Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!decwrl!purdue!bu-cs!bloom-beacon!apple!voder!pyramid!prls!mips!sultra!dtynan
From: dtynan@sultra.UUCP (Der Tynan)
Newsgroups: comp.dcom.modems
Subject: Re: London England and modem speeds
Summary: line optimization
Keywords: How fast can they be? Problems?
Message-ID: <2556@sultra.UUCP>
Date: 4 Oct 88 00:03:55 GMT
References: <311@hrc.UUCP> <41536@pyramid.pyramid.com>
Organization: Tynan Computers, Sunnyvale, CA
Lines: 55

In article <41536@pyramid.pyramid.com>, csg@pyramid.pyramid.com (Carl S. Gutekunst) writes:
> In article <311@hrc.UUCP> dan@hrc.UUCP (Dan Troxel VP) writes:

	[All kinds of stuff about horrible bandwidth deleted]
> 
> >o If so, is there a different way to send the data in a speedily fashion?
> 
> X.25 Public Data Network. We have a 9600bps Telenet line here, and most places
> we call in the U.K. have 9600bps lines to British Telecomm Packet Switch
> Service (PSS). We get a consistent data rate in excess of 500 characters per
> second. Unfortunately, X.25 is hideously expensive; Telenet is charging us
> $1400/month, plus $12 per 64K and $12 per hour of connect time. If you work it
> out, all you need to maintain is a *reliable* 80 cps on a modem to break even.
> If the TrailBlazer worked, we'd completely adandon X.25 to the U.K.
> 
> <csg>

Pay to the order of USENET: $0.02;
I make a *lot* of calls to Ireland, and I can see why the bandwidth isn't
there.  There are a lot of problems with transatlantic communications.  What
is really needed, is a modem (Telebit, are you listening?) which is optimized
for this type of transfer.  With the horrible costs quoted for X.25, someone
could make some money, by modifying the TrailBlazer code.  Some simple
optimizations;
	1.  Bandwidth is allocated dynamically.  For example, a call to London
	    may produce a real low data rate, but a second call a few minutes
	    later may be great.
	2.  The lines are worse than half-duplex.  The primitive switching
	    used to fake full-duplex vox is terrible.  I notice delays of
	    up to a second in switching back.  This can be fatal to a modem.
	    A small packet size, with a very large window may help.
	3.  Echo is prevalent, as well as crosstalk.  This is something that
	    could be filtered out at the modem.
	4.  Ireland is currently switching over to a digital system.  Some of
	    the larger cities are now digital, and the clarity is pretty good.
	    Some of the others, however, are probably using cans!  If possible,
	    make the connection to a large metropolitan area.
	5.  Most of the common-carriers have links to London.  Check each of
	    them (and post the results!).  AT&T may be giving you the old
	    "under-the-sea" cables, whereas someone else may let you use their
	    ultra-super sattelite for the same cost.
There are probably hundreds more optimizations.  What *I'd* like to see, is
some sort of short-wave link (TDMA?), that we could all use, sans licence.
As a hard-and-fast rule, however, whatever common-carrier you choose, will
charge you the same for a lousy quality line, as they will for a good one.
For this reason, the modem should dump the line and restart if the error
rate is pitiful.  Of course, then again, the line quality *should* be better
during business hours (when it is more expensive).  BTW; Carl, you should
think about sub-letting some of your X.25 bandwidth, and spreading the
overhead.
						- Der
-- 
Reply:	dtynan@sultra.UUCP		(Der Tynan @ Tynan Computers)
	{mips,pyramid}!sultra!dtynan
	Cast a cold eye on life, on death.  Horseman, pass by...    [WBY]