Please see next explanation from documentation (I think it could be much more clear to you):Principle: When the BTS transmits a DL layer-2 frame on the air (e.g.
an I-frame with an embedded layer 3 message) the BTS starts the
timer T200 and waits for the acknowledgement frame from the MS.
The acknowledgement frame can be another I-frame sent by the MS
in the UL (if the MS has to transmit a layer-3 DTAP message
anyway) as well as RECEIVE READY (RR) if there is nothing else to
be transmitted on layer 3. If the acknowledgement from the MS is not
received within T200 the transmission of the layer 2 frame is
repeated and T200 restarted. The total number of repetitions is
restricted to N200, a fixed value for the BTS side specified by GSM
which depends on the control channel types:
If T200 expires after the last layer 2 frame repetition the BTS sends
an ERROR INDICATION with cause ‘T200 expired N200+1 times’
(followed by a RELEASE INDICATION) message to the BSC, which
releases the associated resources.