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From: mark@cbosgd.UUCP (Mark Horton)
Newsgroups: net.dcom
Subject: Re: modem evaluation - Telebit Trailblazer
Message-ID: <2060@cbosgd.UUCP>
Date: Tue, 29-Apr-86 17:10:16 EDT
Article-I.D.: cbosgd.2060
Posted: Tue Apr 29 17:10:16 1986
Date-Received: Fri, 2-May-86 09:25:44 EDT
References: <1703@sdcsvax.UUCP>
Organization: AT&T Bell Laboratories, Columbus, Oh
Lines: 49

I haven't used the Telebit modem, but I have used the UDS 9600 A/B,
and this review sounds remarkably similar to my experience.

When I first got the UDS, I had a pair of them talking over a dialup
line.  Since the modems are sync/half duplex, you put a box called
an EC100 on each end - that box converts to async, full duplex, and
does an error correcting protocol as well.  (Total cost: about $2000
for each modem plus $500 for the EC100, or about $5000 for a pair.)

The error correction works well - it's quite nice for terminal use,
as the noisy phone line }i type problems go away.  But this does not
mean you don't need a higher level checksum, since you can still get errors
in the RS232 link between the modem and the computer.  For file transfer,
disabling the checksum is akin to using cu and ~%put at 9600 baud over
a hardwired RS232 link - ill advised.

But the half duplex nature of the connection is VERY important if you are
using it for interactive UNIX type terminal uses.  In particular, echoing
of characters is slow because, for each character you type, it has to send
the character, turn around the line, echo the character, and turn the line
around again.  This can take about 1/2 second per character (you didn't say
how fast the Telebit does turnaround) and is quite annoying for interactive
users.  It feels as if the system you are using is heavily loaded, and the
user gets mentally fatigued pretty fast when using a system like this.
For applications where you don't need echo (UUCP, reading news) it's great.

One thing I tried that I didn't see mentioned was to run TCP/IP/SLIP over
such a connection.  This worked quite well, but again, if you telnet or
rlogin over the link, the same echo problem occurs, and it feels sluggish.

Another problem with the UDS is that it really wants to be used over a leased
line, with a given pair of modems configured for each other.  Using it as a
dialup replacement for general purpose UUCP would never work, since there are
several jumpers that must match the modem on the other end, and you must tune
these jumpers for your telephone connection.  (For example, the line turnaround
time could be adjusted to 30, 50, or 150ms, the baud rate and fallback baud
rate must match, carrier detect level, training methods must all match.)
UDS was very helpful in configuring it over the phone, but this isn't really
intended as a dialup modem.

Eventually I got a 4 wire leased line (cheap since it's within one central
office) and reused the same modems in 4 wire mode.  Now it's essentially a
6 mile 9600 baud RS232 cable, missing most of the control signals.

I'm very interested to see how the V.32 technology works out.  Is it true
that the only one out now (from Concord) runs $2500, but the prices should
be down to $700 "Real Soon Now?"

	Mark