Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!sol.ctr.columbia.edu!cica!iuvax!maytag!xenitec!geoffw
From: geoffw@xenitec.on.ca (Geoffrey Welsh)
Newsgroups: comp.sys.cbm
Subject: Re: kermit vs. xmodem - was Re: kermit problem
Message-ID: <1990Apr10.145731.279@xenitec.on.ca>
Date: 10 Apr 90 14:57:31 GMT
References: <14836@s.ms.uky.edu> <1424@ac.dal.ca>
Reply-To: geoffw@xenitec.UUCP (Geoffrey Welsh)
Organization: XeniTec Consulting Services, Kitchener, ON, Canada
Lines: 49

In article <1424@ac.dal.ca> mike@ac.dal.ca writes:
>Sorry, only Punter Protocol transfers 256 bytes... Standard Xmodem does 128
>bytes, and Xmodem1-K does 1024 at a time.. and xmodem is very much
>faster than kermit... and punter is faster still...

   Let me clear up some ideas people have about 'new Punter' (C1)
protocol: XMODEM may only have a block size of 128 bytes, but it
requires only one ACK per block. C1's block size is 255 bytes (of
which seven are used for block numbers, checksums, etc.), but it
must double-handshake every block:

	send		receive
	<block>
			GOO/BAD
	ACK
			S/B
	<block>

   So, there are 4 turnarounds for every 248 data bytes sent, and
a byte overhead of 2.7%. XMODEM offers 2 turnarounds for every 128
bytes of data sent (4 per 256) and, with 4 or 5 extra bytes per
block depending on checksum or CRC mode, a byte overhead of 3% or
3.7%.

   So, what's this all mean?

   The lower byte overhead suggests that C1 might be faster where
the main time delay is getting the data through, e.g. over a 300
bps modem. However, XMODEM's slight edge in the data squeezed through
per turnaround makes them about even at higher speeds (1200/2400)
and makes it look good at 9600 or over long distance connects. As
has been pointed out, XMODEM-1K will need even fewer turnarounds
so it gets much better performance.

   Your mileage may vary.

   How good each protocol is doesn't count for squat if the CODE
is slow & sloppy. If your fave terminal does Punter much faster
than it does XMODEM, I'm willing to bet that it's because the
author simply copied Steve Punter's C1 routines (which are reasonable,
if not amazing) but used some really poor routines for XMODEM.

   Geoff

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