From technical perspective, dropped calls show the number of abnormal disconnection’s during call setup, SMS, supplementary service activation or during conversation. They are a few different counters for dropped calls, i.e. dropped calls due to low signal strength, bad quality, too high timing advance and miscellaneous is used to get an indication of the reason for possible bad performance.

From a subscriber point of view, dropped calls are those that interrupt an ongoing conversation, i.e. a call dropped on the TCH. If the call is dropped on the SDCCH the user simply re-dials again and hopefully succeeds with the new call setup. For this analysis we will focus on the drop call on the Traffic Channel (TCH).

TCH Drop call can be divided into a few categories:

1. Excessive Timing advance

2. Low Signal Strength – UL/DL/BL

3. BL Bad Quality – UL/DL/BL

4. Sudden Loss

5. Other reason (not standalone counter)

Dropped connection due to failure is counted in counter TFNDROP, which step for the number of abnormally terminated connections. These counters are incremented when the BSC send ‘CLEAR REQUEST’ and when ‘CLEAR COMMAND’ message is received if the code differs from the cause codes ‘CALL CONTROL’ and ‘Handover successful’. If the ‘CLEAR REQUEST’ has been sent previously before ‘CLEAR COMMAND’, the counter will not step.

When a call is abnormally disconnected, that is ‘CLEAR REQUEST’ is sent to the MSC, a check is made in the function Assignment or Handover if any of the following urgency state existed. If more than one type of urgency state are indicated by the locating procedure, the following priority (highest priority first) is used to determine the type of urgency state:

1. Excessive Timing Advance (TA)

2. Low signal strength in downlink and/or uplink

3. Bad quality downlink and/or uplink

4. Sudden loss of connection

When type of urgency is determined, one of counters of this urgency state is stepped.