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10 gsmp&o b-en-gsm network sdcch assignment analysis-word--201009
 

10 gsmp&o b-en-gsm network sdcch assignment analysis-word--201009

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    10 gsmp&o b-en-gsm network sdcch assignment analysis-word--201009 10 gsmp&o b-en-gsm network sdcch assignment analysis-word--201009 Document Transcript

    • GSM Network SDCCH Assignment Analysis
    • i Contents 1 SDCCH Assignment Failure Troubleshooting Guide............................................................................. 1 1.1 Procedure .......................................................................................................................................... 1 1.2 Causes of Several SDCCH Assignment Failure:............................................................................... 2 1.2.1 Inappropriate Tx-Integer Setting Parameter:.......................................................................... 2 1.2.2 Lapd delay leads to high SDCCH assignment failure rate..................................................... 4 1.2.3 Co-BCCH & Co-BSIC interference....................................................................................... 5 1.2.4 Over coverage ........................................................................................................................ 6 1.2.5 Uplink Noise Interference...................................................................................................... 7 1.2.6 MS frequent location update due to poor downlink quality................................................... 7 2 Cases of high SDCCH assignment failure rate........................................................................................ 9 2.1 LAPD delay caused by too many paging.......................................................................................... 9 2.1.1 Too much paging causes LAPD delay.................................................................................... 9 2.1.2 Satellite transmission delay.................................................................................................. 10 2.1.3 Transmission equipment failure causes LAPD delay........................................................... 12 2.2 High SDCCH assignment failure rate caused by other signals with the same BCCH and the same BSIC...................................................................................................................................................... 14 2.2.1 Case 1................................................................................................................................... 14 2.2.2 Case 2................................................................................................................................... 14 2.3 Noise signal access.......................................................................................................................... 16 2.3.1 Noise signal is represented as TA oversteps practical coverage........................................... 16 2.3.2 Noise signal is represented as receiving level is lower than BTS receiving sensitivity....... 18 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 cell....... 19
    • ii 2.4.1 Case 1 ...................................................................................................................................19 2.4.2 Case 2 ...................................................................................................................................19 2.5 SDCCH assignment failure caused by bad network coverage.........................................................20 2.5.1 Case 1 ...................................................................................................................................20 2.5.2 Case 2 ...................................................................................................................................21 2.5.3 Case 3 ...................................................................................................................................22 2.6 SDCCH assignment failure caused by continuous access request of location update.....................23 2.7 SDCCH assignment failure caused by improper-set Tx-Integer......................................................24
    • 1 1 SDCCH Assignment Failure Troubleshooting Guide 1.1 Procedure Upon receiving the complaint of high SDCCH assignment failure, extract performance report from OMC-R Client, filter out trouble cells based on SDCCH assignment failure rate. Adjust TX_INTEGER to 14 for the bad cells. Check whether BCCH Trx of the bad cell share the same Lapd with that of other cell’s BCCH Trx. If it’s the case, make a BCCH shift to other Lapd board to avoid the multiplexing. Check whether there’re cells that use the same BSIC and same BCCH; If it’s the case, modify it. Observe one day. If the problem still exists, take the next step. Collect measurement report of the bad cells and get to know the time span of high SDCCH assignment failure. Analyze basic measurement report. If counter 11687(Number of other access request attempts ) is high while counter 11639(Number of other successful access requests) is zero, it’s probable that TCH handover request of other cell is mistaken for access request of serving cell. If Mplog file contains the error message “Channel Number=Ox88” then it indicates the existence of the same BCCH handover interference, check whether there is the cell with the same BCCH and BSIC. Analyze basic measurement report, check whether counter 11643(Number of sdcch allocation failures including handover) has value, if so then we may suspect for transmission problems . Check whether transmission alarm exists or whether there is lapd error printing by analyzing mplog. Analyze basic measurement report to see whether high SDCCH assignment failure still exists. If it’s the case then check whether TCH assignment failure rate is high too. Generally TCH assignment failure rate is lower than SDCCH assignment failure rate. If
