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Omen Technology Incorporated
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The High Reliability Software
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17505-V Northwest Sauvie Island Road
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Portland Oregon 97231
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VOICE: 503-621-3406 :VOICE
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Modem: 503-621-3746 Speed 1200,2400,19200(Telebit PEP)
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Compuserve:70007,2304 GEnie:CAF
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UUCP: ...!tektronix!reed!omen!caf
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Chapter 0 Rev 10-27-87 Typeset 10-27-87 1
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Chapter 0 ZMODEM Protocol 2
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1. IIIINNNNTTTTEEEENNNNDDDDEEEEDDDD AAAAUUUUDDDDIIIIEEEENNNNCCCCEEEE
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This document is intended for telecommunications managers, systems
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programmers, and others who choose and implement asynchronous file
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transfer protocols over dial-up networks and related environments.
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2. WWWWHHHHYYYY DDDDEEEEVVVVEEEELLLLOOOOPPPP ZZZZMMMMOOOODDDDEEEEMMMM????
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Since its development half a decade ago, the Ward Christensen MMMMOOOODDDDEEEEMMMM
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protocol has enabled a wide variety of computer systems to interchange
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data. There is hardly a communications program that doesn't at least
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claim to support this protocol, now called XXXXMMMMOOOODDDDEEEEMMMM.
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Advances in computing, modems and networking have spread the XMODEM
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protocol far beyond the micro to micro environment for which it was
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designed. These application have exposed some weaknesses:
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o+ The awkward user interface is suitable for computer hobbyists.
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Multiple commands must be keyboarded to transfer each file.
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o+ Since commands must be given to both programs, simple menu selections
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are not possible.
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o+ The short block length causes throughput to suffer when used with
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timesharing systems, packet switched networks, satellite circuits,
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and buffered (error correcting) modems.
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o+ The 8 bit checksum and unprotected supervison allow undetected errors
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and disrupted file transfers.
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o+ Only one file can be sent per command. The file name has to be given
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twice, first to the sending program and then again to the receiving
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program.
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o+ The transmitted file accumulates as many as 127 bytes of garbage.
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o+ The modification date and other file attributes are lost.
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o+ XMODEM requires _c_o_m_p_l_e_t_e 8 bit transparency, all 256 codes. XMODEM
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will not operate over some networks that use ASCII flow control or
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escape codes. Setting network transparency disables important
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control functions for the duration of the call.
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A number of other protocols have been developed over the years, but none
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have proven satisfactory.
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