Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!sdd.hp.com!elroy.jpl.nasa.gov!decwrl!sgi!vjs@rhyolite.wpd.sgi.com
From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.dcom.modems
Subject: Re: PEP turnaround time
Message-ID: <105588@sgi.sgi.com>
Date: 22 May 91 17:56:56 GMT
References: <9105210700.AA14518@ucbvax.Berkeley.EDU> <1991May22.002144.2198@netcom.COM>
Sender: guest@sgi.sgi.com
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 22


> >Have I read that for the Telebit proprietary modulation scheme it takes
> >about 1 second to turn the line around?
 
> Over clean lines the typical single-character turnaround time of PEP is
> around 140-160 milliseconds.

The single-character turn-around time is not what people notice.  Switching
among packet sizes takes a long time.  For example, the ping command reports
ICMP ECHO's over a SLIP line using PEP typically take >1.1 seconds.  If you
turn on the speaker, you can hear the modem laboriously switching between
7 and 18 baud (or whatever the exact numbers).

I've long thought that Telebit missed a bet by not using a more hysteresis
in their packet/baud switching.  If they'd pick a speed and stick to it
longer, much of the jerkiness of interactive traffic over PEP would go
away.  This is obviously the problem with ping's over PEP.  It's the
problem with control-L in a vi window.



vjs