Date: Tue, 6 Oct 1992 06:06:36 GMT
From: gerg@netcom.com (Greg Andrews)
Newsgroups: comp.dcom.telecom
Subject: Re: LD Transmission Quality Comparison
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 764, Message 4 of 4

Looks like I've fallen behind in my TELECOM Digest reading.  Apologies
for responding to a message posted so many days ago ...

john@zygot.ati.com recently wrote:

> ruck@zeta.ee.ufl.edu (John R Ruckstuhl Jr) writes:

>> Can I safely extrapolate -- AT&T LD quality is superior ?

> Regarding modem communications, apparently the answer is 'yes'. I have
> six modems that are communicating with modems around the country and
> occasionally overseas around the clock. Each day I send and receive
> tens of megabytes over long distance circuits.

> 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.

When this problem first appeared, the first conclusion everyone drew
was that the line was bad.  However, the line was perfectly functional
and of high quality, as Telebit's own tests confirmed by holding the
line open and switching the modems to a V.32 connection.  The modems
were able to hold the line effortlessly, even with a sensitive
modulation like V.32, on a line that caused PEP to constantly retrain
until the modems gave up.

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?)

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).

In most cases, the modems on both ends of the phone line needed the
new firmware, though a few calls could get by with just one.
Unfortunately, a compatibility issue with an old version of
TrailBlazer Plus firmware caused many people to turn off the echo
canceller mods by setting S120=16.

If your modem or the other modem has older firmware, or has the echo
canceller mods turned off, you could have been experiencing this
problem when you used Sprint or MCI.

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.


Greg Andrews   UUCP: {amdahl,claris}!netcom!gerg   Internet: gerg@netcom.COM