zmodem file transfer problems

[John G. De Armond]

alexp at chaos.cs.brandeis.edu (Alex Prengel) writes:

>Not sure this is the right group for this, but it was the closest I could find.
>I am having problems sending files from a UNIX mainframe to my PC using zmodem
>(I have used it extensively in PC to PC transfers and found it fast, reliable
>and efficient). When I initiate the transfer from the mainframe (sz foo.bar)
>and start the download on the PC end, the transfer begins but invariably starts
>to generate errors (typically ZERPOS header, but also CRC ERROR and various
>others). The errors start at a very consistent byte count (about 5K as I recall)
>and then repeat frequently (about every 4 to 10K, though the interval is not
>perfectly regular). Although zmodem will retransmit until the file gets through,
>the transfer is much too slow, making it utterly impractical for large files.

I believe the problem to be that the current versions (up to about 9 months
old) disable the handshaking on the modem port.  I see this problem 
commonly on systems where the modem does a baudrate conversion.  Example,
a telebit with the port baud locked at 19.2 and the dialin port allowed to
adjust to the caller's rate.

Knowing whether or not the modem on the other end is of this type is 
crucial information.

In the interim, a quick fix is to use the -w option to limit the window
size to less than the capacity of the modem buffer.  That way the computer-
modem link stops and waits for ACKs.  You won't see any degradation in
performance on the 2400 baud line because of this, as the ack will be 
returned before the buffer completely clears.

In the meanwhile, I'm working on an implementation of zmodem  for a 
commercial application in my work and will be looking at this problem
in detail.

John

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