A method for operating a radio link protocol receiver when receiving datablocks from a transmitter, comprising: receiving a data block; determining in the receiver whether a size of a receive window is less than or equal to a bitmap size, wherein the bitmap size is a maximum size, expressed in a number of bits, of the bitmap to be reported in an acknowledgment message, and based on the determination, if the size of the receive window is greater than the bitmap size, the reported bitmap comprises a partial bitmap covering one of an initial portion of the receive window and a portion other than the initial portion of the receive window; and further determining, in the receiver, which portion of the receive window is reported, wherein said further determining comprises: determining if the a previous acknowledgment message carried a partial bitmap that did not cover the a highest BSN received at the time, and if the difference between: (a) the BSN that has a value one higher than the highest BSN yet received (V), and (b) the highest BSN that was included in the bitmap when the previous acknowledgment message was transmitted, PBSN, is larger than a threshold value, PBSN_Threshold, then the partial bitmap covering a portion other than the initial portion of the receive window is sent provided that PBSN is still larger than the start of the receive window (V); otherwise the partial bitmap covering the initial portion of the receive window is sent; where: BSN is a block sequence number, PBSN is a partial block sequence number, PBSN_Threshold is a threshold value partial block sequence number, V is a receive state variable that denotes the BSN which has a value one higher than the highest BSN yet received, modulo SNS; SNS is a sequence number space that denotes a range [0; SNS-1] in which a block sequence number, BSN, takes its value, and V is a receive window state variable that denotes a lowest BSN not yet received, modulo SNS, and represents the start of a receive window. receiving a data block; determining in the receiver whether a size of a receive window is less than or equal to a bitmap size, wherein the bitmap size is a maximum size, expressed in a number of bits, of the bitmap to be reported in an acknowledgment message, and based on the determination, if the size of the receive window is greater than the bitmap size, the reported bitmap comprises a partial bitmap covering one of an initial portion of the receive window and a portion other than the initial portion of the receive window; and further determining, in the receiver, which portion of the receive window is reported, wherein said further determining comprises: determining if the a previous acknowledgment message carried a partial bitmap that did not cover the a highest BSN received at the time, and if the difference between: (a) the BSN that has a value one higher than the highest BSN yet received (V), and (b) the highest BSN that was included in the bitmap when the previous acknowledgment message was transmitted, PBSN, is larger than a threshold value, PBSN_Threshold, then the partial bitmap covering a portion other than the initial portion of the receive window is sent provided that PBSN is still larger than the start of the receive window (V); otherwise the partial bitmap covering the initial portion of the receive window is sent; determining if the a previous acknowledgment message carried a partial bitmap that did not cover the a highest BSN received at the time, and if the difference between: (a) the BSN that has a value one higher than the highest BSN yet received (V), and (b) the highest BSN that was included in the bitmap when the previous acknowledgment message was transmitted, PBSN, is larger than a threshold value, PBSN_Threshold, then the partial bitmap covering a portion other than the initial portion of the receive window is sent provided that PBSN is still larger than the start of the receive window (V); otherwise the partial bitmap covering the initial portion of the receive window is sent; where: BSN is a block sequence number, PBSN is a partial block sequence number, PBSN_Threshold is a threshold value partial block sequence number, V is a receive state variable that denotes the BSN which has a value one higher than the highest BSN yet received, modulo SNS; SNS is a sequence number space that denotes a range [0; SNS-1] in which a block sequence number, BSN, takes its value, and V is a receive window state variable that denotes a lowest BSN not yet received, modulo SNS, and represents the start of a receive window.