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Chapter 6 Rev 10-27-87 Typeset 10-27-87 11 |
Chapter 6 ZMODEM Protocol 12 |
7. ZZZZMMMMOOOODDDDEEEEMMMM BBBBAAAASSSSIIIICCCCSSSS |
7.1 PPPPaaaacccckkkkeeeettttiiiizzzzaaaattttiiiioooonnnn |
ZMODEM frames differ somewhat from XMODEM blocks. XMODEM blocks are not |
used for the following reasons: |
o+ Block numbers are limited to 256 |
o+ No provision for variable length blocks |
o+ Line hits corrupt protocol signals, causing failed file transfers. In |
particular, modem errors sometimes generate false block numbers, false |
EOTs and false ACKs. False ACKs are the most troublesome as they cause |
the sender to lose synchronization with the receiver. |
State of the art programs such as Professional-YAM and ZCOMM overcome |
some of these weaknesses with clever proprietary code, but a stronger |
protocol is desired. |
o+ It is difficult to determine the beginning and ends of XMODEM blocks |
when line hits cause a loss of synchronization. This precludes rapid |
error recovery. |
7.2 LLLLiiiinnnnkkkk EEEEssssccccaaaappppeeee EEEEnnnnccccooooddddiiiinnnngggg |
ZMODEM achieves data transparency by extending the 8 bit character set |
(256 codes) with escape sequences based on the ZMODEM data link escape |
character ZDLE.[1] |
Link Escape coding permits variable length data subpackets without the |
overhead of a separate byte count. It allows the beginning of frames to |
be detected without special timing techniques, facilitating rapid error |
recovery. |
Link Escape coding does add some overhead. The worst case, a file |
consisting entirely of escaped characters, would incur a 50% overhead. |
The ZDLE character is special. ZDLE represents a control sequence of some |
sort. If a ZDLE character appears in binary data, it is prefixed with |
ZDLE, then sent as ZDLEE. |
The value for ZDLE is octal 030 (ASCII CAN). This particular value was |
chosen to allow a string of 5 consecutive CAN characters to abort a ZMODEM |
__________ |
1. This and other constants are defined in the _z_m_o_d_e_m._h include file. |
Please note that constants with a leading 0 are octal constants in C. |
Chapter 7 Rev 10-27-87 Typeset 10-27-87 12 |
Chapter 7 ZMODEM Protocol 13 |
session, compatible with YMODEM session abort. |
Since CAN is not used in normal terminal operations, interactive |
applications and communications programs can monitor the data flow for |
ZDLE. The following characters can be scanned to detect the ZRQINIT |
header, the invitation to automatically download commands or files. |
Receipt of five successive CAN characters will abort a ZMODEM session. |
Eight CAN characters are sent. |
The receiving program decodes any sequence of ZDLE followed by a byte with |
bit 6 set and bit 5 reset (upper case letter, either parity) to the |
equivalent control character by inverting bit 6. This allows the |
transmitter to escape any control character that cannot be sent by the |
communications medium. In addition, the receiver recognizes escapes for |
0177 and 0377 should these characters need to be escaped. |
ZMODEM software escapes ZDLE, 020, 0220, 021, 0221, 023, and 0223. If |
preceded by 0100 or 0300 (@), 015 and 0215 are also escaped to protect the |
Telenet command escape CR-@-CR. The receiver ignores 021, 0221, 023, and |
0223 characters in the data stream. |
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