Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!olivea!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: PEP vs. v.32
Message-ID: <3828.27d3d869@hayes.uucp>
Date: 5 Mar 91 17:42:01 GMT
References: <6927.27D24513@zswamp.fidonet.org>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 43

In article <6927.27D24513@zswamp.fidonet.org>,
root@zswamp.fidonet.org (Geoffrey Welsh) writes: 

>    My disappointment has doubled.  At first I believed that Telebit & co. were 
> simply not pushing the technology.  Now I that I have reason to believe that 
> they are, I can't avoid the conclusion that they are doing so poorly!
> 
>    If there are 511 carriers operating at a maximum of 8 baud with 4 bit QAM, 
> the maximum possible raw throughput is in the vicinity of 16000 bps, a figure 
> which I have seen Telebit modems approach on clean lines.  If, as you say, 
> they're using a 64 point constellation, the maximum raw throughput should 
> be around 24000 bps, and I've never seen a TB come even close to that.

Of course, the problem is that your assumptions are wrong.  If you 
had 511 carriers with 8Hz frequency separation, you'd need at least 
4088Hz of bandwidth -- and a typical telephone channel only has 
about 3400Hz of usable bandwidth (and that's pushing it).  

In reality, DAMQAM only uses 395 carriers, sending about 7.6 symbols
per second per carrier, with 6 bits per carrier MAXIMUM, for just a
tad over 18,000bps maximum throughput (less protocol overhead).  Two
additional carriers are used for "pilot tones" for timing recovery.
To use all 397 tones at their maximum bit capacity would require a
perfect circuit over a bandwidth of 3100Hz.  In the real world, such
circuits simply do not exist.   Even if they did, PEP still couldn't 
achieve this full throughput, because you have to subtract out 
roughly 3% for protocol overhead, and a few percent (I don't know 
exactly how much) for line turnarounds to acknowledge data (it is, 
after all, a half-duplex modulation scheme).  I'd say that 16,000bps 
was pretty good, considering.

Not that I'd compliment Telebit or anything.  God forbid.

What _I_ want to see is their new V.fast proposal with echo 
cancellation that supposedly really does run at 24,000 bps!  But, 
based on their track record of not submitting promised CCITT 
contributions on schedule, I'll believe it when I see it.

-- 
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