Performance Troubleshooting -CS - SDCCH Drop(RF) v0.2

BSS Performance Troubleshooting Instructions

SDCCH Drop (RF Reason)

Version 0.2

2 Copyright 2007 Nokia Siemens Networks.All rights reserved.

document description

Title and version Performance troubleshooting instructions - BSSReferenceTarget Group NPO engineers (working in NOA projects)Technology and SW release

2G, 2.5G

Related Service Items

Radio network optimization / Capacity Extension Management

Service Item numberAuthor Karri SunilaDate -Approver Eric Kroon

CHANGE RECORD

This section provides a history of changes made to this document

VERSION DATE EDITED BY SECTION/S COMMENTS

0.1 08.03.2010 Karri Sunila ALL First draft:0.2 10.03.2010 Karri Sunila All Modifications based on comments

Copyright © Nokia Siemens Networks. This material, including documentation and any related computer programs, is protected by copyright controlled by Nokia Siemens Networks. All rights are reserved. Copying, including reproducing, storing, adapting or translating, any or all of this material requires the prior written consent of Nokia Siemens Networks. This material also contains confidential information which may not be disclosed to others without the prior written consent of Nokia Siemens Networks.


Table of Contents

Table of Contents......................................................................................3

1. Purpose and Scope....................................................................4

2. SDCCH Drop (RF Reason).........................................................52.1 SDCCH Drop KPIs..................................................................................................52.2 Referencing KPIs to be checked.............................................................................62.3 Referencing parameters..........................................................................................72.4 Referencing Features..............................................................................................8

3. Troubleshooting..........................................................................83.1 Alarms.....................................................................................................................83.2 Quality / Interference (Rx_Level_Statistics table)...................................................83.3 Coverage.................................................................................................................93.4 Imbalance................................................................................................................93.5 SDCCH Blocking (blck_5a)...................................................................................103.6 Call related DX causes (Clear code observation).................................................103.7 Neighbor plan........................................................................................................113.8 Parameters............................................................................................................12


1. Purpose and ScopePurpose:

This document is an optimization guideline with the purpose to instruct NPO engineers working in NOA projects to deliver traffic and capacity optimization; a working item within NOA - Radio network optimization – GERAN services (SI 7248). 1

This document is meant for INTERNAL USE ONLY

Scope:

The scope of the document is the following:

Show different ways how to detect that capacity management is required by using BSS KPIs (monitoring, mainly BM and Planning KPIs)

Note! All the networks are different and thus is heavily recommended to analyze network properly before optimization. Also properly optimization strategy should be created, will it be better to use special cell level optimized parameter values and loose control of the network or use some general (not cell level optimized) values.

1 Please note that potentially for the capacity optimization a separate service will be created within the NOA portfolio.


2. SDCCH Drop (RF Reason)Dropped call ratios, TCH and SDCCH, are one of the most important indicators to monitor the network quality. In following chapters drop call rate related KPIs are described and some troubleshooting instructions, how to find the reason for dropped calls.

There are many times several reasons for KPI degradation, so all step should be checked to be sure that reasons for dropped calls will be found.

2.1 SDCCH Drop KPIs

SDCCH drop formula used in this document is a benchmarking KPI, sdr_4

RF reason related counters are

o 1003 sdcch_radio fail

o 1004 sdcch_rf:old_ho

Typical reason for SDCCH RF drops are:

o Bad interference

o Bad coverage

o UL problems (imbalance)

o SDCCH Blocking

Note! Clear code analysis is strongly recommended to see which reasons are causing SDCCH drops


2.2 Referencing KPIs to be checkedDropped call is not a reason, it is always a consequence. In this chapter SDCCH drop related KPsI are mentioned.

2.2.1 Quality (ulq_2a,dlq_2a, Rx_Level_Statistics table)

Bad quality is typically measured as BER (bit error rate) or as FER (frame erasure rate). The FER Measurement provides the uplink frame erasure rate (UL FER) from each codec of each TRX in the BTS.. In this document quality (DL and UL) is measured using BER measurements.

Bad quality is effecting heavily on Drop call rate. RX level – quality distribution can be seen below. The distribution (lowest picture) shows that there are quite a lot bad quality samples which will cause problems in the network.

o Voice quality will be decreasedo PS quality (throughput ) will be decreasedo Dominance areas are not working properlyo Drops + HO failures will be increased

q0 q1 q2 q3 q4 q5 q6 q7-100dBm 10645 8516 6813 5450 4360 3488 2791 2232-95dBm 47043 37634 30108 24086 9865 7543 5643 2345-90dBm 56204 44963 35971 28776 16574 5676 845 65-80dBm 200863 160690 128552 102842 17654 3653 145 28-70dBm 12785 10228 8182 6546 456 112 24 23-47dBm 4583 1123 583 452 261 76 26 2

Bad interference problem → signal level good (<-80dBm) and sometimes no better cell available. If better cell available and quality samples are 4 or worse → HO (reason quality or interference, depends on the parameter) Interference is causing drops.

