Path: utzoo!mnetor!tmsoft!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: Practical Peripherals 9600 SA
Message-ID: <3703.2771fa36@hayes.uucp>
Date: 21 Dec 90 12:04:06 GMT
References: <1990Dec17.144237.19679@isis.cs.du.edu> <5134@husc6.harvard.edu> <7264@plains.NoDak.edu> <~+L^*1*@rpi.edu>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 37

In article <~+L^*1*@rpi.edu>, yee@pawl.rpi.edu (Lester W Yee)
writes: 

> I have a question:  Sometimes when there is a long pause, I can see garable
> characters on the screen.  The computer is waiting for the otcomputer  
> (host) computer to get data.  I see garable characters, but it's like they 
> are not there.  ON WWIV BBSes which I call, after seeing the garable string,
> I immediately check the message that I was typing in, and the characters 
> weren't there.  It's sort of like these were invisible characters, that MY 
> computer saw, but the host computer didn't see anything strange. 

V.22bis modems use frequency division multiplexing to acheive 
full-duplex transmission.  The calling modem transmits on the low 
frequency band (1200Hz carrier), and the answering modem transmits 
on the high frequency band (2400Hz carrier).  The calling modem is 
thus more in the center of the channel, where the amplitude response 
and delay is more "flat"; the high channel, transmitted on by the 
answering modem, is generally somewhat more impaired and more 
susceptible to noise.  It is therefore not uncommon for you to 
receive garbage characters that the other end doesn't see -- because 
they weren't generated by you and ECHOED by the other side, but were 
generated by noise in the high band which only the reciever on the 
high band (in your calling modem) would see.

You could just have a slightly noisy phone line, or a slightly less 
than perfect modem (no modem is really perfect in terms of never 
seeing errors), etc.  It's nothing to do with you software, I'm 
pretty sure.  It is these minor, annoying line noise problems that 
modem-based error control (V.42, MNP) was designed to eliminate.

	-- Toby

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-449-8791  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