A transmitter is capable of transmitting layer 2 protocol data units (PDUs). Each PDU has an n-bit sequence number. A base sequence number BT(A) is obtained that marks a beginning sequence number of a transmitting window of the transmitter. A current sequence number VT(S) is obtained that marks a sequence number of a PDU that is next to be transmitted by the transmitter. 2.sup.n is added to a difference of the current sequence number VT(S) and the base sequence number VT(A) to yield a first value. A second value is obtained that is a modulus of the first value with 2.sup.n. A test value is then obtained that is the second value divided by a size of the transmitting window. Polling is triggered when the test value is greater than or equal to a polling value. The polling value represents a percentage of the transmitting window that has been transmitted.