On circuit switch service, when a call is abnormally disconnected, a Clear Message with cause code Call Control be treated as normal Disconnection is sent to the MSC – named Clear Request Message.
Refer to Ericsson system, the following Urgency condition is checked at that time and the relevant counter is incremented as a consequence:
- Excessive TA
- Low Signal Strength
- Bad Quality
- Sudden Drop
As named, stright forward meaning for the dropped call is described for the first three items. However, Sudden Drop is quite not easy to understand.
Sudden Loss are drops that have not been registered as bad quality, signal strength, timing advance. The term Sudden Loss is used because if the network cannot establish a connection with the lost MS after a pre-defined period, the sudden loss counter is incremented if the last reported measurement from the MS does not fulfill any of the reasons mentioned.
A connection is marked as Sudden loss if none of the three types of urgency states (that is excessive TA, low signal strength or bad quality) are indicated and the locating procedure indicates missing measurement results from the MS.
Drops due to ‘Other’ reasons are generally associated with hardware problems and disturbances, number of drops due to ‘Other’ reasons is obtained by subtracting the drops with known reasons from the total number of drops.
Main contributors in sudden and other TCH drop:
- Very sudden and severe drop in signal strength, such as when subscribers enter into buildings, elevators, parking garages, etc.
- Very sudden and severe occurrence of interference or bad quality.
MS running out of battery during conversation. - Handover Lost.
- BTS HW faults.
- Synchronization or Abis link fault (transmission faults).
- MS Faults.