Path: utzoo!utgpu!water!watmath!clyde!bellcore!rutgers!mcnc!unccvax!dya
From: dya@unccvax.UUCP (York David Anthony)
Newsgroups: sci.electronics
Subject: Re: 9600 baud modems
Message-ID: <1047@unccvax.UUCP>
Date: 25 Jul 88 16:49:47 GMT
References: <1127@nunki.usc.edu> <478@ns.UUCP> <1044@unccvax.UUCP> <506@ns.UUCP> <1988Jul23.203605.20716@utzoo.uucp>
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 43

In article <1988Jul23.203605.20716@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
> > 	This is how **some** 9600 baud modems work. Real 9600 baud
> > modems work by quadrature amplitude modulation...
> 
> Real 14000 baud modems, mind you, work by the multi-carrier approach
> originally described.  ("Real" meaning "Telebit Trailblazer", of course.)
> Why buy 9600-baud QAM when you can have 14000-baud PEP? :-)

	Not true at all. CCITT specification V.33, which applies
to trellis-encoded 14,400 bit/sec and 12,000 bit/sec (corresponding
to 6-bit per symbol and 5-bit per symbol encoding, respectively) 
specifies only QAM of the carrier. A V.33 modem, when operating
over a suitable dial-through circuit, **guarantees** a throughput
of 14,400 bits per second, period.  

	Unless and until Telebit Trailblazer, or any other of these
"tier-two" high speed modems can demonstrate a throughput of
14,400 bits/sec using random synchronous data, these modems 
will be rightfully considered to be "second class" compared to
the UDS 14.4 Trellis and the Rockwell R144HD. 

	Having transmitted medical images (essentially random
data) through every kind of modem known to God, I can with 
certainty state that the Telebit Trailblazer does not even achive
achieve 9600 bits/sec on random data.  

	The Trailblazer does deal with certain line impairments
better than the earlier QAM modems.  However, the two technologies
are **not the same**, despite what you can do with Fourier transforms
and their ilk. 

	Of course, most people do not send random data (or nearly
random data), the adaptive compression and equalization of the
Trailblazer (plus many manufactuers' nice big "fool UUCP" buffers)
can make burst rates of 14,400 bits/sec possible.  As far as I
am concerned, the only measure of a modem's performance is the
worst case continuous speed at a specified bit error rate. 
Why not run crypt on a file like /vmunix and send that through
the Trailblazer and a UDS. Hmmmmmmmmmmmm?

Yours for truth in modem advertising;

York David Anthony