WXMODEM is a full-duplex "sliding windows" protocol for file transfer operations. It allows the transmission of up to four XMODEM 128-byte data blocks before requiering an acknowledgment and uses a CRC for error checking. It was developed for use on packet-switching networks in wich the flow of data in small packets. By enabling multiple data blocks to be transmitted before requring an acknowlegment from the reciving device, transmission efficency is increased.