Really bad interference problem → signal level is really good (<-70dBm) and usually no better cell available → no HO → samples can be seen in the table. Interference is causing drops.

Situation is “network is working properly” If there are quality 4 or worse samples → quality HO. Most of the samples are q4 samples. If lots of q5..q7 samples → interference problem and interference must be analyzed / removed. If quality HOs but no q5..q7 samples → better cell is available → no interference problems. In these signal levels overlapping exists and if handover reason is no PBGT, it will be quality HO. By parameter amount of quality HOs can be adjusted




Bad quality samples due to signal level problems. If PBGT overlapping is not existing → lots of quality HOs + level HOs (margin are lower than in PBGT).

Not interference problem, more coverage problem.

DL_q0 DL_q1 DL_q2 DL_q3 DL_q4 DL_q5 DL_q6 DL_q7-100dBm 12057 2827 3108 3952 4783 6200 7013 8156-95dBm 44818 4811 5041 5866 6587 7223 7259 6781-90dBm 98587 7107 7400 8334 8470 8781 7825 6162-80dBm 225919 7450 7731 8445 7726 7441 5695 3369-70dBm 88708 1014 971 998 751 688 689 367-47dBm 15881 84 109 122 104 167 199 184

There are almost as much samples Q5 and Q7 samples as Q 4 samples → even interference is really bad or there is no better cell available ( no ho’s after bad quality samples). These kind of interference cells should be optimized, otherwise there are lots of drops etc


2.2.2 SDCCH Blocking (blck_5a)

The SDCCH is not involved in paging but it is the channel used for Location Area Updates. (Routing Area Updates are performed via a TBF on a PDTCH.) Unlike the PCH and AGCH, the SDCCH is used for various transactions:

o Call set-upo Short Message Service (SMS)o Location Area Updateso Queuing (during call-setup)

First of all, it should be checked what is causing SDCCH blocking.

o SMS, Location updates e (sms_9,RS_205, trf_370, RS_216)

Note! SDCCH Blocking should be checked also in target cells

2.2.3 Coverage (Rx_Level_Statistics table)

Bad coverage means that received signal level is near the (MS/BS) sensitivity level. Here the quality will be decreased due to bad signal level and like bad quality, this bad quality will cause user perceived problems.

Bad coverage is effecting heavily on SDCCH drop call rate. If there are bad coverage dips in the cell, quality will be decreased and call will be dropped. See picture below, bad quality samples are due to bad coverage.

DL_q0 DL_q1 DL_q2 DL_q3 DL_q4 DL_q5 DL_q6 DL_q7-100dBm 7055 1398 1374 1906 2163 2003 1468 832-95dBm 20109 1307 1274 1161 694 332 211 84-90dBm 34531 1053 745 587 273 131 94 41-80dBm 107539 875 518 630 161 98 113 47-70dBm 177614 283 316 663 61 32 29 9-47dBm 58718 78 91 198 54 40 54 32

There are bad quality samples only due to signal level problems.

Note! It should be checked that there is no imbalance problems between UL and DL. Many times UL is the weaker one, so UL coverage should be also checked.

2.3 Referencing parametersParameter assessment should be done to make sure that there are no any parameter discrepancies which could cause drops. Some parameters are effecting on SDCCH drop rate.. More information, see troubleshooting


2.4 Referencing Featureso Dynamic SDCCHo Increased Dynamic SDCCHo AMR HR

3. TroubleshootingDropped call is always a consequence, not a reason. In this chapter some troubleshooting items, which should be checked when analyzing SDCCH Drop KPI, are mentioned.

There are many times several reasons for KPI degradation, so all step should be checked to be sure that reasons for dropped calls will be found.

3.1 AlarmsCheck that there are no any ** or *** active alarms. Connection to NetAct is needed.

3.2 Quality / Interference (Rx_Level_Statistics table)Bad quality is common reason for TCH / SDCCH dropped calls.

o If bad quality in good signal level,(>-75dB)

Frequency changes (new frequency plan) should be done. This is the worst situation, because no better cell available

Interference HO threshold can be adjusted to for example -75dB to make the difference between quality HO and interference HO more clear

o If bad quality in signal level,(-75dB…..-100dB)

Frequency changes can be done to reduce interference Checked that no missing adjacency cells. Missing neighbors can be

one reason for bad quality, serving areas are not working properly.

o If bad quality in bad signal level,(<-100dB) Bad quality due to bad coverage. Improve coverage, see 3.3 Check also imbalance between UL and DL. Which coverage UL or DL

is more important to improve Parameters (Rx Lev Min Cell (SL), Rx Lev Access Min (RXP)) can be

adjusted to avoid call setups / HOs in very bad coverage.


o Broken components, quality results are strange, see picture below

Q4 samples are strange, no typical interference. More quality4 samples than quality3 samples. HW problem, after TRX reset TRX started to work properly.

