SNA maximum transmit window (TW). The maximum number of outstanding I-format link protocol data units (LPDUs). It is also called the maximum transmit window count or the maximum window out count. The TW count allows the sender to transmit frames before that sender is forced to halt and wait for an acknowledgement. Therefore, the receiver should be able to absorb that number of frames, either in its service access point (SAP) buffers or within the buffers in workstation memory.