0004133373@mcimail.com (Donald E. Kimberlin) (06/21/91)
continues a thread Mark Miller started ("What the Heck is Digital
Fiber Optic Quality Anyway?">, quoting Mark, then responding:
> Mark Miller wrote:
>> as I understand it, virtually all long distance calls are sent in a
>> digital format.
> I believe this is in error. Although AT&T is actively replacing the
> analog facilities within their VAST system, I'm sure they have a LONG
> way to go.
> It wouldn't surprise me to find that only HALF of their traffic makes
> the long haul digitally -- perhaps even less.
Au contraire, Monsieur Redelfs! Not only has digital transmission
been what supplied the explosive expansion of public telecommunica-
tions in North America and worldwide; it is also been so increasingly
cheap to install and operate that the vast majority of analog plant
has been taken out of service. The real reasons for this were money
reasons; no more, no less. (Can you say "Cheaper ... VASTLY Cheaper!"?)
In the case of AT&T, some may recall that almost two years ago,
AT&T took a large write-down to "retire obsolete plant". That was the
end of the analog era for AT&T. And, MCI did a similar thing about a
year later, with similar effect. Sprint's claim to fame is that they
started up building a new network that was all digital from the
beginning. (But in fact, they were really only expanding nationwide
and replacing the far smaller analog plant their antecedents had built
beginning back with Southern Pacific Communications Company in the
1970's..)
And the "digital revolution" has gone far beyond the interstate
common carriers, too. The Local Exchange Companies (LEC's) have
similarly been tossing out analog plant at an incredible rate,
replacing and expanding it with digital transmission plant. Only one
reason: Cheaper.
Continuing Mark's questions and Jim's replies:
>> is this "fiber optic quality" spiel just some marketing drival
Jim responded:
> ....there is certainly a lot of truth to the implication that data
> transmitted via optical fiber is usually of a higher quality than that
> which is not.
I have to take issue here, Jim. Once your inherently analog speech
(or analog modem data signals) have been digitized, any difference
in error rate due to the underlying transport medium is largely
invisible to you. The "last mile" to your house is,with rare
exception, still analog copper, while "the last few miles" across your
LEC is still very likely digital transmission on copper. This means
your "fiber optic quality" phone calls have to transit several miles
of copper at each end, anyway.
If you happen to be on a campus or in an industrial office of any
size, it's pretty likely your signal is digitized as it leaves your
premises, for purely economic reasons. On just two wire pairs that
once held only two local analog phone lines, your LEC can get 24
circuits. That's done with electronics that costs less than all the
copper it would take.
If your location is really large (a thousand people or so, and
new, such that a lo: of copper would have been required), it's likely
your LE C has run fiber right into the premises, because fiber gives
them a further economic advantage over larger copper cables ... plus
room to expand electronically and not have to pull in even more cable
in the future.
And it needn't be either fiber or copper, either. There are MANY
circuit-miles of microwave radio that bears digitized signals, too.
Again, once your signal has been digitized, you can't tell what the
transport medium is. Digital transmission is, at once, both a
blessing and a curse. When it's working, it works perfectly.
Luckily, that's most of the time. But when it fails, it fails
flat...and it can go from perfect to failure in microseconds, and even
toggle between the two conditions at microsecond rates.
One thing to be said for analog: When it's about to fail, you can
hear the noise rising, perhaps in sufficient time to do something
about it. Not so with digital. What makes digital such an
improvement is :hat it's rather unforgiving to its installers. They
usually can't get digital to work if they don't do the job with
reasonable skill. Old analog could be made to "work," however badly,
leading an incompetent installer to tell you that you should be
grateful you got anything through!
"Olde Analog" really wan't as bad as it was made out to be. Its
image problem is that so many people got stuck with so much badly-
installed and maintained stuff, they think thxt was intrinsic to
analog transmission.
Just for a frame of reference: One of my projects in 1971 (Egads!
