Troubleshooting for GSM KPIs (SD Block & SD Drop)

KPIs to be monitored:

·            SD Blocking

·            SD Drop

SDCCH Channel:

·   SDCCH channel is a dedicated channel which is using for LAC updation, Call Setup, SMS in Ideal mode. It works in UL & DL

SD Blocking:

·   SD Blocking means that you are not getting SD resource for the call origination. When MS connects with Network then RACH and AGCH are provided. After AGCH, SDCCH is provided but if SDCCH is not provided at this time due to some problem or due to unavailable of SD by BSC.

KPI Formula in Ericsson:

·            SDCCH CONGESTION = (CCONGS / CCALLS) * 100

·            CCONGS - Congestion counter.

·            CCALLS - Channel allocation attempt counter (on SDCCH).

Reason for SD Blocking:

·            LAC Boundary

·            High Volume of SMS

·            SD utilization is high

·            Time Slot faulty

·            Adaptive configuration of logical channel switch off

·            Wrong SD Dimension

·            Incorrect CHAP Settings

·            Hardware Issue

Solution for Removal of SD Blocking:

·   Check the no of SD channels available, if less, then increase SD channel while TCH Blocking should be taken care.

·   Check LAC boundary, if location update is more, then change the LAC of that site and set C2 and HYS.

·   Use of dynamic SDCCH (it is a BSC parameter)

·   Shift SD to new time slot

·   Adaptive configuration of logical channel switch ON

·   Check for T3212 value

Need to check which parameter:

1. CHAP (Channel Allocation Profile): Its Immediate Assignment Process on TCH, It    provides different channel allocation strategies,

        CHAP 0: Immediate assignment on TCH is not permitted,

        CHAP 1: Immediate assignment on TCH is last preference, where in TCH is allocated at immediate assignment only when there is no Idle SDCCH is available

CHAP2: Immediate assignment on TCH is first preference where in SDCCH may only be allocated when there are no idle TCH is available.

2. Adaptive configuration of logical channel (ACLP): The purpose of this features dynamic reconfiguration of Idle TCH Channel to SDCCH Channel, when there is SDCCH High load

S LEVEL Defines: Reconfigure of an Idle TCH to an SDCCH will take place; Default 0 Congested rate for a cell is increase S LEVEL 2

S TIME Define Minimum Time Interval between SDCCH, Can be reconfigured back to TCH, Default value 20s, Range 15s to 3600s

3. T3212: Periodic update timer value:  High volume of LAC Border can cause SD Congestion so optimize the periodic registration timer.  Irrespective of the location, coverage, activity, the mobile has to update its location to the MSC after a defined time/period.

4. CRH (Cell Reselection Hysteresis) :Receiving Signal strength hysteresis for required cell reselection over location area border, In order to overcome the Ping-Pong effects in cell reselection across location area borders, CRO and PT can also used to delay reselection in LAC Borders.(Location area code is an identity number given to the site of a base station)

SD DROP:

                When SD is assigned for a mobile during call connection process and during this time due to any problem or any mismatch occurs by which SD loss occurs, It is between allocation of SD and before TCH allocation.

KPI Formula in Ericsson:

·   SDCCH Drop Rate = (CNDROP-CNRELCONG/CMSESTAB)*100

·   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 Tran-coder congestion.

·   CMSESTAB - Total number of successful MS channel establishment on SDCCH.

Reason for SD Drop:

·            Overshooting

·            Shift the SD time slot

·            Interference

·            It may be uplink or downlink issue in which cells foe UL put TMA in that cell and DL provide tilt

·            HW Issue

·            Wrong parameter planning

·            Bad coverage

·            MAIO mismatch

·            High Pathless

·            High LAPD utilization

·            Wrong Power control settings

·            Check the Timer T 3101

·            Check the Timer T 200(20ms)

Solution for Removal of SD Drop:

Interference:

·            Check the BCCH Plan(C/I or C/A)

·            Co-BSIC & Co- BCCH

·            To find out proper frequency to reduce interference

Overshooting:

·            LAC Planning

·            If a cell is picking call from long distance, check the sample log according to TA

·            Cell orientation need to defined according to clutter

Bad Coverage:

·            If the drop call is due to low signal strength uplink, check the receive path of this particular TRX. Check receiver sensitivity, VSWR, feeder connection and etc. Drops due to Low Signal Strength.

·            If the drop call reason is due to low signal strength downlink, then, check the transmit path. Check cards, feeder and etc.

Hardware Fault:

·            Check Alarms.

·            TRX condition.

·            Check Path Imbalance.

·            VSWR of the Cell.

·            Connector Connection.

·            Sometimes you will find issues on BCCH TRX. In this case BCCH shift from one to other TRX will reduce SD drop

Need to check Which Parameter:

Drop Reason mainly Low signal strength (UL & DL), Bad Quality (UL & DL) and Excess Timing Advance

And High Interference (Co-BCCH & Co-BSIC), Wrong Power control Settings and Too High of CRH Can result in SDCCH drops

Power control settings: Lack of good power control settings for SDCCH can lead to excess drop. Two types of power control 1.MS Power control 2.BTS Power Control

SDCCHREG: Function of this parameter Enable (1)/Disable (0), Enable the power control to minimize the drops

INIDES (Initial desires signal strength): For the SDCCH UL and DL, Default value -70dbm, especially UL impact of drop rate, because extremely sensitive to interference, for INDIES from -70 dbm to -85dbm this will reduce the cumulative power emitted by mobile closer to base station (which need not transmit at very high power to communicate with the BS) and this will reduce the interference

Due to ICM Band (CDMA):

·            Some time SD drops takes place due to near sites of CDMA.

·            Check the ICM band value of that site.

·            Use BPF (Band pass filter).