of one time is consistent and FN is continuous. If BCCH of one cell shares the same frequency with TCH of another cell, handover access on TCH can be considered as random access request which will lead to SDCCH assignment failure. Check problem signaling, we find RA is the same, TA is consistent while FN is continuous. Thus we can conclude that great number of Channel Request is false access caused by handover access of co-BCCH cell.Whats more, if the two cell share the same BCCH and is close, downlink interference may occur which will also lead to SDCCH assignment failure. 1.2.4 Over coverageThere are two situation causing over coverage

1 Coverage area is large leading to poor downlink reception quality of cell edge. Because sensitivity of BTS is high than that of MS, BTS can receive Channel Request sent by MS, and MS cant receive Imm Assign sent by BTS.2 Cell coverage is large which results in co-BCCH and co-BSIC with the cell far away. To solve the problem, antenna engineering parameter needs to be adjusted for coverage control purpose. TA_allowed can also be used to solve the problem but it may cause MS to fail to get access to the network. Therefore, threshold of TA_allowed shall be greater than actual coverage range of the cell. Repetitor influence shall be taken into account when calculating cell coverage range.

We do site TA_Allowed adjustment in order to avoid reselection failure due to TA TA_Allowed. We do experiment concerning this problem, and the result is when MS selects a cell with strongest power, and if TA_allow area update is unsuccessful, MS will select another cell with less strong powerC1>0access of this cell is allowed. Interval of cell reselection is decided by TX_Integer and maximum resend time and is generally several seconds. Computing method is as followsCell reselection interval = TX_Integer * MaxRetrans +T3126Judging from the result, we can see that cell that fails to reselect will be punished, as a result, reselection will not fail due to TA_Allowed. Remark Other manufacturers have similar parameters like TAallowed. For example, Nortel has RNDACCTIMADVTHRESHOLD and it is described as followsavoid SDCCH assignment by making the parameter link to actual coverage area range and setting proper threshold to filter false RACH request. It shows that for cells with small radius coverage, RNDACCTIMADVTHRESHOLD is set to be 35Km and RACH misjudgement system demodulate the noise to be RACH burst by mistake is almost 30% of all RACH requests. When rndAccTimAdvThreshold is changed to be 2, RACH misjudgement seldom occurs.1.2.5 Uplink Noise InterferenceBTS receiving sensitivity is generally between -112dbm and -125dbm, and random access signaling with less sensitivity is noise interference which will definitely lead to SDCCH assignment failure. RACHMin-dbmis a parameter set to filter noise signaling. Random access signaling with receiving level lower than RACHMin will be discarded as noise interference. By adjusting RACHMin, SDCCH assignment success rate can be greatly increased.

False noise access signaling reflects in two aspects: one, receiving level is week; the other, TA is greater than actual coverage range. Hence RACHMin integrated with TA_allowed can reduce the influence of noise interference to a larger extent.

noticeRachMin shall be carefully set. If its too high, it may affect paging success rate.1.2.6 MS frequent location update due to poor downlink qualityBecause receiving sensitivity of BTS is high than that of MS, BTS can receive Channel Request sent by MS, and MS cant receive Imm Assign sent by BTS.Especially when MS is put in closets or under the pillows. If MS needs to do location update, Channel Request may be frequent launched which will result in SDCCH assignment failure because Imm Assign message fails to reach.

2 Cases of high SDCCH assignment failure rate2.1 LAPD delay caused by too many paging2.1.1 Too much paging causes LAPD delay1. Problem description

We discover that ZTE BSC3 SD assignment success rate is relatively low in a certain place while analyzing its performance parameters and the rate is especially low on late busy hour, only about 60%.

