ZMODEM

ZMODEM is an inline <a href="/facts/Comparison_of_file_transfer_protocols/XGBF8I9C">file transfer protocol</a> developed by <a href="/facts/Chuck_Forsberg/1GQ7kf8H">Chuck Forsberg</a> in 1986, in a <a href="/facts/Project/7pXnTsmF">project</a> funded by <a href="/facts/Telenet/VCgNaKux">Telenet</a> in order to improve file transfers on their <a href="/facts/X.25/yaGPqw2Y">X.25</a> network. In addition to dramatically improved performance compared to older protocols, ZMODEM offered restartable transfers, auto-start by the sender, an expanded 32-bit <a href="/facts/Cyclic_redundancy_check/u2exo4sv">CRC</a>, and <a href="/facts/Escape_character/pTSAozpa">control character quoting</a> supporting <a href="/facts/8-bit_clean/2j1DTzgg">8-bit clean</a> transfers, allowing it to be used on networks that would not pass control characters.
In contrast to most transfer protocols developed for <a href="/facts/Bulletin_board_system/22d9c9s2">bulletin board systems</a> (BBSs), ZMODEM was not directly based on, nor compatible with, the seminal <a href="/facts/XMODEM/TIadURGQ">XMODEM</a>. Many variants of XMODEM had been developed in order to address one or more of its shortcomings, and most remained <a href="/facts/Backward_compatible/FbHFBu5y">backward compatible</a> and would successfully complete transfers with "classic" XMODEM implementations. This list includes Forsberg's own <a href="/facts/YMODEM/eLmKVq2b">YMODEM</a>.
ZMODEM eschewed backward compatibility in favor of producing a radically improved protocol. It performed at least as well as any of the high-performance varieties of XMODEM, did so over links that previously did not work at all, like X.25, or had poor performance, like <a href="/facts/Telebit/ULT9p1Bc">Telebit</a> modems, and included useful features found in few other protocols. ZMODEM became extremely popular on <a href="/facts/Bulletin_board_system/22d9c9s2">bulletin board systems</a> (BBS) in the early 1990s, becoming a standard as widespread as XMODEM had been before it.

ZMODEM open-in-new

ZMODEM