Date: Mon, 12 Oct 1992 10:25:33 -0600
From: rickie@trickie.ualberta.ca (Richard Nash)
Newsgroups: comp.dcom.telecom
Subject: Re: LD Transmission Quality Comparison
Organization: TELECOM Digest
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 777, Message 4 of 7

andys@sbi.com writes:

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

>> 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?)

Detecting the guard tone is the basis under which all echo-cancellers
work.  What isn't described, is when do they drop back in? As soon as
the modem guard tone is once again detected?

>> 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 above statement is rather arrogant! Echo cancellers have been
around for several decades and the operational specifications for them
are well known by telco engineering standards groups. Telebit PEP
modulation scheme has not. Perhaps the echo canceller manufacturers
should be issued with crystal balls that allow them to see how future
contraptions will affect their product?  Only an engineer, would have
the audacity to presume that a marketed product *they* designed was
free of problems. Almost everyone who has attempted to use a modern
modem has encountered some degree of operational difficulty with it.
As is well known, data transmission greater than 1200 baud on a
switched circuit is a recent thing. Modem manufacturers have spent
considerable R&D to engineer products with faster data rates *around*
the limitations of the switched network.  Some modem designs are
better thought out than others.  Obviously the engineers at Telebit
never took the echo canceller interaction into account because they
were unaware of it!  As described, the problem was corrected by
Telebit.  This scenario of revising a product to address a previously
unanticipated problem is very common as anyone in the manufacturing
industry knows. Adaptability!

> 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.

Sounds like the Total Quality movement has hit your company, eh?  This
kind of thinking reminds me of the time I was on a hardware course,
and the work assignment was to manually enter a small hand written
program.  A fellow student had quickly coded the program and dumped it
into the core.  When he attempted to execute the program, it failed to
run properly.  Being a rather hyper type of person, he immediately
afixed the blame on the computer rather than his hand loop.

In case you haven't noticed, Telebit modems do their own thing. My
v.32bis modem doesn't talk to the Telebit in it's native tongue.  The
never ending battle over whose product will become the defacto
standard is an issue that will only die when the demand for the
product also dies. Perhaps when data modulation is no longer required?
(ISDN?)


Richard Nash     Edmonton, Alberta Canada T6K 0E8
UUCP:         trickie!rickie@ersys.edmonton.ab.ca