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o+ Lack of public domain documentation and example programs have kept
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proprietary protocols such as RRRReeeellllaaaayyyy,,,, BBBBllllaaaasssstttt,,,, and others tightly bound
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to the fortunes of their suppliers. These protocols have not
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benefited from public scrutiny of their design features.
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Chapter 2 Rev 10-27-87 Typeset 10-27-87 2
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Chapter 2 ZMODEM Protocol 3
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o+ Link level protocols such as XXXX....22225555,,,, XXXX....PPPPCCCC,,,, and MMMMNNNNPPPP do not manage
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application to application file transfers.
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o+ Link Level protocols do not eliminate end-to-end errors. Interfaces
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between error-free networks are not necessarily error-free.
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Sometimes, error-free networks aren't.
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o+ The KKKKeeeerrrrmmmmiiiitttt protocol was developed to allow file transfers in
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environments hostile to XMODEM. The performance compromises
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necessary to accommodate traditional mainframe environments limit
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Kermit's efficiency. Even with completely transparent channels,
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Kermit control character quoting limits the efficiency of binary file
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transfers to about 75 per cent.[1]
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A number of submodes are used in various Kermit programs, including
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different methods of transferring binary files. Two Kermit programs
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will mysteriously fail to operate with each other if the user has not
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correctly specified these submodes.
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Kermit Sliding Windows ("SuperKermit") improves throughput over
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networks at the cost of increased complexity. SuperKermit requires
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full duplex communications and the ability to check for the presence
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of characters in the input queue, precluding its implementation on
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some operating systems.
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SuperKermit state transitions are encoded in a special language
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"wart" which requires a C compiler.
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SuperKermit sends an ACK packet for each data packet of 96 bytes
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(fewer if control characters are present). This reduces throughput
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on high speed modems, from 1350 to 177 characters per second in one
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test.
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A number of extensions to the XMODEM protocol have been made to improve
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performance and (in some cases) the user interface. They provide useful
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improvements in some applications but not in others. XMODEM's unprotected
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control messages compromise their reliability. Complex proprietary
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techniques such as CCCCyyyybbbbeeeerrrrnnnneeeettttiiiicccc DDDDaaaattttaaaa RRRReeeeccccoooovvvveeeerrrryyyy((((TTTTMMMM))))[2] improve reliability,
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but are not universally available. Some of the XMODEM mutant protocols
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have significant design flaws of their own.
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o+ XXXXMMMMOOOODDDDEEEEMMMM----kkkk uses 1024 byte blocks to reduce the overhead from transmission
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delays by 87 per cent compared to XMODEM, but network delays still
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__________
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1. Some Kermit programs support run length encoding.
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2. Unique to DSZ, ZCOMM, Professional-YAM and PowerCom
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Chapter 2 Rev 10-27-87 Typeset 10-27-87 3
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Chapter 2 ZMODEM Protocol 4
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degrade performance. Some networks cannot transmit 1024 byte packets
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without flow control, which is difficult to apply without impairing the
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perfect transparency required by XMODEM. XMODEM-k adds garbage to
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received files.
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o+ YYYYMMMMOOOODDDDEEEEMMMM sends the file name, file length, and creation date at the
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beginning of each file, and allows optional 1024 byte blocks for
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improved throughput. The handling of files that are not a multiple of
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1024 or 128 bytes is awkward, especially if the file length is not
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known in advance, or changes during transmission. The large number of
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non conforming and substandard programs claiming to support YMODEM
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further complicates its use.
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o+ YYYYMMMMOOOODDDDEEEEMMMM----gggg provides efficient batch file transfers, preserving exact file
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length and file modification date. YMODEM-g is a modification to
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