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