    • GSM Network SDCCH Assignment Analysis 2 TCH assignment failure rate is high, it suggests that the cell is having some interference or poor coverage. So reduce BCCH interference and improve the coverage. If TCH assignment failure rate is good, observe signaling tracing of the cell for 10 minutes, convert it to be MA10 format and filter Channel Request messages. If there are many Immediate Assign failure of Channel Request with high TA, it suggests there is overshooting 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. If SDCCH assignment failure increases suddenly, check whether it is periodic (High SDCCH in fixed time of each day), if it’s the case then the coverage area is blind spot. When SDCCH assignment failure increases suddenly but not periodic, confirm whether it’s accompanied by TCH assignment failure. If it’s the case then the cell may have sudden strong interference. If SDCCH assignment failure is suddenly increased instead of periodic and without accompanied with sudden 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 counter 11645 then most probably failures are caused by disconnection due to Mobile Station uplink poor quality, which is common in GSM network.. 1.2 Causes of Several SDCCH Assignment Failure: 1.2.1 Inappropriate Tx-Integer Setting Parameter: The interval (TS number) between the two channel request messages is a random number in {S,S+1,…,S+T-1}. In which, T is the TS number which represents by TxInteger, S value are as follows TS number TxInteger represents S (CCCH not combines with SDCCH) S (CCCH combines with SDCCH) 3, 8, 14,50 55 41 4, 9, 16 76 52 5,10,20 109 58 6,11,25 163 86
    • Chapter 1 0BSDCCH Assignment Failure Troubleshooting Guide 3 7,12,32 217 115 TxInteger value TS number represents 0 3 1 4 2 5 3 6 4 7 5 8 6 9 7 10 8 11 9 12 10 14 11 16 12 20 13 25 14 32 15 50 From the above two grids, we get the relation between TxInteger with the interval between request message of the two channels TxInteger Interval(ms) (CCCH not combined with SDCCH) Interval(ms) (CCCH combined with SDCCH) 12 501ms~593ms,109~129 slot 267ms~359ms,58~78 slot 13 750ms~865ms,163~188 slot 396ms~511ms,86~111slot 14 998ms~1146ms,217~249 slot 529ms~676ms,115~147 slot 15 253ms~483ms,55~105 slot 189ms~419ms,41~91 slot Generally , Tx-Integer is 14 by default. When transmission link delay is large, TxInteger is small, it may cause many MS access requests. Abis one-way signaling transmission delay is 60ms~100ms. For your notice,delay of one immediate assignment without the UM delay as follows Channel Required uplink- 60ms
    • GSM Network SDCCH Assignment Analysis 4 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 if Tx integer is set to 15 it corresponds that the interval between two-channel request is about 300ms. Ie. MS resends Channel Request before receiving Imm Assign. At the same time when MS sents the second attempt for channel request it receives Channel Request Imm Assign of the first attempt and complete access procedure, during this process the Channel Request of the second attempt fails. 1.2.2 Lapd delay leads to high SDCCH assignment failure rate There are several causes leading to lapd delay, If Lapd1*4 multiplex is adopted, it’s probable that multiple BCCH share the same lapd which will lead to great lapd throughput and finally time delay. Large lapd flow results in delay. For example, inappropriate LAC allocation will lead to great paging number which will result in lapd overflow. Transmission equipment has great delay. For example, ABIS adopts satellite transmission. 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 it’s vicious circle. If lapd delay reaches certain degree which leads to MS resending Channel Request, SDCCH assignments may fail. Please refer to the following diagram:
    • Chapter 1 0BSDCCH Assignment Failure Troubleshooting Guide B T SMS B S C Channel Request Channel Required Channel Active Channel Active Ack mm Assign(OK) Imm Assign Cmd Channel Request(Re-Send) TxInteger Channel Required Channel Active Channel Active Ack Imm Assign Cmd Imm Assign(Fail) S change to SDCCH Lapd Delay 1.2.3 Co-BCCH & Co-BSIC interference There are two situations, which lead to co-BCCH & co-BSIC interference: 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), two cells of same bcch and bsic may cause false access. Therefore we should avoid co-BCCH and co-BSIC. When one cell uses the BCCH same as other cells TCH/BCCH and CO-BSIC, during the time of handover, “ handover access” message happened in the TCH TS will be decoded by both the targeted cell as well as other cell that uses the same BCCH/BSIC and so the cell which is not the original target cell will also decode as Channel Request message. Hence the case will result in great SDCCH assignment failure. When MS initiate Channel Request, RA is random and not consistent. MS access 5
    • GSM Network SDCCH Assignment Analysis 6 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. For checking these type of problem we can analyze the signal trace, when we find RA is the same, TA is same 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. Thus if the two cell share the use the same BCCH and is close, downlink interference may occur which will also lead to SDCCH assignment failure. 1.2.4 Over coverage There are two situation causing over coverage, 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 can’t receive Imm Assign sent by BTS. 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. Repeater influence shall be taken into account when calculating cell coverage range. We do adjust TA_Allowed parameter 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 . If TA_allow area update is unsuccessful, MS will select another cell with less strong power(C1>0)access 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 follows, Cell reselection interval = TX_Integer * MaxRetrans +T3126 Judging 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.
