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Chapter 3
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X/YMODEM Protocol Reference 10-27-87 9
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4. XMODEM PROTOCOL ENHANCEMENTS
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This chapter discusses the protocol extensions to Ward Christensen's 1982
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XMODEM protocol description document.
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The original document recommends the user be asked whether to continue
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trying or abort after 10 retries. Most programs no longer ask the
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operator whether he wishes to keep retrying. Virtually all correctable
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errors are corrected within the first few retransmissions. If the line is
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so bad that ten attempts are insufficient, there is a significant danger
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of undetected errors. If the connection is that bad, it's better to
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redial for a better connection, or mail a floppy disk.
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4.1 Graceful Abort
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The YAM and Professional-YAM X/YMODEM routines recognize a sequence of two
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consecutive CAN (Hex 18) characters without modem errors (overrun,
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framing, etc.) as a transfer abort command. This sequence is recognized
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when is waiting for the beginning of a block or for an acknowledgement to
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a block that has been sent. The check for two consecutive CAN characters
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reduces the number of transfers aborted by line hits. YAM sends eight CAN
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characters when it aborts an XMODEM, YMODEM, or ZMODEM protocol file
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transfer. Pro-YAM then sends eight backspaces to delete the CAN
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characters from the remote's keyboard input buffer, in case the remote had
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already aborted the transfer and was awaiting a keyboarded command.
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4.2 CRC-16 Option
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The XMODEM protocol uses an optional two character CRC-16 instead of the
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one character arithmetic checksum used by the original protocol and by
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most commercial implementations. CRC-16 guarantees detection of all
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single and double bit errors, all errors with an odd number of error
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bits, all burst errors of length 16 or less, 99.9969% of all 17-bit error
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bursts, and 99.9984 per cent of all possible longer error bursts. By
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contrast, a double bit error, or a burst error of 9 bits or more can sneak
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past the XMODEM protocol arithmetic checksum.
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The XMODEM/CRC protocol is similar to the XMODEM protocol, except that the
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receiver specifies CRC-16 by sending C (Hex 43) instead of NAK when
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requesting the FIRST block. A two byte CRC is sent in place of the one
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byte arithmetic checksum.
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YAM's c option to the r command enables CRC-16 in single file reception,
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corresponding to the original implementation in the MODEM7 series
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programs. This remains the default because many commercial communications
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programs and bulletin board systems still do not support CRC-16,
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especially those written in Basic or Pascal.
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XMODEM protocol with CRC is accurate provided both sender and receiver
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Chapter 4 XMODEM Protocol Enhancements
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X/YMODEM Protocol Reference 10-27-87 10
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