What is Dropped Call?
All cell resources are available but calls are failing, then we have a call drop scenario. This could be caused by software errors, congestion, C7 link failures,HW problems or many other reasons.
If a call is abnormally disconnected, a Clear Request is sent to the MSC .If the Call is disconnected in a normal Fashion then Clear Message with cause code Call Control is sent. It is important to establish what types of calls are failing, and over what percentage of the network it is occurring.
SDCCH CHANNEL :
SDCCH is a dedicated channel which is using for LAC updation, Call setup, SMS in idle mode. It works in UL & DL.
1-SD Blocking:
SD blocking means that you are not getting SD resource for the call origination. When MS connects with NW then RACH and AGCH are provided. After AGCH,SDCCH is provided but if SDCCH is not provided at this time due some problems or due to unavailable of SD by BSC ,it’s called as SD Blocking. There are no of reasons for that.If such a case arises the customer will not be able to originate any call.
Difference Between Blocking and Congestion:
It is very important to know difference between blocking and congestion. Some people think that both are same, but they differ from each other. If all the SD resources are full and not available for SD assign then its come into congestion. If at a particular time call is attempted and it fails then it known as Blocking.
Reasons for SD Blocking:
Some of them are-
-LAC boundary.
-HW Prob.
Solutions for removal of SD Blocking:
- Here are some steps by which we can reduce SD Blocking-
1.Check the No. of SDCCH channel Available, if less then increase SD channel taking care that there is no TCH Blocking.
2.Check LAC boundary, If location update is more then change the LAC of that site and set C2 and HYS.
3.Use of Dynamic SDCCH (It is a BSC parameter and will be applied on whole BTS).
4.Hardware check / shift SD to new time slot
5.Some times BMA and HYS parameters are useful to remove SD Blocking.
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2-SD Drop:
As per diagram shows when we assigned SD for call origination and at that time due to some problem or any mismatch comes by which SD loss occurs, it is called as SD Drop.
It occurs between allocation of SD and before TCH allocation. Sometimes SD drop occurs because queuing is not activated in the system.
If SD drop is high plz look on parameters like- overshooting , shift the SD time slot , may be hardware issue, interference, change the values of RXP, PMAX, may be issue of uplink or downlink issue in that cells for UL put a TMA in that cell and for DL provide tilt ,re orient that antenna.
If SD Drop is high:
If SD drop is high plz look on parameters like:
-Overshooting
-Shift the SD time slot
-Hardware issue
-Interference
-Change the values of RXP PMAX
-It may be uplink or downlink issue in which cells for UL put a TMA in that cell and for DL provide tilt
-Re orient that antenna
Reasons of SD Drop:
Hardware Fault.
-Interference.
-MAIO mismatch.
-Bad Coverage.
-High TR Fail.
-Outage.
-Overshooting.
-Abis Drop.
-High Path Loss.
-Wrong Parameter Planning.
-Due to ICM Band(CDMA)
-High LAPD Utilization
-Heavy blocking and DR feature being used extensively
Solutions for removal of SD Drop:
Interference:
-Check the BCCH Plan (C/I or C/A).
-Co-BSIC & Co BCCH.
-Use latest ND 111 and MapInfo to find out proper frequency to reduce interference.
Arrange Drive Test:
The best way to find the real issues for Interference makes DT.
-Check interference by Interference scanning.
-Check clean BCCH by frequency scanning.
Overshooting:
LAC Planning.
-If a cell is picking call from long distance, Check the sample log according to TA.
-Cell orientation need to be defined according to clutter.
-Mount position
-Effective Tilt.
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.
-Use MapInfo or Google Earth to find location of sites.
High LAPD Utilization:
Check LAPD util report from OSS, and define 32 kbps signaling instead of 16kbps
Hardware Fault:
Check Alarms.
-TRX condition.
-Check Path Imbalance.
-VSWR of the Cell.
-Connector Connection.
-Some times you will find issues on BCCH TRX.In this case BCCH shift from one to other TRX will reduce SD drop.
Due to ICM Band:
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).
-Use the spectrum analyzer.
Check for parameter:
Check the Timer T 3101
-Check the Timer T 200(20ms)
-T11 Expired(10 s)