Path: utzoo!mnetor!uunet!husc6!uwvax!oddjob!gargoyle!ihnp4!cbosgd!mandrill!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: comp.sys.amiga
Subject: Re: 19.2k baud modems
Message-ID: <855@neoucom.UUCP>
Date: 19 Dec 87 16:10:45 GMT
References: <7403@ccicpg.UUCP> <503@cord.UUCP>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 60
Summary: I have a trailblazer


I'd suggest reading comp.dcom.modems to follow the lively
discussion of the Telebit Trailblazer and other modems of various
types.

The Trailblazer  supports the normal 0-300, 1200, and 2400 buad
transmission formats up to v22.bis standard.  If desired MNP error
correction can be applied to 1200 and 2400 baud connections with
other modems that support MNP.  With MNP in effect, transmission
errors virtually disappear; MNP also achieves a 2:1 data
compression, resulting in an effective 4800 baud connection.

In addition to ISO transmission modes, the Trailblazer also has a
proprietary Trellis coded multi carrier system.  Data are
transmitted up up to 512 carriers of 7.6 Hz bandwidth.  An initial
handshake procedure determines what mode is it be used.  If the
"FAST" Trellis transmission mode is selected, the line profile is
examined to decide which of the 512 carriers will be employed.  The
Trellis mode is half duplex.  In theory, if all 512 carriers are in
operation, the instantaneous throughput is 19200 bits/sec.
Typically between me and the vax about 20 miles away, I get 14500
to 16500 bits/sec as claimed by the status report from the modem
(ATs70?).  When two Trailblazers are talking in Trellis mode, AT
commands can be sent to the remote Trailblazer via a supervisory
data channel (AT%...).  This latter feature is very handy.

An optional ROM can be installed to support the uucp g protocol or
kermit.  With the option ROM, the Trailblazers have the ability to
spoof the local host into nearly continuous throughput.  The
Trailblaizers have faily be internal buffers.  They can send
appropriate checksums back to the local host to keep filling their
buffers before a large block is actually sent between the modems.
I've found that I typically get 550 characters / sec in this mode
for uucp.  Most of the limit seems to be with the vax which just
can't keep sending data for sustained periods any faster than that.
Nonetheless, this is about 5-1/2 times the throughput that I get
with a Hayes 1200 modem.  As such, the Telebit should be able to
save its cost in terms of reduced phone bills in about threee months.
(Assuming a commercial rate in-state long distance news feed that
operates 4 hours a day.)

For me, the Trailblaizer has made talking to our vax *possible*.
Our dial-up suffers a strong high-frequency roll-off somewhere in
the web of wiring between the CO that serves the vax and the CO
that serves me.  As such a Hayes 1200 is useless.  The answer
carrier is the higher frequency, which is attenuated by the poor
circuit.  The result is a continuous stream of ~r~r~r_{... at my
end.  The Trailblazer has an adaptive equalizer that is able to
work around the line impairment at even 1200 baud from a Hayes
modem on the vax.  Of course a Trailblazer will not cure all types
of line impairment.  The TelCo was totally unsympathetic with our
complaints about the line.  They said (roughly quoting) "If it's
good enough to hold a voice conversation, that's all we care about.
Tough."

Naturally, I don't have any financial interest in Telebit other
than reduced financial assests on my account.  It is a product that
does indeed do what it say it will in the glossy ads.

--Bill