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+ Lack of public domain documentation and example programs have kept
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proprietary protocols such as Blast, Relay, and others tightly bound
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to the fortunes of their suppliers.
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+ Complexity discourages the widespread application of BISYNC, SDLC,
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HDLC, X.25, and X.PC protocols.
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+ Performance compromises and complexity have limited the popularity of
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the Kermit protocol, which was developed to allow file transfers in
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environments hostile to XMODEM.
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The XMODEM protocol extensions and YMODEM Batch address some of these
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weaknesses while maintaining most of XMODEM's simplicity.
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YMODEM is supported by the public domain programs YAM (CP/M),
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YAM(CP/M-86), YAM(CCPM-86), IMP (CP/M), KMD (CP/M), rz/sz (Unix, Xenix,
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VMS, Berkeley Unix, Venix, Xenix, Coherent, IDRIS, Regulus). Commercial
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implementations include MIRROR, and Professional-YAM.[1] Communications
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Chapter 3
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X/YMODEM Protocol Reference 10-27-87 6
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programs supporting these extensions have been in use since 1981.
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The 1k block length (XMODEM-1k) described below may be used in conjunction
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with YMODEM Batch Protocol, or with single file transfers identical to the
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XMODEM/CRC protocol except for minimal changes to support 1k blocks.
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Another extension is the YMODEM-g protocol. YMODEM-g provides batch
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transfers with maximum throughput when used with end to end error
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correcting media, such as X.PC and error correcting modems, including 9600
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bps units by TeleBit, U.S.Robotics, Hayes, Electronic Vaults, Data Race,
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and others.
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To complete this tome, edited versions of Ward Christensen's original
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protocol document and John Byrns's CRC-16 document are included for
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reference.
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References to the MODEM or MODEM7 protocol have been changed to XMODEM to
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accommodate the vernacular. In Australia, it is properly called the
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Christensen Protocol.
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3.1 Some Messages from the Pioneer
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#: 130940 S0/Communications 25-Apr-85 18:38:47
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Sb: my protocol
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Fm: Ward Christensen 76703,302 [2]
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To: all
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Be aware the article[3] DID quote me correctly in terms of the phrases
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like "not robust", etc.
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It was a quick hack I threw together, very unplanned (like everything I
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do), to satisfy a personal need to communicate with "some other" people.
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ONLY the fact that it was done in 8/77, and that I put it in the public
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domain immediately, made it become the standard that it is.
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__________________________________________________________________________
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1. Available for IBM PC,XT,AT, Unix and Xenix
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2. Edited for typesetting appearance
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3. Infoworld April 29 p. 16
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Chapter 3
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X/YMODEM Protocol Reference 10-27-87 7
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