    • Chapter 1 0BSDCCH Assignment Failure Troubleshooting Guide 7 Remark: Other manufacturers have similar parameters like TAallowed. For example, Nortel has RNDACCTIMADVTHRESHOLD and it is described as follows,avoid 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 Interference BTS receiving sensitivity is generally between -112dbm and -125dbm and random access signaling with less sensitivity has noise interference, which will definitely lead to SDCCH assignment failure. RACHMin(-dbm) is 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: 1- receiving level is week, 2- TA is greater than actual coverage range. Hence RACHMin integrated with TA_allowed can reduce the influence of noise interference to a larger extent. Notice: RachMin shall be carefully set. If it’s too high, it may affect paging success rate. 1.2.6 MS frequent location update due to poor downlink quality Because receiving sensitivity of BTS is high than that of MS, BTS can receive Channel Request sent by MS, and MS can’t receive Imm Assign sent by BTS.Especially when MS is put in pockets 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.
    • 9 2 Cases of high SDCCH assignment failure rate 2.1 LAPD delay caused by too many paging 2.1.1 Too much paging causes LAPD delay 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%. Analysis After checking statistic we find out that almost each cell’s SD assignment failure rate is high, and the bad assignment isn’t caused by cell’s radio parameters; We can see that SD channel has no congestion generally from statistic, the congestion rate is only 0.02%; ZTE BSC1/2/4 SD assignment success rates are all above 95%, that’s 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. Adjustment and effect Contact 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 Object Signaling channel congestion rate (%) Total call attempts in signaling channel Total overflow times of signaling channel SDCCH assignment success Number SDCCH assignment failure Number SD assignment success rate
    • GSM Network SDCCH Assignment Analysis 10 March 10th Bsc3 0.04 171155 69 96228 74855 56.25% March 11th Bsc3 0.04 173784 72 97894 75796 56.36% March 13th Bsc3 0.05 158272 87 86770 71373 54.87% March 14th Bsc3 0 105145 5 100085 5045 95.20% March 15th Bsc3 0 101817 0 98763 3045 97.01% 2.1.2 Satellite transmission delay Problem description There are total 4 sites, TBT-G, TBT-D, GWD-G and GJR-G,all 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_1 10921 2008 E-GSM900 GWD1 50.31 2_1_92_2 10922 2008 E-GSM900 GWD2 50.64 2_1_92_3 10923 2008 E-GSM900 GWD3 50.3 2_1_252_1 12521 2008 E-GSM900 TBT1 55.74 2_1_252_2 12522 2008 E-GSM900 TBT2 50.2 2_1_252_3 12523 2008 E-GSM900 TBT3 63.57 2_1_326_1 13261 2008 E-GSM900 GAR1 49.6 2_1_326_2 13262 2008 E-GSM900 GAR2 50.26 2_1_326_3 13263 2008 E-GSM900 GAR3 50.54 2_1_999_1 12524 2008 DCS1800 TBT4 50.24 2_1_999_2 12525 2008 DCS1800 TBT5 49.38 2_1_999_3 12526 2008 DCS1800 TBT6 49.01 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: We can see that the average time required to successfully activate one channel is 0.58s.
