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Why call drop happens

I had received lots of requests about demonstration of Telecom case studies, why this happens, how this happens etc among which most hot topic was  Why call drop happens or Reasons for call drop or How to overcome call drop . Well, frankly the whole can not be discussed in a single post or in some few paragraphs. But for the sake of discussion we can continue this by stating some overviews, not digging into much details.

Now a days, call drop or technically TCH drop is considered to be a major issue which is prior to be fined by TRAI to all operators. So if you found if any network operator facing lots of TCH drops, then its really bad impact on the network, there must be something wrong technically. So, you should either try to lodge a request to optimize the network condition or simply port to another. Though I must tell the ist one you should always give a try.


(1)Low signal strength on uplink or downlink or both links:

call drops counter due to low signal strengths. During the last measurement report before the call drop when signal strength is below LOWSSDL or LOWSSUL , the drop counters. LOWSSDL & LOWSSUL these BSC parameters which are statistic property parameters & they dont play a role in behavior of calls. If both UL & DL signal strengths below the threshold value, only drop due to low SS BL will be pegged. Bad tunnel coverage cause many dropped calls as well as so called coverage holes. Bad indoor coverage also results in dropped calls. Also building shadowing could be a vital reason. Earlier, I had already posted a demonstration about call drop due to Bad Signal strength.

Now action needed to consider:

  • check coverage plots.
  • check output power
  • check link budget & power balance
  • check antenna configuration, installation.
  • Perform drive test & site surveys.
  • check TRX/TS with high CONERRCNT.


  • Adding repeater to increase coverage could be a good solution for examples of tunnels.
  • Change to a better antenna with higher gain .
  • If there are large coverage holes, then add new base stations.
  • Block/ Deblock TRX.


(2)Poor quality on Down or uplink or both links:

call drops counter due to bad signal quality. During the last measurement report before the call drop when signal strength is below BADQDL or BADQUL , the drop counters. BADQDL & BADQUL these BSC parameters which are statistic property parameters & they dont play a role in behavior of calls. If both UL & DL signal strengths above the threshold value, only drop due to BAD quality BL will be pegged.

Problems due to bad quality are associated with co-channel interference on BCCH or TCH. Faulty MAIO assignments can cause frequency collisions, also there are chances of external interference.

Now action needed to consider:

  • check C/I or C/A plots.
  • check frequency plans.
  • check MAIO,HSN, HOP parameters.
  • check for external interferences.
  • perform drive tests.


  • change BCCH frequency
  • change BSIC.
  • change MAIO, HOP, HSN.
  • record RIR or on-site frequency scanning to identify sources of interferences.


(3)Sudden loss of connection:

Drops due to sudden loss or due to the registration of signal being low in strength , excessively timing advance (TA), bad hardware conditions (faulty or failure) are the main reasons which indicates missing measurement reports from MS. suppose, when a subscriber enter into buildings, elevators, underground garages; there are lots of chances of signal to be faded which leads to interference, resulting the signal quality bad. Also Handover failures, BTS HW faults, A-Bis link faulty ( transmission faults), MS faults these can be counted as reasons of loss to connectivity.

Now what should be the actions:

  • check BTS error logs, alarms or fault codes.
  • check CONERRCNT per TRX and TS.
  • Need to check Transmission link (A-bis).
  • check for DIP slips
  • check LAPD congestion.
  • correlate Handover lost to drops due to sudden loss.


  • Fix HW faults & alarms.
  • Reset TRX with high CONERRCNT
  • To ensure that sync & A-bis links are stable
  • change RBLT with high DIP slips.
  • change CONFACT or increase transmission capacity.
  • investigate HO lost problem.


(4)Excessive TA:

TCH drop counters due to excessive timing advance when during the time of disconnection, the last TA value recorded was higher than the TALIM parameter. this drop reason is very common to isolated areas or islands where coverage is provided widely.

Now action needed to consider:

  • check if cell parameter TALIM is < “63”


  • set TALIM close to a value of 63
  • Make tilt to antenna height / output power (for co-channel cells)


These all are the reasons technically, which is quite meaningful if you’re from Telecommunication Industry. Share what do you think.



4 Responses
  • Brijesh Patel
    December 13, 2016

    Wow… that’s really interesting never knew these all reasons. But now i came to know all because of you So i am thankful to you.

      December 13, 2016

      Thanks Brijesh ! Lots of things from Telecommunication industry to know, even I’m also a fan of learning 🙂

  • Viraj Patel
    April 28, 2017

    What is the difference between Soft Handover and hard Handover?

      April 28, 2017

      Thanks Viraj for your query !
      Soft & hard handover are two different types of handover.
      In simple words, Hard means break before make, Soft handover is make before break. I assumed you have some knowledge already about handover procedure, so hopefully this short info will help you.

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