From: andys@internet.sbi.com (Andy Sherman)
Newsgroups: comp.dcom.telecom
Subject: Re: LD Transmission Quality Comparison
Date: 9 Oct 92 13:32:01 GMT
Organization: TELECOM Digest
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 771, Message 3 of 3

On 6 Oct 92 06:06:36 GMT, gerg@netcom.com (Greg Andrews) said:

> john@zygot.ati.com recently wrote:

>> Every so often, I find that a better rate can be had from MCI or from
>> Sprint by using some plan or another. Each time I have started routing
>> calls over anyone other than AT&T, all hell breaks loose. I find
>> failed conversations aplenty. UUCP sends me messages right and left
>> peppered with 'LOGIN FAILED' or 'CONVERSATION FAILED' or 'LOST LINE'.
>> Frequently, throughput falls from 1400 CPS to something like 300-400
>> CPS. And invariably, my overall bill goes WAY up. Why? First there is
>> the lower throughput. Then there are the billing errors.

> I can't speak to the billing errors, but the modem symptoms you
> describe seem to this Telebit technician like a well-known interaction
> between Telebit's PEP modulation and the echo cancellers used my MCI,
> Sprint, and others.

> Rather than poor quality, the problem was being caused by the echo
> cancellers interfering with the modem transmissions.  When the modems
> stopped sending data to train themselves to the line conditions, the
> echo cancellers would turn themselves off.  The difference in the
> modem 'conversation' between data transfer mode and training mode was
> enough for the echo cancellers to change their behavior.  The modems
> saw an undisturbed line when they trained, so they couldn't adapt.
> Why didn't AT&T lines do this?  They used different brands of echo
> cancellers.  (Perhaps because their network hadn't used fiber optics
> until very recently?)

First off, AT&T has had *some* optical fiber in the network for some
time.  But the particular medium of digital transmission should matter
not one whit for how to do echo cancellation.  The propagation delays
are the same for all terrestrial links.

Yes AT&T uses an echo cancellation scheme that is different from that
used by the other carriers.  Wanna take a guess as to which brand?
Most of the AT&T long distance network is switched by #4 ESS(R)
switches, which have integral echo cancellation on all trunks that are
greater than some given distance.  (Some one of my former colleagues
can tell us if that number is proprietary).  These echo cancellers
automatically drop out as soon as a modem guard tone is detected.
That's why none of the modems had a problem on AT&T.  (It was once
alleged in this forum, by some dweeb or another, that AT&T never had a
modem problem because AT&T didn't do echo cancellation, but that was
B.S.)

> Telebit worked with the engineers from a couple of the echo canceller
> manufacturers for several months.  Eventually a solution was found
> where the modems would be able to keep the echo cancellers disabled.
> The modem firmware was updated to add the 'echo canceller mods'
> starting with version 7.00 (BC7.00, GE7.00, and GF7.00, though the
> T1000 uses FA2.10).

Had the echo cancellers done the right thing to begin with, Telebit
would never have had the problem.

> The point I've been trying to make is that your connection troubles
> might have been caused by this interaction and not by the quality of
> the LD carrier.  It would be unfair to criticize the LD carrier
> because the original PEP modems couldn't keep their echo cancellers
> off the line.

Really?  I consider the inability of echo cancellers to detect a modem
to be a quality issue.  It is not a quality, customer driven solution
to force customers to modify their CPE because the network
infrastructure is of inferior quality.  Remember that there was one
carrier whose infrastructure never had the problem and whose network
could and can deal with the unmodified Telebit modems.  The problem
was not that the PEP modems couldn't turn off the echo cancellers, the
problem was that the echo cancellers couldn't (and can't) detect the
PEP modem without forcing the modem manufacturer to add non-standard
stuff.  You don't consider that a quality metric?  I do.

Besides all that, every now and then somebody posts a throughput test
for the big three carriers, and AT&T almost always wins.  That also is
John Higdon's experience, but I don't know if he's ever posted the
numbers or any $$/KB_transferred figures derived from them.

Needless to say, I speak for neither my former nor current employers.


Andy Sherman   Salomon Inc  -  Unix Systems Support  -  Rutherford, NJ
(201) 896-7018  -  andys@sbi.com or asherman@sbi.com
"These opinions are mine, all *MINE*.  My employer can't have them."