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From: julian@uhccux.uhcc.hawaii.edu (Julian Cowley)
Newsgroups: comp.binaries.ibm.pc.d,comp.sys.ibm.pc
Subject: Re: DSZ rz & sz
Message-ID: <2927@uhccux.uhcc.hawaii.edu>
Date: 3 Jan 89 22:28:51 GMT
References: <54@VAX1.CC.UAKRON.EDU> <2919@uhccux.uhcc.hawaii.edu> <1516@umbc3.UMD.EDU>
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Organization: University of Hawaii at Manoa
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In article <1516@umbc3.UMD.EDU> cs374326@umbc3.UMD.EDU (Peter Johansson) writes:
>But in the end, it really shouldn't matter all that much.  Remember, the
>only disadvantage to a small packet size in Zmodem is the overhead (header,
>CRC, etc) for each packet, and you do not have to worry about inter-packet
>delays as you do with x- and y-modem.

That's true, as long as you have a relatively error-free line,
but if you have a bad telephone line, then it makes a lot of
difference.  Just as with X/Ymodem, the sender has to retransmit
an entire block (or subpacket) if the receiver says it didn't
receive it correctly the first time.  The larger the block,
then, the longer it takes for the retransmission.  At 1200 baud,
a 1024 byte block takes at least a couple of seconds, less
overhead.  Therefore, the real trick is finding the block size
that permits best thoughput, and is usually dependent on the
line speed and error level.  Do you send large blocks that has
less overhead but longer retransmission time, or do you send
small blocks that have more overhead but shorter retransmission
time?  Higher line speeds mean that larger blocks can be used
because they take less time to retransmit.  Conversely, higher
error rates mean that larger blocks waste time because it will
mean more retransmissions.

The recommendations that were in the Zmodem manual that I read
gave the figures that I quoted in my previous article, but as
you pointed out, it looks as though they have changed,
especially if you found them in a program such as sz.  This
could be because the the recommendations for adjusting the
packet size during transmission are still evolving.  I'll have
to download the Zmodem documentation or the sz program from
Simtel20 again to find out for sure.

julian@uhccux.uhcc.hawaii.edu
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julian@uhccux.bitnet
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