KPI Analysis_SD Drop Rate
6. SDCCH Drop Rate 6.1 Define 6.2 Measure 6.2.1 SDCCH Drop Rate The correct formula for measuring SDCCH Drop Rate is SDCCH Drop Rate = [(CNDROP-CNRELCONG)/ (CMSESTAB)]*100 % Equation 5 Some of the documentation gives the formula for SDCCH Drop Rate as (CNDROP/CMSESTAB)*100, problem with this equation is, it doesn’t take out dropped SDCCH due TCH or Transcoder Congestion. Alternately you can use both these formulae to measure the percentage contribution of TCH or Transcoder Congestion as the primary cause of high reported SDCCH drop rate.
CNDROP: total number of dropped SDCCH channels in a cell (for the measurement period). CNRELCONG: total number dropped (released) connections on SDCCH due to TCH or Transcoder congestion. CMSESTAB: total number of successful MS channel establishment on SDCCH. 6.2.2 SDCCH Drop Reason There are 6 different stats available in Ericsson system which gives the individual percentage contribution of each possible factor that can contribute towards the net SDCCH Drop: Drop Reason, Low Signal Strength Uplink (%). Drop Reason, Low Signal Strength Downlink (%). Drop Reason, Bad Quality Uplink (%). Drop Reason, Bad Quality Downlink (%). Drop Reason, Excess TA (%). Drop Reason, Other (%). BSC 1 SDCCH Drop Cause19.073.380.377.26Drop Reason, Low SS (%)Drop Reason, Bad Quality (%)Drop Reason, Excessive TA (%)Drop Reason, Other (%) Figure 8 : SDCCH Drop Cause at BSC level
For the above given BSC, close to 77.26% of the drops happening on SDCCH is due to the category Other Reasons. Once we get the major contributing factor towards the SDCCH drop either at the BSC level or at the cell level, it becomes easy to use this information and find the root cause for excess SDCCH drop. Ericsson Confidential GSM NETWORKS KPI ROOT CAUSE 28 (50) ANALYSIS Prepared (also subject responsible if other) No. ESA/SK Subhash Panikar ESA/SK 06:0027 Approved Checked Date Rev Reference Amos Phahla 2006-10-17 PA1 REP00271_A
33 Power control is covered in detail under the section of TCH Drop Rate. 34 Either by frequency scan using a drive test tool or (preferably) by using
the record test neighbour frequency signal strength feature (both these
methods are covered in the TCH Drop Rate section of this document). 35 Use the cell parameter BSCTXPWR Other Reasons
6.3 Analyse 6.3.1 Fish bone diagram for the root cause analysis for high SDCCH drop rate SDCCH Drop RateHigh TASignal StrengthTCH/transcoder CongestionInterference
Figure 9: Fish bone diagram for the root cause analysis of high SDCCH drop rate
Signal Strength: Excess percentage of SDCCH Drop due to, low signal strength on the uplink and downlink: cause could be poor RF coverage; here use options like MS and BS power control revision (possibly power control33 settings are set too aggressive), antenna down tilts and antenna height reductions (foot print reduction), MS and BS transmit power (set low causing a link imbalance), use of TMA or planning a new site to take care of the weak coverage area. Interference: Excess percentage of Interference of SDCCH Drop due to, bad quality on the uplink and downlink: co channel and adjacent channel interference is the root cause here, isolate the interferer 34 and change frequency ; alternately do a coverage optimization of either the serving cell or the interferer. TCH / Transcoder Congestion: in one of the stats [(CNDROP/CMSESTAB)*100] used by Ericsson to measure SDCCH Drop Rate, SDCCH released due to TCH or Transcoder resource congestion is also treated as a dropped SDCCH connection (technically this is not a dropped SDCCH connection, as SDCCH was successfully assigned and was also successfully used to reach to TCH and the connection was “released as a normal network assisted release due to lack of availability of further resources” and not due to issues on SDCCH). Use TCH congestion optimization methods to bring down high SDCCH drop due to TCH congestion. High TA: use down tilts, height reduction or BS power reduction35 to take off high TA coverage of the cell. Power Control Settings: Lack of good power control settings for SDCCH can lead to excess drop. All the cell parameters that control MS and BS Power Control (covered under the section of TCH Drop Rate) impacts equally the SDCCH power control too. Cell parameters unique to SDCCH power control are SDCCHREG which enables (SDCCHREG=1) or disables (0) power control on SDCCH. By default this parameter is disabled but it is recommended to enable power control on SDCCH. The second parameter from the set of BSS power control parameter that has an impact on SDCCH alone (and not TCH) is INIDES which defines the desired initial signal strength for the SDCCH on DL and UL , default value for the same is -70 dBm (with a range of -100 dBm to 47 dBm). Optimising INDES , especially on the uplink will have an direct impact on SDCCH drop rate , this is because UL is the weakest of both the links and is extremely sensitive to interference ; now if there exists a high percentage of traffic close to the cell (which need not transmit at very high power to communicate with the BS) we can bring down the figure for INDES from -70 to say -85 dBm , this will reduce the cumulative power emitted by mobiles closer to the base station and thus reducing the “high noise floor” created for mobiles trying to approach the same cell from far off zones of the cell coverage area. This will Ericsson Confidential GSM NETWORKS KPI ROOT CAUSE 29 (50) ANALYSIS Prepared (also subject responsible if other) No. ESA/SK Subhash Panikar ESA/SK 06:0027 Approved Checked Date Rev Reference Amos Phahla 2006-10-17 PA1 REP00271_A
reduce the number of interference related drops on SDCCH from far off zones. UPDWNRATIO, this cell parameter controls the rate at which the MS and BS gets powered up and down, default setting for this parameter is 200 (which means power up step size in dB for UL/DL is twice that of power down step size, say power down step size per power control interval is 2 dB on the UL, then power up step will be 4 dB). Its always a good strategy to have an” aggressive settings for power control from a signal level point of view and make both UL and DL very sensitive to quality issue” , this strategy will make sure that the MS and BS transmits optimum power at all the times but will power up immediately on quality issues caused by interference. This approach is usually seen to bring down both SDCCH and TCH drop rate in a BSC. More on effective power control strategy is covered in the later part of this document. • Adaptive Configuration of Logical Channel: In some of the earlier software release versions, excess SDCCH drop due to “other reason” was at times (especially during peak busy traffic hours with high CP load at the BSC) seen to be due to the feature “Adaptive Configuration of Logical Channel”. This issue seems to be minimised on the later generation BSS hardware / Software release versions. If all other causes for excess “SDCCH drop due to other reason” don’t appear to be the root cause, try disabling this feature from a test cell to measure the impact on SDCCH drop due to other reason. • Dip Status: Digital Path (DIP) or the E1 connection to the site as well as the E1 connection to the BSC is often cause of SDCH drop due to “other reason”. High BER (Bit Error Rate) or high Frame Loss due to unstable transmission (check for high T200 expiry from layer 3 messages of CTR measurements or drive tests) is often seen to be the cause for high SDCCH drop due to other reasons. • Other Reasons: Possible causes for high SDCCH drops due to other reasons could be due to incorrect power control settings, Adaptive Configuration of Logical Channels, DIP status, Hardware faults at the BTS, frequency interference problems (causing sudden drops) or C7 link between the BSC and the MSC having link congestions or link stability issues.