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ZMODEM is an asynchronous communication protocol that provides faster data transfer rates and error detection than Xmodem. ZMODEM supports enormous block sizes and, following a communication failure, allows transfers to resume from where they stopped.
ZMODEM was developed by Chuck Forsberg in 1986. In addition to improved performance and recovery following a communications failure, it supported expanded 32-bit cyclic redundancy checks and control character quoting, as well as autostart by senders. It was also widely used in bulletin board systems.
ZMODEM also supported the sliding window for better performance. Sliding window protocols permit the sending machine to move on to the next packets without waiting for an acknowledgment. The receiver then sends acknowledgment or no acknowledgment signals together with the packet number. The sender then processes the messages as required, thereby reducing latency to zero, and only incurring the cost of small overhead data.
ZMODEM design is an engineering compromise between conflicting requirements. It permits programs to initiate file transfers. It also allows senders to submit commands to receiving programs, while file names are entered only once. ZMODEM supports menu selection where wild card names are used with batch transfers. Only very few keystrokes are required to initiate transfers. ZMODEM will step down to Ymodems if the other end does not support ZMODEM.