Path: utzoo!utgpu!watserv1!maytag!xenitec!zswamp!root
From: root@zswamp.uucp (Geoffrey Welsh)
Newsgroups: comp.dcom.modems
Subject: Hardware flow control and Compression
Message-ID: <81.283BB3AC@zswamp.uucp>
Date: Thu, 23 May 91 01:19:00 EDT
Organization: Izot's Swamp BBS (FidoNet), Kitchener, Ontario

In a letter to All, Randy Clarke (csquared@ccwf.cc.utexas.edu ) wrote:

 >The problem we've run into is this:  Apparently,
 >compression only works if RTS/CTS flow control is available.  

   Umm, that's a bit misleading.  It implies that turning off flow control 
also switches data compression off, which *might* be the case but in all 
probability isn't.

   Flow control allows you to speak to the modem at speeds exceeding the 
actual link speed without fear of overrunning the modem's buffers.  This is 
important if you expect the modem to compress your data and achieve higher 
throughput, since the modem can't transmit data any faster than you send it to 
the modem in the first place.

   So even if a 9600 bps modem with data compression is able to send your 
ASCII text at 1700 CPS, a computer stuck at 9600 bps to ensure integrity will 
never see more than 960 CPS (minus some overhead).

 >My question is this:  Why doesn't compression work with
 >standard XON/XOFF software control?

   Unless you have really oddball modems, it should.  I've done it with 
several brands.

 >From what I've read, it has something to do with setting
 >up a sychronous connection between the two modems.

   Most error-correction protocols (which serve as foundations for data 
compression protocols) don't use synchronous framing (i.e. they strip start 
and stop bits), giving you an incidental 20% boost in performance *before* 
data compression... *IF* you are talking to your modem at a speed higher than 
the reported physical connect speed.

   First of all, tell your modem *not* to adjust "DTE rate" (its RS-232 port 
speed) when it makes a connection, and tell your software not to adjust to 
the reported connect speed.  Now configure your RS-232 port to operate at the 
modem's highest supported RS-232 speed (for newer USRobotics Couriers and most 
V.32bis, that's 38,400 bps; for most 9600+ bps modems, 19,200 bps; for 2400 
bps modems, that's usually 9600 bps).

   This is known as 'locking your port speed'.

   Now simply configure your host computer and your modem to use the same 
type of handshaking (RTS/CTS hardware, XON/XOFF software, or ENQ/ACK 
software)... I suspect that you have this correct already.
 

--  
Geoffrey Welsh - Operator, Izot's Swamp BBS (FidoNet 1:221/171)
root@zswamp.uucp or ..uunet!watmath!xenitec!zswamp!root
602-66 Mooregate Crescent, Kitchener, ON, N2M 5E6 Canada (519)741-9553
"He who claims to know everything can't possibly know much" -me