Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!zaphod.mps.ohio-state.edu!wuarchive!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: Hardware flow control and Compression
Message-ID: <3949.28392152@hayes.uucp>
Date: 21 May 91 13:32:02 GMT
References: <49273@ut-emx.uucp>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 39

In article <49273@ut-emx.uucp>, csquared@ccwf.cc.utexas.edu (Randy
Clarke) writes: 

>      We're currently trying to set up some v.42bis modems on our Xyplex
> terminal server at work.  The problem we've run into is this:  Apparently,
> compression only works if RTS/CTS flow control is available.  As it stands
> now, the Xyplex does not support this form of flow control (No pin outs
> on the RJ-45 connector).   My question is this:  Why doesn't compression
> work with standard XON/XOFF software control?  From what I've read, it
> has something to do with setting up a sychronous connection between the
> two modems.  Could someone please explain what kind of handshaking takes
> place to make hardware flow control necessary or at least point me to
> a FAQ file which explains v.42bis compression in some detail.

There's no reason why data compression wouldn't work with XON/XOFF 
flow control instead of RTS/CTS, or even with no flow control at all 
(although you'd risk losing data in that case).  The reason RTS/CTS 
is _preferred_ (but not _mandatory_) is that it is fully transparent 
to the type of data you're sending -- you can send the XON and XOFF 
characters as data, without them interfering with the local flow 
control mechanism.  But so long as you never transfer XON or XOFF as 
data (meaning, you use file transfer protocols such as Kermit or 
ZMODEM that transparentize these characters), then there's no reason 
why you can't use XON/XOFF flow control with modems that have error 
control and data compression.

If the particular modems you have happen to not support XON/XOFF 
flow control while V.42bis is being used, then that is a flaw in the 
design of THOSE modems.  All Hayes V.42bis modems, and the vast 
majority of V.42bis modems from other manufacturers, DO support 
XON/XOFF protocol along with V.42bis data compression.

	-- Toby

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-840-9200  Telex 151243420
Hayes Microcomputer Products Inc. | Fax     +1-404-447-0178  CIS   70271,404
P.O. Box 105203                   | UUCP uunet!hayes!tnixon  AT&T    !tnixon
Atlanta, Georgia  30348  USA      | Internet       hayes!tnixon@uunet.uu.net