Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!romp!shaffer!jon
From: jon@shaffer.UUCP (Jon Doran)
Newsgroups: comp.dcom.modems
Subject: Re: Full duplex / Half Duplex with Telebits
Summary: Half Duplex with Telebit modems
Message-ID: <2486@shaffer.UUCP>
Date: 28 Jul 89 20:08:01 GMT
References: <630@lakart.UUCP> <7461@cbmvax.UUCP>
Organization: IBM AWD, Austin, TX
Lines: 19

Telebit uses half duplex connections to avoid complicated echo cancellation.
At each point where a modem is connected to a phone line, there is a small
amount of echo.  Thus a typical connection has two sources of echo.

Now echo cancellation on a V32 connection is not THAT hard.  V32 uses two
frequencies near the center of the bandwidth, and the echo tends to shift in
the band.  But Telebit's PEP (not their V32!) does not use single character
modulation, they use 512 frequencies distributed over the band.  So the echo
can appear to be valid traffic even after the frequency shift.

The only easy solution at this point is to use half duplex connections, the
echo is ignored completely.  This does not cut into your bandwith; V32 used
two frequencies for full duplex so Telebit would either need 1024 frequencies
or would need to halve their transmission rate.

BTW, a little T2500 trivia: Did you know that the T2500 is a 7.3 baud modem??

Jon Doran
IBM AWD, Austin Tx