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In summation, ZMODEM performance in the presence of errors resembles
that of X.PC and SuperKermit. Short bursts cause minimal data
retransmission. Long bursts (such as pulse dialing noises) often
require a timeout error to restore the flow of data.
15. PPPPAAAACCCCKKKKEEEETTTT SSSSWWWWIIIITTTTCCCCHHHHEEEEDDDD NNNNEEEETTTTWWWWOOOORRRRKKKK CCCCOOOONNNNSSSSIIIIDDDDEEEERRRRAAAATTTTIIIIOOOONNNNSSSS
Flow control is necessary for printing messages and directories, and
for streaming file transfer protocols. A non transparent flow
control is incompatible with XMODEM and YMODEM transfers. XMODEM
and YMODEM protocols require complete transparency of all 256 8 bit
codes to operate properly.
The "best" flow control (when X.25 or hardware CTS is unavailable)
Chapter 15 Rev 10-27-87 Typeset 10-27-87 34
Chapter 15 ZMODEM Protocol 35
would not "eat" any characters at all. When the PAD's buffer almost
fills up, an XOFF should be emitted. When the buffer is no longer
nearly full, send an XON. Otherwise, the network should neither
generate nor eat XON or XOFF control characters.
On Telenet, this can be met by setting CCIT X3 5:1 and 12:0 at bbbbooootttthhhh
ends of the network. For best throughput, parameter 64 (advance
ACK) should be set to something like 4. Packets should be forwarded
when the packet is a full 128 bytes, or after a moderate delay
(3:0,4:10,6:0).
With PC-Pursuit, it is sufficient to set parameter 5 to 1 at both
ends after one is connected to the remote modem.
<ENTER>@<ENTER>
set 5:1<ENTER>
rst? 5:1<ENTER>
cont<ENTER>
Unfortunately, many PADs do not accept the "rst?" command.
For YMODEM, PAD buffering should guarantee that a minimum of 1040
characters can be sent in a burst without loss of data or generation
of flow control characters. Failure to provide this buffering will
generate excessive retries with YMODEM.
TTTTAAAABBBBLLLLEEEE 1111.... Network and Flow Control Compatibility
______________________________________________________________________________
| Connectivity | Interactive| XMODEM| WXMODEM| SUPERKERMIT| ZMODEM |
_|________________________________________|____________________________|__________________|____________________|____________________________|____________________|_
_|____________________|______________|_________|__________|______________|__________|
|Direct Connect | YES | YES | YES | YES | YES |
_|____________________|______________|_________|__________|______________|__________|
|Network, no FC | nnnnoooo | YES | (4) | (6) | YES (1)|
_|____________________|______________|_________|__________|______________|__________|
|Net, transparent FC| YES | YES | YES | YES | YES |
_|____________________|______________|_________|__________|______________|__________|
|Net, non-trans. FC | YES | nnnnoooo | no (5) | YES | YES |
_|____________________|______________|_________|__________|______________|__________|
|Network, 7 bit | YES | nnnnoooo | no | YES (2) | YES (3)|
_|____________________|______________|_________|__________|______________|__________|
(1) ZMODEM can optimize window size or burst length for fastest
transfers.
(2) Parity bits must be encoded, slowing binary transfers.
(3) Natural protocol extension possible for encoding data to 7 bits.
(4) Small WXMODEM window size may may allow operation.
(5) Some flow control codes are not escaped in WXMODEM.
(6) Kermit window size must be reduced to avoid buffer overrun.
Chapter 15 Rev 10-27-87 Typeset 10-27-87 35
Chapter 15 ZMODEM Protocol 36
16. PPPPEEEERRRRFFFFOOOORRRRMMMMAAAANNNNCCCCEEEE CCCCOOOOMMMMPPPPAAAARRRRIIIISSSSOOOONNNN TTTTAAAABBBBLLLLEEEESSSS