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Chapter 9 ZMODEM Protocol 24 |
9.4 FFFFuuuullllllll SSSSttttrrrreeeeaaaammmmiiiinnnngggg oooovvvveeeerrrr EEEErrrrrrrroooorrrr FFFFrrrreeeeeeee CCCChhhhaaaannnnnnnneeeellllssss |
File transfer protocols predicated on the existence of an error free |
end to end communications channel have been proposed from time to |
time. Such channels have proven to be more readily available in |
theory than in actuality. The frequency of undetected errors |
increases when modem scramblers have more bits than the error |
detecting CRC. |
A ZMODEM sender assuming an error free channel with end to end flow |
control can send the entire file in one frame without any checking of |
the reverse stream. If this channel is completely transparent, only |
ZDLE need be escaped. The resulting protocol overhead for average |
long files is less than one per cent.[7] |
9.5 SSSSeeeeggggmmmmeeeennnntttteeeedddd SSSSttttrrrreeeeaaaammmmiiiinnnngggg |
If the receiver cannot overlap serial and disk I/O, it uses the |
ZZZZRRRRIIIINNNNIIIITTTT frame to specify a buffer length which the sender will not |
overflow. The sending program sends a ZZZZCCCCRRRRCCCCWWWW data subpacket and waits |
for a ZZZZAAAACCCCKKKK header before sending the next segment of the file. |
If the sending program supports reverse data stream sampling or |
interrupt, error recovery will be faster (on average) than a protocol |
(such as YMODEM) that sends large blocks. |
A sufficiently large receiving buffer allows throughput to closely |
approach that of full streaming. For example, 16kb segmented |
streaming adds about 3 per cent to full streaming ZMODEM file |
transfer times when the round trip delay is five seconds. |
10. AAAATTTTTTTTEEEENNNNTTTTIIIIOOOONNNN SSSSEEEEQQQQUUUUEEEENNNNCCCCEEEE |
The receiving program sends the AAAAttttttttnnnn sequence whenever it detects an |
error and needs to interrupt the sending program. |
The default AAAAttttttttnnnn string value is empty (no Attn sequence). The |
receiving program resets Attn to the empty default before each |
transfer session. |
The sender specifies the Attn sequence in its optional ZSINIT frame. |
The AAAAttttttttnnnn string is terminated with a null. |
__________ |
7. One in 256 for escaping ZDLE, about two (four if 32 bit CRC is used) |
in 1024 for data subpacket CRC's |
Chapter 10 Rev 10-27-87 Typeset 10-27-87 24 |
Chapter 10 ZMODEM Protocol 25 |
Two meta-characters perform special functions: |
o+ \335 (octal) Send a break signal |
o+ \336 (octal) Pause one second |
11. FFFFRRRRAAAAMMMMEEEE TTTTYYYYPPPPEEEESSSS |
The numeric values for the values shown in boldface are given in |
_z_m_o_d_e_m._h. Unused bits and unused bytes in the header (ZP0...ZP3) are |
set to 0. |
11.1 ZZZZRRRRQQQQIIIINNNNIIIITTTT |
Sent by the sending program, to trigger the receiving program to send |
its ZRINIT header. This avoids the aggravating startup delay |
associated with XMODEM and Kermit transfers. The sending program may |
repeat the receive invitation (including ZRQINIT) if a response is |
not obtained at first. |
ZF0 contains ZCOMMAND if the program is attempting to send a command, |
0 otherwise. |
11.2 ZZZZRRRRIIIINNNNIIIITTTT |
Sent by the receiving program. ZF0 and ZF1 contain the bitwise or |
of the receiver capability flags: |
#define CANCRY 8 /* Receiver can decrypt */ |
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