2. Analysis1 After checking statistic we find out that almost each cells SD assignment failure rate is high, and the bad assignment isnt caused by cells radio parameters;

2 We can see that SD channel has no congestion generally from statistic, the congestion rate is only 0.02%;

3 ZTE BSC1/2/4 SD assignment success rates are all above 95%, thats a normal figure. Only BSC3 is abnormal. Because BSC3 is separately controlled by MSC7, after contacting China Unicom we find out that SD assignment success rates of all BSC controlled by MSC7 (including Siemens BSC) are about 60%, and MSC7 paging success rate is also low. We get to know from China Unicom that there is only one LAC controlled by MSC7. We have to page all cells in LAC while paging, then more traffic, more paging. 3. Adjustment and effectContact Siemens and ask them to add one more LAC to MSC7 and update LAC SN of cells controlled by part of BSC (Siemens). After the adjustment, BSC3 SD assignment success rate comes back to normal and reaches above 95%.Time ObjectSignaling channel congestion rate (%)Total call attempts in signaling channel Total overflow times of signaling channel SDCCH assignment success rateSDCCH assignment failure rateSD assignment success rate

March 10th Bsc30.0417115569962287485556.25%

March 11th Bsc30.0417378472978947579656.36%

March 13th Bsc30.0515827287867707137354.87%

March 14th Bsc3010514551000855045

March 15th Bsc30101817098763304597.01%

2.1.2 Satellite transmission delay1. Problem description

There are total 4 sites, TBT-G, TBT-D, GWD-G and GJR-Gall of which are controlled by BSC01 but by different periphery modules. From the following performance parameter form we can see that these sites SD assignment failure rates are as high as 50% or even higher.

Object CI LAC FreqBand CellName SDCCH ASS Failure Rate

2_1_92_1109212008E-GSM900GWD150.31

2_1_92_2109222008E-GSM900GWD250.64

2_1_92_3109232008E-GSM900GWD350.3

2_1_252_1125212008E-GSM900TBT155.74

2_1_252_2125222008E-GSM900TBT250.2

2_1_252_3125232008E-GSM900TBT363.57

2_1_326_1132612008E-GSM900GAR149.6

2_1_326_2132622008E-GSM900GAR250.26

2_1_326_3132632008E-GSM900GAR350.54

2_1_999_1125242008DCS1800TBT450.24

2_1_999_2125252008DCS1800TBT549.38

2_1_999_3125262008DCS1800TBT649.01

2. Analysis

We record signaling on Abis interface of TBT1, 4, 5, 6, GAR and GWD. Set TBT5 signaling as an example, the analysis is as follows:

1 We can see that the average time required to successfully activate one channel is 0.58s.

2 We figure out from the following signaling whether the two signaling is Channel Required sent by the same mobile phone

We can calculate frame numbers of the two channels by T1, T2 and T3. The formula is FNT1*26*51+((T3-T2)mod 26)*51+T3The frame number difference of the two channels is 3245432227227framesTracking the whole process of the first channel request we can see it is a complete signaling process to turn off the mobile phone. Tracking the whole process of the second channel request we can see immediate assignment failure, BSC doesnt receive Establish Indication message, and T3101 is overtime and then channel is released.

The two signaling has the same Access delay: 3. Meanwhile the max retransmission times that system configured is 4, TX Integer14T32S217. Then the interval for any mobile phone to send two Channel Require messages in one call is a random one from 217 to 248 timeslot, which means the shortest interval between two requests sent by mobile phone is 1001ms, and the longest interval is 1144ms. The interval between these two Channel Require messages received by BSC is 1.906-0.875=1.031s. We suppose that uplink and downlink signaling transmission delays between BTS and BSC are the same, then signaling length of the whole immediate assignment process is 0.58*2=1.16s, which is close to 1.031s. According to upper calculation of frame number, the real interval between these two messages is 227 frames (1048), and then we deduce that the two Channel Require messages are sent by one mobile phone in one call attempt.

3. Conclusion

We have to use satellite to transmit because these sites are far away from the urban area. The satellite transmission delay on one direction is around 260ms, and then transmission delay of 4 signaling is 1040ms, which accords with the upper signaling analysis.