3.3 CoverageBad coverage, like bad quality is common reason for SDCCH drops

o If bad UL signal level,(<-100dB) samples . Check imbalance between UL and DL. If lots of UL imbalance, UL

coverage improvement can improve network performance.o Add MHA to improve UL coverageo Check that receiver is working properly, no broken components

o If bad DL signal level,(<-100dB) samples . Check imbalance between UL and DL. If lots of DL imbalance, DL

coverage improvement can improve network performance.o Check that transmitter is working properly, no broken

componentso Check that power parameter (PMAX) is correct

o If no imbalance and bad coverageo Add more gain antennaso Use Flexi BTS o Add new sectoro Add new site

Parameters (Rx Lev Min Cell (SL), Rx Lev Access Min (RXP)) can be adjusted to avoid call setups / HOs in very bad coverage.

o Broken components (lots of sample <-100dB), see picture below

Almost all UL samples in bad coverage. DL is working fine => HW problems, broken combiner, TRX etc. No alarms.


3.4 ImbalanceImbalance can be reason for SDCCH drops, particularly if SDCCH drop reason is sdcch_abis_fail_call.

Imbalance should be checked, which one (UL or DL) is limiting one and this (UL or DL) should be improved. See bad coverage

3.5 SDCCH Blocking (blck_5a)First of all, check what is causing SDCCH blocking

o SMS, Location updates e (sms_9,RS_205, trf_370, RS_216)

These transactions are of variable length as they depend on elements other than the BSS. The following activities can be done for better SDCCH functionality (check also RS130)

o Increase of Periodic location update timer / Periodic RA update timer (PRAU) (it can be risky to change these parameters on network level)

o Increase of MS Reachable timer (MSRT)o More SDCCH allocation

Expand configuration, remove combined SDCCH Dynamic SDCCH or increased Dynamic SDCCH feature usage

o Combined RAU (NMO-I with Gs for (E)GPRS) (Resume feature decreases the amount of RAUs)

o LA/RA re-planningo Optimizing overshooting

More detailed information, see reference

3.6 Call related DX causes (Clear code observation)Clear Code Observation contains information on the clear codes that indicate a failure in a call. The failure may lead to call dropping or inform about handover problems.

SDCCH drop in RF reason can be analyzed with following counters:

o 1003 SDDCH_RADIO_FAIL Updated in n call phases 1-8 only, see call phases below.

o 1004 SDDCH_RF_OLD_HO Updated in n call phases 9-11 only

Call Phases:

1. Paging/initial MS on CCCH 2. MM signalling 3. Basic assignment 4. Release 5. FACCH assignment 6. SMS establishment on TCH 7. SMS establishment on SDCCH


8. Ciphering 9. External handover for source side 10. Internal handover intra for source side 11. Internal handover inter for source side 12. External handover for target side 13. Internal handover intra for target side 14. Internal handover inter for target side 15. Conversation

It is good to know which DX causes are triggered in call phase 1-8 or 9-11, so which is more detailed reason (DX cause) for SDCCH drop (RF)

More information, see document “Call Related DX Causes in BSC”

3.7 Neighbor planGood neighbor plan in playing important role. If some overshooting and due to that some neighbor cells are missing, there will be definitely be some dropped calls.

o Check all neighbor cells in planning tool (visual check is recommended)

Check Timing advance distribution. If high distance samples and only nearest cells as a neighbor cells, these will be some missing neighbors

GREEN = source

RED =neighbor cell

BLUE = no neighbor

Note! If cell is located near the sea, neighbor planning is more challenging. Signal in the sea might come far away, so neighbor planning should be based on that information.

o Check that BCCH –BSIC plan is properly done

If lots of sdcch_abis_fail_call, this can be the reason, see figure belowe


o

3.8 ParametersIt is important that parameter values are the same as these are planned. If there are some parameter discrepancies, these should be correct before more detailed optimization.

Parameter assessment should be done to get clear picture what kind of parameters values are in the network. More detailed information how to do parameter assessment can be found from here

https://sharenet-ims.inside.nokiasiemensnetworks.com/Open/380106805

Some parameters, which can improve drop call rate, are mentioned below. It is important that engineer, who really understands how these parameters are effecting on network, can adjust these parameters. With incorrect values DCR can be even higher.

o Periodic LA/RA update timerso MS Reachable timer (MSRT)o If lots of t3101 expired samples

adjust t3101 timer values (SDCCH reservation time)o BCCH + BSIC reuse

If lots of SDCCH abis fail calls

https://sharenet-ims.inside.nokiasiemensnetworks.com/Open/380106805

Documents

Performance Troubleshooting -CS - SDCCH Drop(RF) v0.2