Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!cbmvax!grr
From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.dcom.modems
Subject: Re: Full duplex / Half Duplex with Telebits
Message-ID: <7461@cbmvax.UUCP>
Date: 28 Jul 89 00:03:55 GMT
References: <630@lakart.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 31

In article <630@lakart.UUCP> dg@lakart.UUCP (David Goodenough) writes:
> As is well recorded, when a pair of TB modems are PEP'ing at each other, only
> one end gets to talk at a time. Now, the modulation used by the TB uses many
> different tones and presumably the demodulation does some real clever trickery
> with DSP to achieve the equivalent of a bandpass filter with a *REAL* high
> Q factor for each tone. Now, why is it not possible for one end to use some
> of the tones, and the other end to use some others, and achieve full duplex
> communicaions, at the cost of reduced transfer rates (i.e. 18000 in one
> direction, or 10000 in one direction, and 8000 in the other).
> 
> Since the system is able to dynamically select a subset of the tones anyway,
> it would seem that most of the ability is in place. However, it can't be
> done, and I'll bet there is a good reason. What is that reason?

The technical issues are that running in a full-duplex modes requiers
better filter/line hybrids and/or a guard band between frequencies to
minimize the effect of the relativly strong transmitted back channel on
the adjacent receive channels.  Now it may be that Telebit felt that
this hurt worse then the costs associated with running the packets in
half-duplex style.  You also have to decide whether to implement a fixed
B/W back channel supported by conventional modem techniques or to do
dynamic channel splitting based on thruput requirements.  Trying to
timeshare the DSP resource to handle simultaneous transmit/receive
processsing could be a major effort.

On the other hand, perhaps they simply weren't interested in this area
and thought they had the reverse thruput issues covered adequately...
-- 
George Robbins - now working for,	uucp: {uunet|pyramid|rutgers}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@uunet.uu.net
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)