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From: brian@sdcsvax.UUCP
Newsgroups: net.dcom
Subject: modem evaluation - Telebit Trailblazer
Message-ID: <1703@sdcsvax.UUCP>
Date: Wed, 23-Apr-86 12:49:15 EST
Article-I.D.: sdcsvax.1703
Posted: Wed Apr 23 12:49:15 1986
Date-Received: Sat, 26-Apr-86 04:10:29 EST
Organization: UCSD wombat breeding society
Lines: 78

I recently had a pair of Telebit Trailblazer modems here for
evaluation.  These are the multiple-carrier devices that claim 9600
bits/sec or better on a normal dial-up phone line.  The price seemed to
be around $2000-$2500 or so, but was clearly adjustable.

They apparently to work by packetising your input, spreading the bits
out over a bunch of low-baud-rate carriers, reassembling that at the
far end, and unpacketising it.  There is some sort of error checking
and correction (retransmission??) as part of the
packeting/unpacketing.  Or at least, that is how the salescritter
explained it to me - the manual was kinda thin on that point.

These things are sophisticated - they have enough circuitry in them
that the wall-mount power supply is larger than most, and there is a
(thankfully quiet) cooling fan.  I'm told it has a 68000 in it.

I tested them both in interactive dial-up service (vi, rn, and other
typical things you'd do in a normal Unix session) and in file transfer
(uucp, kermit, xmodem) applications.

In interactive use, they are FAST!  It's lovely to see your screen
paint in just a second or so.  Its really GREAT for paging through a
file or reading news.  BUT:  These modems aren't full-duplex in the
FAST mode.  They apparently simulate full-duplex by turning the line
around quickly, but there is a very noticeable echo delay when using
them with a terminal.  This is especially true when using 'vi', which
often generates a burst of cursor positioning commands in response to a
single keystroke.  It looks jumpy.  In the several days that I had the
modems hooked up, I found the 1-3 second delay quite disconcerting and
annoying.

Note that in 300, 1200, or 2400 bits/sec mode, there is no delay - it
works just like my USR Courier - perhaps better.

In microcomputer file transfer use (kermit and xmodem) it works pretty
well.  Unfortunately, it doesn't seem to run all that fast!  Apparently
the packetising and turnaround delay are enough that the protocol
doesn't run at anywhere near full speed because its waiting for the
acknowledgement of the previous packet.  Undoubtably a protocol that
allows for more than one packet to be in flight would work much
better.  Telebit has a version of Crosstalk for the IBM PC that they
claim makes very fast file transfers - I guess its tuned for use with
their modem's time delays and error correction characteristics.  I
didn't test it.

With uucp, these modems work really well.  However, the uucp transfers
weren't much faster than those at 480 char/sec.  I've seen (somewhere
in the dark past of the net) a note that showed that normal uucp gained
very little speed running above 4800 baud.  It is probably that effect
that I'm seeing here, rather than anything the modem is doing.  Thus
there is some question in my mind as to whether it would be worth
having this fast a modem for uucp use.

The error correction seems to work very well.  I can't recall having
seen even one garbled or spurious character in the 3 or 4 days I was
using the modem.  We're on a NT DMS-100 telephone switch, and its real
unusual to see that low an error rate.

Also, the modem maintains a couple of statistics, including the average
bit rate being achieved.  It seemed to run around 17Kbit for my usage,
and slightly lower for another person who tried them from a bit farther
away (presumably a slightly different quality of the phone
connection).

Conclusions: for windowed protocols that don't falter on line
turnaround delays, and for interactive use where you either can use
local echo or tolerate long echo delays, these will work well.  I'm not
going to buy any of them now, because the price/performance ratio for
our applications just doesn't seem to make it worthwhile.  (Especially
since 9600 baud modems are supposed to be just around the June corner
at about $700 each, or so the salescritters would have me believe.)

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   "There is more harmony in films than in life."
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