    • Chapter 2 1BCases of high SDCCH assignment failure rate 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 FN=T1*26*51+((T3-T2)mod 26)*51+T3 The frame number difference of the two channels is 32454-32227=227(frames) Tracking 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 doesn’t receive Establish Indication message, and T3101 is overtime and then channel is released. 11
    • GSM Network SDCCH Assignment Analysis The two signaling has the same Access delay: 3. Meanwhile the max retransmission times that system configured is 4, TX Integer=14(T=32,S=217). 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. 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 delay 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. 12
    • Chapter 2 1BCases of high SDCCH assignment failure rate 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 13
    • GSM Network SDCCH Assignment Analysis 14 breaks. 2.2 High SDCCH assignment failure rate caused by other signals with the same BCCH and the same BSIC 2.2.1 Case 1 Problem description: In a place the high SDCCH assignment failure rate problem hasn’t been solved for a long time, and SDCCH assignment failure rates of lots of cells in the whole network are higher than 25%. Solution: The problem can’t 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. 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. 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) Reset cell’s 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. The basic solution is to avoid signal with same frequency and same BSIC. 2.2.2 Case 2 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.
    • Chapter 2 1BCases of high SDCCH assignment failure rate 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 15
    • GSM Network SDCCH Assignment Analysis 16 checking the cell’s surrounding frequencies and BSIC. Analysis conclusion: Indian Spice doesn’t 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 cell’s 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 cell’s SDCCH assignment success rate is low. The following table shows the cell’s basic data measured on April 26th busy hour. Time Alias 11644(Number of SDCCH Assignment Successful) 11645(Number of SDCCH Assignment Failure) 2007-4-26 19:15 ASHOKA PILLAR-1 191 15 2007-4-26 19:30 ASHOKA PILLAR-1 190 24 2007-4-26 19:45 ASHOKA PILLAR-1 177 33 2007-4-26 20:00 ASHOKA PILLAR-1 192 26 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: SN TA Reason Time when immediate assignment being sent 1 6 location update 05-49-52.640 2 7 location update 3 7 location update 4 6 MTC 5 5 location update 6 5 MTC 05-56-43.343 7 0 MTC 05-57-24.531 8 4 MTC 06-00-02.109 9 1 MTC 06-00-02.890 10 63 MOC 06-05-56.828 11 61 MOC 06-06-12.656
    • Chapter 2 1BCases of high SDCCH assignment failure rate 17 12 59 MOC 06-06-23.578 13 5 location update 06-06-43.078 14 53 MOC 06-06-57.718 15 49 location update 06-07-22.578 16 46 MOC 06-07-42.718 17 43 MTC 06-08-08.515 18 41 MOC 06-08-14.765 19 40 MTC 06-08-20.578 20 18 Call reset 06-08-22.203 21 38 MOC 06-08-27.546 22 36 location update 06-08-45.062 23 5 MTC 06-08-52.140 24 5 MTC 06-08-52.625 25 34 MOC 06-08-52.796 26 5 MTC 06-08-53.281 27 35 MOC 06-08-55.375 28 36 MTC 06-08-55.562 29 35 MOC 06-08-55.984 30 34 MTC 06-08-56.578 31 32 MOC 06-09-11.640 32 30 MTC 06-09-24.546 33 27 MTC 06-09-38.031 34 27 MTC 06-09-38.578 35 27 MTC 06-09-39.109 36 0 MOC 06-09-57.171 37 24 MOC 06-09-57.828 38 10 MOC 06-11-15.406 39 2 MOC 06-12-12.781 40 0 MOC 06-12-52.671 41 0 MOC 06-12-53.218 42 1 location update 06-15-13.140 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.
