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YMODEM wherein ACKs for data blocks are not used. YMODEM-g is
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essentially insensitive to network delays. Because it does not support
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error recovery, YMODEM-g must be used hard wired or with a reliable
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link level protocol. Successful application at high speed requires
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cafeful attention to transparent flow control. When YMODEM-g detects a
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CRC error, data transfers are aborted. YMODEM-g is easy to implement
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because it closely resembles standard YMODEM.
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o+ WWWWXXXXMMMMOOOODDDDEEEEMMMM,,,, SSSSEEEEAAAAlllliiiinnnnkkkk,,,, and MMMMEEEEGGGGAAAAlllliiiinnnnkkkk have applied a subset of ZMODEM's
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techniques to "Classic XMODEM" to improve upon their suppliers'
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previous offerings. They provide good performance under ideal
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conditions.
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Another XMODEM "extension" is protocol cheating, such as Omen Technology's
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OOOOvvvveeeerrrrTTTThhhhrrrruuuusssstttteeeerrrr((((TTTTMMMM)))) and OOOOvvvveeeerrrrTTTThhhhrrrruuuusssstttteeeerrrr IIIIIIII((((TTTTMMMM)))). These improve XMODEM throughput
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under some conditions by compromising error recovery.
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The ZMODEM Protocol corrects the weaknesses described above while
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maintaining as much of XMODEM/CRC's simplicity and prior art as possible.
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3. ZZZZMMMMOOOODDDDEEEEMMMM PPPPrrrroooottttooooccccoooollll DDDDeeeessssiiiiggggnnnn CCCCrrrriiiitttteeeerrrriiiiaaaa
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The design of a file transfer protocol is an engineering compromise
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between conflicting requirements:
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3.1 EEEEaaaasssseeee ooooffff UUUUsssseeee
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o+ ZMODEM allows either program to initiate file transfers, passing
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commands and/or modifiers to the other program.
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o+ File names need be entered only once.
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o+ Menu selections are supported.
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Chapter 3 Rev 10-27-87 Typeset 10-27-87 4
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Chapter 3 ZMODEM Protocol 5
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o+ Wild Card names may be used with batch transfers.
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o+ Minimum keystrokes required to initiate transfers.
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o+ ZRQINIT frame sent by sending program can trigger automatic downloads.
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o+ ZMODEM can step down to YMODEM if the other end does not support
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ZMODEM.[1]
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3.2 TTTThhhhrrrroooouuuugggghhhhppppuuuutttt
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All file transfer protocols make tradeoffs between throughput,
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reliability, universality, and complexity according to the technology and
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knowledge base available to their designers.
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In the design of ZMODEM, three applications deserve special attention.
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o+ Network applications with significant delays (relative to character
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transmission time) and low error rate
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o+ Timesharing and buffered modem applications with significant delays
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and throughput that is quickly degraded by reverse channel traffic.
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ZMODEM's economy of reverse channel bandwidth allows modems that
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dynamically partition bandwidth between the two directions to operate
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at optimal speeds. Special ZMODEM features allow simple, efficient
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implementation on a wide variety of timesharing hosts.
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o+ Direct modem to modem communications with high error rate
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Unlike Sliding Windows Kermit, ZMODEM is not optimized for optimum
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throughput when error rate and delays are both high. This tradeoff
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markedly reduces code complexity and memory requirements. ZMODEM
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generally provides faster error recovery than network compatible XMODEM
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implementations.
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In the absence of network delays, rapid error recovery is possible, much
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faster than MEGAlink and network compatible versions of YMODEM and XMODEM.
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File transfers begin immediately regardless of which program is started
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first, without the 10 second delay associated with XMODEM.
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__________
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