T200 is an air interface timer regulating the communication between MS and BTS. It defines the rate of repetition of a message (on BTS side) on the air if it is not acknowledged by MS.
If the message was repeated N200 times and no answer was received by MS, an error indication with cause T200 expired is provided to BSC.
T200=ny1 * N200 where ny1 is configurable on BTS database