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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 */