2.1.3 Transmission equipment failure causes LAPD delay1. Problem description: massive SDCCH assignment failures occur in 3 cells of a site in India accompanied with lots of TCH assignment failures. Basic measure data are shown in the following figure.

2. Problem analysis: commonly SDCCH assignment failure represents transmission failure. After checking mpLog print we find out that the site has lots of LAPD Errors get printed: Site 36, BTS 3, TRX 2 catch errorF: receive SABME in Mulframe or TimerRec, Site 36, BTS 1, TRX 0 catch errorF: receive SABME in Mulframe or TimerRec. Then we check alarm, there are lots of transmission alarm:

Therefore we can confirm that there is transmission problem that causes lots of LAPD breaks. 2.2 High SDCCH assignment failure rate caused by other signals with the same BCCH and the same BSIC 2.2.1 Case 11. Problem description: In a place the high SDCCH assignment failure rate problem hasnt been solved for a long time, and SDCCH assignment failure rates of lots of cells in the whole network are higher than 25%.

2. Solution: The problem cant be solved by replacing all the hardware. When TA=20, a signal from another cell with the same frequency and the same BSIC is received that causes SDCCH assignment failure. Therefore we re-plan over 10 cells BSIC in the whole network. After re-planning, parameters of all cells whose BSIC has been modified come back to normal. 3. Conclusion of fault analysis: Within 5km, if a mobile phone locates in an area covered by two cells with the same BCCH and the same BSIC, then SDCCH assignment failure may occur. The failure is triggered by these two cells with the same BCCH and the same BSIC, two cells SDCCH timeslots are synchronous. After mobile phone synchronizes with BTS and chooses to be accessed into one of these two cells, it will be interference to another cell.

4. So we have two solutions for high SDCCH assignment failure problem (within a certain multiplexing distance, high SDCCH assignment failure rate caused by other signals with the same BCCH and the same BSIC) 1) Reset cells CMM whose assignment failure rate is high, reset clock to avoid SDCCH timeslot synchronization, so as to decrease the impact. This is just a temporary solution. In field situation we shall modify parameters and then change them back, it is the reason to reset CMM. 2) The basic solution is to avoid signal with same frequency and same BSIC.

2.2.2 Case 21. Problem description: SDCCH assignment failure rate keeps high in a cell in Indian spice network, but its TCH assignment rate is normal. Basic measure data are shown in the following figure.

2. Problem Analysis: after tracking signaling of troubled cell, we find out that SDCCH assignment failure is caused by lots of random signal access whose TA>37.

We figure out that there is a cell 20Km away using same frequency and BSIC after checking the cells surrounding frequencies and BSIC.

Analysis conclusion: Indian Spice doesnt have much frequency resource and its sites distribute densely, therefore it is inevitable that cells use same frequency and BSIC within 34km. We have to change antenna down tilt angle or modify TA_Allowed to shrink cells coverage, so as to solve SDCCH assignment failure caused by cells use same frequency and BSIC.

2.3 Noise signal access 2.3.1 Noise signal is represented as TA oversteps practical coverage 1. Problem description: a cells SDCCH assignment success rate is low. The following table shows the cells basic data measured on April 26th busy hour. TimeAlias11644(Number of SDCCH Assignment Successful)11645(Number of SDCCH Assignment Failure)

2007-4-26 19:15ASHOKA PILLAR-119115

2007-4-26 19:30ASHOKA PILLAR-119024

2007-4-26 19:45ASHOKA PILLAR-117733

2007-4-26 20:00ASHOKA PILLAR-119226

2. Problem analysis: after analyzing signaling on Abis interface, the summary of immediate assignment failure signal accesses into TA (in channel required) is as follows: SNTAReasonTime when immediate assignment being sent

