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Reply To: call drop due to expiry of T200 timer


1st Query
T200 is a basic timer in the LAPDm & LAPD protocol (default value 240ms). T200 is the time between retransmissions, T200 is started for every transmission of a frame, and if the timer expires the frame is retransmitted. If the error persists after N200 retransmissions a link failure is detected and the data link is re-established. If the contact between the BSC and the BTS (or BTS & the MS)is broken the protocol will detect a link failure in approximately one second when using default values, calculated as (N200+1)*T200 + transmission and processing time.

2nd Query
It is just the other way round.Data link failure is the cause of HO failure

3rd Query
As HO process is break before make,so the data link in case of HO is from MS to target BTS.
After the HO command ,if a data link failure is observed,HO faiure happens,& then either return to old channel or call drop happens.