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A file transfer protocol with error checking and crash recovery. Developed by Chuck Forsberg. Its transfer rate is similar to YMODEM-g. Like YMODEM-g, ZMODEM does not wait for positive acknowledgement after each block is sent, but rather sends blocks in rapid succession. If a ZMODEM transfer is cancelled or interrupted for any reason, the transfer can be resurrected later and the previously transferred information need not be resent.

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A file transfer protocol that was very popular during the days of analog modem transmission between bulletin boards because it handled noisy and changing line conditions very well, including satellite transmission. Funded by the Telenet packet network and developed by Chuck Forsberg in the mid-1980s, Zmodem uses a sliding window design, which is faster than the Xmodem and Ymodem protocols that came before it. The TCP protocol also uses a sliding window (see sliding window).

Using variable length blocks and CRC error correction, Zmodem first sends file name, date and size before sending the data. The receiver continuously sends back the starting location of the next part of the file that is required so that crash recovery can be easily implemented by the file transfer program if the transmission fails in the middle. Many popular communications programs offer Zmodem as a file transfer option.

Industrial Zmodem
As a compact and proven protocol, Zmodem is used for embedded systems. Although residing in the public domain specifically for Unix systems, the heart of the protocol is available as Industrial Zmodem, a commercial product from Forsberg's company. For more information, visit
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