16location update05-49-52.640

27location update

37location update

46MTC

55location update

65MTC05-56-43.343

70MTC05-57-24.531

84MTC06-00-02.109

91MTC06-00-02.890

1063MOC06-05-56.828

1161MOC06-06-12.656

1259MOC06-06-23.578


1453MOC06-06-57.718


1646MOC06-07-42.718

1743MTC06-08-08.515

1841MOC06-08-14.765

1940MTC06-08-20.578

2018Call reset 06-08-22.203

2138MOC06-08-27.546


235MTC06-08-52.140

245MTC06-08-52.625

2534MOC06-08-52.796

265MTC06-08-53.281

2735MOC06-08-55.375

2836MTC06-08-55.562

2935MOC06-08-55.984

3034MTC06-08-56.578

3132MOC06-09-11.640

3230MTC06-09-24.546

3327MTC06-09-38.031

3427MTC06-09-38.578

3527MTC06-09-39.109

360MOC06-09-57.171

3724MOC06-09-57.828

3810MOC06-11-15.406

392MOC06-12-12.781

400MOC06-12-52.671

410MOC06-12-53.218


The surrounding sites of the site distribute densely with distances lesser than 1km. TA values are unreal on the table, we suppose there are fake signals. Further more, assignment failure in half an hour almost focuses on 5 minutes. We shall use TA_Allowed parameter to filter fake signals. 2.3.2 Noise signal is represented as receiving level is lower than BTS receiving sensitivity 1. Problem description: a cells SDCCH assignment failure rate keeps high but TCH assignment rate is ok. 2007-12-23 0:00 ~ 24:00

UserLabelObject identifierCell and Location Area Cell(LAC-CI)SDCCH assign successful numberSDCCH assign failure numberSDCCH assign failure rateTCH Assignment Success NumberTCH assign failure numberTCH assign failure rate

SUNKADAKATTE-PIPLINE-3Bsc107-Site35-Bts3LAC107-CI1735314479449023.6346781222.54

2. Problem analysis: the cell uses EDGE TRX; in this version, random accessed receiving level can be reported in physical context by Channel Request. Through observing the cells signaling tracking data, we find out there are lots of Channel Request messages whose receiving level is -135dbm0x87which cause large numbers of SDCCH assignment failures.

We can confirm that most of these Channel Requests are noise signal; we can solve it through setting RACHMin.

2.4 High SDCCH assignment failure rate because frequency of target channel to hand over is the same with that of BCCH of the problem cell, and target cell has same BSIC with that of problem cell2.4.1 Case 1

Problem description: the following are a cells signaling. We can see that there are continuous Channel Requests with same RA, TA, and their frame numbers are consecutive. All of the SDCCH assignments corresponding to these Channel Requests are failed with no exception. Besides, other access request attempt times in basic measurement are extremely high. Then we can confirm that all these abundant and consecutive Channel Requests are fake access caused by incoming handover of co-channel cells.

2.4.2 Case 2

A cells SDCCH assignment failure rate soars on busy hour, but TCH assignment failure rate is low, network performance parameters are in the following table:

UserLabelObject identifierCell and Location Area Cell(LAC-CI)PmdatatimeSDCCH assign failure rateTCH assign failure rate

GOLLARAHTTI-XCEL-2Bsc107-Site64-Bts2LAC116-CI1764219:00-20:0015.850.68



After tracking the sites signaling we find out there are lots of consecutive abnormal random access whose Channel Request RA and TA are the same and frame number are consecutive.

We find out there is a cell uses same frequency and same BSIC 14km away from the site after checking frequency planning. The problem is solved by re-planning the frequency.

2.5 SDCCH assignment failure caused by bad network coverage2.5.1 Case 1

Problem description: a cell has high SDCCH assignment failure rate problem accompanied with high TCH assignment failure rate. Outgoing handover attempts are frequent and call drop rate is high. Customer has complaints about it. The problem isnt solved after resetting TRX and the site.Problem analysis: we can see from the cells basic measure that access reasons are various for SDCCH assignment failure, both MOC access and MTC access account for a certain proportion. The number of uplink sampling whose RQ3 is quite large, uplink quality is very bad. Therefore we can confirm that the cells uplink signal has interference or the coverage is unfavorable.

