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The Call Drop Rate is assumed in many contracts as one of the important criteria to verify a network during its acceptance phase.
The goal of call drop analysis is to identify cells suspected to have excessive call drops problems (cells that overcome a define call drops rate threshold during their busy hour time)
A second target is to identify potential problems analyzing the drop typologies mostly occurring in the cells of the network. This identification is useful to better define the problem that better fit for the further investigations.
Call is dropped because the MS overcame the threshold of the max distance allowed in that cell set in the database.
Drops due to DMR
This kind of drops is due to unexpected sequences on L2 LAPD messages. In high interference cases a L2 message of interfering mobile is interpreted as
message of the victim MS and the call of the victim MS is dropped. L2 messages of different MS are mixed up (high interferences in the network with limited
number of RF for BCCH and no ciphering is active).
Drops due to SER
This kind of drops is due to unexpected sequences on L2 LAPD messages. In high interference cases a L2 message of interfering mobile is interpreted as
message of the victim MS and the call of the victim MS is dropped. L2 messages of different MS are mixed up (high interferences in the network with limited
number of RF for BCCH and no ciphering is active).
timer MS radio Freq Power Capability Indication – hardcoded at 20s at call set-up and HO; expires if no HO CMP or ASS CMP (first L3 message from MS) is
received – should be an event that never appear – depends on the setting of T8 and T10; If timer T_MSRFPCI is bigger than T8 and T10 then drops due to
T_MSRFPCI timer expiration could indicate possible troubles on the signaling between BSC and target BTS or possible BSC congestion or A interface problem.
Drops due to Other
This drop occurs if the T8 expires in the BSC during an outgoing Inter-BSC handover procedure.
T8 is started when the BSC sends the HO CMD to the MS during an outgoing HO procedure. This timer waits for the receipt of a HO COMP (for outgoing Intra-BSC
HO), a CL CMD with cause 'handover successful' (for outgoing Inter-BSC HO) or (in the unsuccessful case) for the HO FAILURE message. If this timer expires, the
call has most probably dropped during the HO procedure.
N200 - restricts the total number of repetitions of the messages controlled by T200 it is a fixed value for the BTS side specified by GSM which depends on the control channel types: N200(SDCCH)=23, N200(SACCH)=5, N200(FACCH/FR)=34, N200(FACCH/HR)=29