    • GSM Network SDCCH Assignment Analysis 2.3.2 Noise signal is represented as receiving level is lower than BTS receiving sensitivity Problem description: a cell’s SDCCH assignment failure rate keeps high but TCH assignment rate is ok. 查询时间:2007-12-23 统计时段:0:00 ~ 24:00 UserLabel Object identifier Cell and Location Area Cell(LAC-CI) SDCCH assign successful number SDCCH assign failure number SDCCH assign failure rate TCH Assignme nt Success Number TCH assign failure number TCH assign failure rate SUNKADA KATTE -PIPLINE-3 Bsc107-Sit e35-Bts3 LAC107-CI1 7353 14479 4490 23.63 4678 122 2.54 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 cell’s signaling tracking data, we find out there are lots of Channel Request messages whose receiving level is -135dbm(0x87)which 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. 18
    • Chapter 2 1BCases of high SDCCH assignment failure rate 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 cell 2.4.1 Case 1 Problem description: the following are a cell’s 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 cell’s SDCCH assignment failure rate soars on busy hour, but TCH assignment 19
    • GSM Network SDCCH Assignment Analysis failure rate is low, network performance parameters are in the following table: UserLabel Object identifier Cell and Location Area Cell(LAC-CI) Pmdatatime SDCCH assign failure rate TCH assign failure rate GOLLARAHTTI-XCEL-2 Bsc107-Site64-Bts2 LAC116-CI17642 19:00-20:00 15.85 0.68 GOLLARAHTTI-XCEL-2 Bsc107-Site64-Bts2 LAC116-CI17642 21:00-22:00 12.78 0.71 GOLLARAHTTI-XCEL-2 Bsc107-Site64-Bts2 LAC116-CI17642 20:00-21:00 11.27 1.36 After tracking the site’s 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 coverage 2.5.1 Case 1 Problem description: a cell has high SDCCH assignment failure rate problem 20
    • Chapter 2 1BCases of high SDCCH assignment failure rate 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 isn’t solved after resetting TRX and the site. Problem analysis: we can see from the cell’s basic measurement report 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 RQ>3 is quite large, uplink quality is very bad. Therefore we can confirm that the cell’s uplink signal has interference or the coverage is unfavorable. 2.5.2 Case 2 Problem description: a cell’s 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. UserLabel Object identifier Handover success rate (%) SDCCH assignmen t success number SDCCH assignment failure number SDCCH assignment failure rate TCH assignment success number TCH assignment failure number TCH assignment failure rate C11658*100/ C11611 GAYATRIPU RAM-MYS-3 Bsc23-Site19-Bt s3 20 1915 3001 58.67 1033 1325 56.19 21
    • GSM Network SDCCH Assignment Analysis Problem analysis: after checking the cell’s 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 cell’s own coverage is bad, but the cell has overlapping problem and co-channel interference. 2.5.3 Case 3 A cell’s SDCCH assignment failure rate reaches as high as 20% on busy hour. UserLabel Object identifier Cell and Location Area Cell(LAC-CI) Pmdatatime SDCCH assign failure rate TCH Assignment Success Rate NANJANGUD-1 Bsc36-Site19-Bts1 LAC1036-CI36191 2007-12-17 20:00-21:00 10.8 81.75 NANJANGUD-1 Bsc36-Site19-Bts1 LAC1036-CI36191 2007-12-17 21:00-22:00 21.52 85.32 NANJANGUD-1 Bsc36-Site19-Bts1 LAC1036-CI36191 2007-12-18 20:00-21:00 18.28 82.17 NANJANGUD-1 Bsc36-Site19-Bts1 LAC1036-CI36191 2007-12-18 21:00-22:00 22.24 84.7 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 cell’s has downlink interference. We can solve it through re-planning frequencies, or using downlink power control of surrounding cells. 22
    • Chapter 2 1BCases of high SDCCH assignment failure rate 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 cell’s 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. 23
    • GSM Network SDCCH Assignment Analysis 2.7 SDCCH assignment failure caused by improper-set Tx-Integer Problem description: a cell’s 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. 24 DATETIME BSC_NAME BSCID CELL_ID SITE_NAME MYHOUR SD_ASSN_FAIL_RATE 13-Dec-07 JAYANAGAR-BSC 102 12282 THAYAGRAJNAGAR-2-s 21 30.21 Problem analysis: after tracking the cell’s 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.
    • Chapter 2 1BCases of high SDCCH assignment failure rate 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 cell’s SDCCH assignment failure rate decreases lower than 10% after changing Tx-Integer into 14. 25