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From: phil@portal.UUcp (Phil Sih)
Newsgroups: net.dcom
Subject: Re: modem evaluation - Telebit Trailblazer
Message-ID: <137@portal.UUcp>
Date: Fri, 25-Apr-86 05:54:16 EDT
Article-I.D.: portal.137
Posted: Fri Apr 25 05:54:16 1986
Date-Received: Sun, 27-Apr-86 05:48:57 EDT
References: <1703@sdcsvax.UUCP>
Organization: Portal Communications, Cupertino, CA
Lines: 90
Summary: remarks on trailblazer

In article <1703@sdcsvax.UUCP>, brian@sdcsvax.UUCP (Brian Kantor) writes:
> I recently had a pair of Telebit Trailblazer modems here for
> evaluation.  These are the multiple-carrier devices that claim 9600
...
> 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 was quite surprised to find out the Trailblazer has more CPU power in 
it than just about any PC out there today. Specifically it has a 68K and
a TI 320 signal processing CPU. I'm even more surprised to hear it has
a fan in a wall-mount power supply! (Never seen that yet.)

...
> 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
...
> modems hooked up, I found the 1-3 second delay quite disconcerting and
> annoying.
> 

Telebit is supposed to be working on or have completed a new rev of their
modem which I was told significantly reduces this problem. You should make
sure you don't have old product. Lower bit rates are FDX.

> 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.

The limit on file xfers in this case may not be the modem/comm line. I heard
Telebit needed the special version of Crosstalk because the PC could not
keep up with the data rate from the modem. Also, since the Trailblazer
protocol is reliable, there is no need for host level packet acks. You
just dump the data and it comes out correct on the other end. That's what
their "Packetized Enesmble Protocol" is supposed to do for you. This makes
me think they just cut out the file level packet acks all together in
their Xtalk.

> 
> 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.
> 

If anyone has any information on this I would very much like to hear about
it.

> 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).

I heard a president from one very well know Scotts Valley micro software
company took one to Europe, hooked the sucker up and got 8-9kbps throughput.
Variable performance is to be expected with this product. It adjusts the
rate at which it can send information over the line based on the channel
characteristics and noise.  There have been some good articles in mags like
Datacommunications covering this and Telebit has some reasonable lit.

> 
> 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.)
> 

They could sell it for a lot less, but part of the problem is there are
not enough applications out there yet that can adequately exploit a >10kbps
dialup connection that is reliable.  I would like to see more myself.

Thanks for your report, this is the first one I have seen from a real
end user.

PS I don't work for Telebit.    - Phil