2.5.2 Case 2

Problem description: a cells SDCCH assignment failure rate reaches as high as 58% on busy hour, accompanied with 56% TCH assignment failure rate, handover success rate is only 20%. After tracking signaling we find out there are lots of consecutive Channel Requests whose TA are 63 get accessed abnormally. UserLabelObject identifierHandover success rate (%)SDCCH assignment success numberSDCCH assignment failure numberSDCCH assignment failure rateTCH assignment success numberTCH assignment failure numberTCH assignment failure rate C11658*100/C11611

GAYATRIPURAM-MYS-3Bsc23-Site19-Bts3201915300158.671033132556.19

Problem analysis: after checking the cells performance data of these days, we find out that its TCH assignment failure rate, call drop rate and handover failure rate keep high but without any alarm. DT engineer discovers that not only the cells own coverage is bad, but the cell has overlapping problem and co-channel interference.

2.5.3 Case 3

A cells SDCCH assignment failure rate reaches as high as 20% on busy hour.UserLabelObject identifierCell and Location Area Cell(LAC-CI)PmdatatimeSDCCH assign failure rateTCH Assignment Success Rate

NANJANGUD-1Bsc36-Site19-Bts1LAC1036-CI361912007-12-17 20:00-21:0010.881.75




TCH assignment failure rate is as high as 20% while SDCCH assignment failure rate rises. These two parameters are ok when traffic is low, which means the cells has downlink interference. We can solve it through re-planning frequencies, or using downlink power control of surrounding cells.

2.6 SDCCH assignment failure caused by continuous access request of location update Problem description: in some boundary sites and suburban sites of city A, their SDCCH assignment failure rates soar without any rule, but meanwhile cells other parameters are normal.

The following are signaling and basic measure data recorded when SDCCH assignment failure rate is high. We can see from signaling that the reason for one MS keeps sending access attempts is Channel Request due to location update, but all attempts are failed.

2.7 SDCCH assignment failure caused by improper-set Tx-Integer Problem description: a cells ordinary SDCCH assignment failure rate keeps around 20%, hits 30% on busy hour. But other parameters (such as TCH assignment failure rate, handover success rate) are all normal.

DATETIMEBSC_NAMEBSCIDCELL_IDSITE_NAMEMYHOURSD_ASSN_FAIL_RATE

13-Dec-07JAYANAGAR-BSC10212282THAYAGRAJNAGAR-2-s2130.21

Problem analysis: after tracking the cells signaling, we find out there are couples of Channel Request messages of the cell commonly appearing together (with same TA and for same reason). The Imm Assign corresponding to the first Channel Request is successful, but the Imm Assign corresponding to the second Channel Request is failed.

Set colored messages in the upper figure as an example; the FN of the first Channel Request message is 964, the FN of the second Channel Request message is 1086, the FN difference is 124. While Tx-Integer=12. Then we can confirm these two Channel Request messages are sent by one MS. Because there is a certain delay on transmission link, then MS resends Channel Request.

The cells SDCCH assignment failure rate decreases lower than 10% after changing Tx-Integer into 14.

Channel Request

Channel Required

MS

Channel Active

Channel Active Ack

Imm Assign(OK)

Imm Assign Cmd

BTS

Channel Request(Re-Send

TxInteger

Lapd

Delay

Channel Required

Channel Active

Channel Active Ack

Imm Assign Cmd

Imm Assign(Fail)

MS change to SDCCH

BSC

1This document contains proprietary information of ZTE Corporation and is not to be disclosed or used except in accordance with applicable agreements.

High SD Failure Analysis

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