Two decades ago! I'm getting old!) involved 25 analog four-wire
circuits that ran 2400 bps 24 hours a day seven day a week, 365 days a
year, between Manhattan Island and San Francisco. Each of the 25 data
streams had a TDM on it that ran 46 international Telex channels at 50
Baud ... no error correction, no nothing. Couldn't stop; had to
transmit everything presented "live."
On one channel of each of the 46-channel TDMs, we ran the
CCITT-standard international test message, "The Quick Brown Fox Jumps
Over The Lazy Dog 12345 67890." (It's in the CCITT book that way.) We
kept the telegraphic transmitters and receivers calibrated by the
standards books, and scored EVERY character error that occurred. Our
typical character errors were three per 24-hour DAY. One time, one of
them dropped ZERO characters for THREE DAYS!.
This kind of performance was with "ordinary" analog circuits ...
circuits that merely met the tariff limits and plant objectives of
both AT&T and Western Union (we bought some from each, and Western
Union's was all microwave all the way across the country). Oh, we
didn't have circuits that were "in spec" when we started out. We had
to refuse and test and retest and nagotiate and threaten and deal
hard. But magically, once they found out we would take no less than
what they advertised and contracted for, the latter two-thirds of our
circuits "met spec" when presented to us.
So much for "analog being deficient." It's only as deficient as
you will accept. Talk about "modern applciations NEEDING digital" is
just sophistry. In fact, in a moment, you'll see that analog is
coming back.
Jim continues with:
> As far as I know, ALL transmissions over fiber ARE digital, whereas
> NOT all (at LEAST!) transmissions over OTHER type of plant is.
This seems a bit garbled, but I take it to mean that"other plant
is likely analog." N{t so, as explained previously. However, until
very recently, fiber as used for multichannel "telephone network"
transmission distances of mroe than a mile or so had only digital
electronics to use on it. Our developments in coherent modulation
techniques at light frequencies were roughly where radio was at the
turn of the century, simply "blasting away" with poorly-shaped,
relatively unstable light generators. This has the effect of
REQUIRING digital transmission on fiber optics as a practical matter.
The jargon for converting older copper plant and microwave to digital
operation is called "digital over-build" in the trade. (There! You
learned some "inside talk!")
But now where are we in 1991? When you read about "erbium
amplifers" being applied to fiber cable, you're reading about ANALOG
devices being applied to fiber ... because the technique has now been
improved such that purer, cleaner lightwave signals can be generated
economically ... and AMPLIFIED. "Digital transmission" implies
regenerating the signal at intervals along the line, retransmiting a
new, cleaned-up pulse signal; getting rid of distortions and noises
that can accumulate along the way.
Jim concludes with:
> Hear a pin drop? So what?!! A fat lot of good that kind of "quality"
> means when there's a Sports Illustrated "SneakerPhone" on one end and
> yet another CheapieChirper phone on the other!!
Absolutely correct, Jim. The vast majority of the public just
messes up what's delivered to them anyway. The thing that made a "pin
drop" audible was that four-wire transmission has already spread close
enough to your phone that transmission levels could be bumped up (with
less noise also) so you could hear it without the noi~e. Whether
fiber or radio or copper was used to bear the signal was of little
importance. Of course, since Sprint had an all-fiber plant, they were
at the time FORCED to use digital operation by the lightwave
technology of the time. In sense, Sprint's marketeers probably led
themselves to believe that fiber was "the only way."
The real truth is that in the world of natural physics, there is
no such thing as a digital function. Every electrical signal has some
small amount of "slope" as it rises, some "overshoot" when it reaches
full level, some amount of "slope" when it falls, and some degree of
"bounc" when it hits bottom. While these can be ignored in many
applications, they are artifacts of the intrinsic analog nature of
their transmission medium.
And now, even fiber optics is finding the wa~ to maximize capacity
is to use analog techniques underlying your "digitized" transmission.
If you read the fiber optic engineering journals, they're now saying
hurrahs for analog technology ... and advertising for analog-smart
engineers and designers, because the universities stopped teaching
"analog stuff" a while back, and all the "old analog people" have
retired.
Were Shakespeare here today, he might say, "All the world's an
analog stage upon which digital play only a bit part." <Groan.>