TELECOM%usc-eclb@brl-bmd.UUCP (06/29/83)
TELECOM AM Digest Wednesday, 29 June 1983 Volume 3 : Issue 32
Today's Topics: NY Legislature Vs. The Phone Company
Telephone Tag
Robert Weitbrecht - In Memoriam
Old Satellites
Gameline Modem - Cheap Because It's Half Duplex
Modems / Satellites / Mailing Lists
Satellite Inquiry
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Date: 26 Jun 83 18:36:50 PDT (Sunday)
From: Newman.ES@PARC-MAXC.ARPA
Subject: Re: NY legislature vs. the phone company
What a ridiculous bill. Southern California had an area code split
last year and will have another one next year. The first one split
Orange County and San Diego into two area codes; the second will split
Los Angeles from the San Fernando Valley.
The only problems this has caused are a few small municipalities which
were to be split down the middle by the area code boundary. These
were adjusted by moving the line slightly, at the cost of forcing some
people to get new phone numbers. Nobody here would dream of proposing
such a bill. Is the NY State Assembly made of technological
illiterates??
/Ron
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Date: Mon, 27 Jun 83 10:51 PDT
From: Deutsch.PA@PARC-MAXC.ARPA
Subject: Telephone tag
To: HEDRICK@RUTGERS.ARPA
We have exactly the same problem you do with people moving around a
lot from one office to another. My solution is even more idealistic
than yours: have everyone (who wants to) carry a little beeper-like
device, and have a sensor permanently installed in each phone. If you
get within N feet of a phone, its sensor knows you are there, and the
sensor sends that information to the forwarding mechanism. Of course,
the beeper has a switch on it that lets you choose not to receive
calls this way, or to forward them to a receptionist, or (ideally)
switch the caller to a digital audio recorder, or whatever.
We had a phone system of approximately the kind you want some years
back, built by Danray Corp. (don't know if they're around any more).
It was completely programmable -- every signal you could possibly want
to get your hands on was routed into or out of a Data General Nova.
If it had worked reliably, we would have loved it to death.
Please let us know what you come up with.
Peter Deutsch
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Date: 27 Jun 1983 1706-PDT
From: ROODE at SRI-NIC (David Roode)
Subject: In Memoriam
To: Telecom at USC-ECLC
Location: EJ296 Phone: (415) 859-2774
A recent item in the SRI employee newsletter states that "Robert
Weitbrecht, a deaf physicist who had worked in SRI's Communications
Laboratory from 1958 to 1969, died recently as a result of a car
accident. He invented a telecommunications device for the
hearing-impaired which was the forerunner of the acoustic-coupled data
modems we know today. A scholarship fund is being established in his
name by Weitbrecht Communications Company in San Carlos."
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Date: 27 June 1983 22:29 EDT
From: "Marvin A. Sirbu, Jr." <SIRBU @ MIT-MC>
Subject: old satellites
Satellites "wear out" when their stabilizing thrusters run out of
fuel. They then drift away from their assigned orbital slot. Also,
they may start tumbling so that their solar cells and antennae aren't
oriented properly. They then run out of power, or have their antenna
pointed so they can't be heard.
Marvin Sirbu
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Date: 27 June 1983 22:37 EDT
From: "Marvin A. Sirbu, Jr." <SIRBU @ MIT-MC>
Subject: TELECOM Digest V3 #30
The reason the Gameline modem is cheap is that it's not full duplex.
The reason Vadic's and 212A's are expnesive is that they are. The
British television industry has been selling a cheap (under $100)
1200/75 modem as part of a Prestel-equipped television set for several
years.
Marvin Sirbu
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Date: Monday, 27-Jun-83 14:18:59 PDT
From: Lauren Weinstein <vortex!lauren@LBL-CSAM>
Subject: modems / satellites / mailing lists
Greetings. There's no big trick to "medium" speed HALF-duplex modems,
up to 2400 bps or so. Half-duplex, to put it very briefly, is much,
much simpler to implement than full-duplex -- there's very little
comparison, really. So, I would wager that the cheapo half-duplex
modem mentioned in a previous digest will not have any bearing on the
technical development of full-duplex modems.
More technical details on request.
As for "deactivated" satellites... The standard policy now is to
simply switch off the transponders and let the bird "sit". Until the
satellite is REALLY dead, the command receiver would probably still be
workable, IF you knew the codes and had the appropriate equipment to
work an uplink. These birds typically would not be of much use to
anyone, since one of the primary reasons for declaring a bird "dead"
is the exhaustion of the fuel for the steering rockets. Without this
fuel, the satellite cannot be maintained in an exact geosync orbit,
and will eventually drift, presumably to a lower orbit and eventual
disintegration in the atmosphere. While this sounds like a
"collision" risk, there is actually one hell of a lot of open space up
there. Of course, as WMartin suggested, the orbital slot used by the
satellite can be immediately used by a new satellite, since a given
degree of geosync orbit does represent a lot of space. As long as the
original satellite isn't transmitting, there won't be any interference
problems.
WMartin humorously suggested that such satellites might be blown up
when their useful life is over. While this isn't done with commercial
communications satellites, there is much work currently underway on
the so-called "killer" satellites which would perform this task as
their primary mission.
--Lauren--
P.S. HOME-SAT and TELETEXT will soon be combined in one new Internet
list to be called VIDEOTECH, which I will moderate. This new list
will be activated as soon as some technical issues can be worked out,
and I will announce it officially at that time.
--LW--
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Date: 27 Jun 1983 19:58:28-PDT
From: Robert P. Cunningham <cunningh@Nosc>
Subject: Re: Satellite Inquiry
I asked some of the same questions when in Norm Abramson's class on
satellite data communications. Here's some of the answers, though if
anyone else wants to correct me, that's fine--this info is mostly
second-or-third-hand, and I might have misunderstood a few things.
The useful life of a satellite is set mainly by the insurance
companies that insure it. If it fails, they're the ones who have to
foot the bill for a replacement bird. For something like Westar, that
could run in the neighborhood of $40 million. If you run the
satellite longer than the insured period (seems to be almost uniformly
7 years), and it fails...tough; the insurance company does not
replace.
Although transponders tend to go bad over time, and there is
decreasing efficiency from the solar cells that power the things, the
first thing to go is (usually) station-keeping fuel. Without
occasional, care- fully controlled bursts from the gas jets, a
geosynchronous satellite starts to 'wobble' in orbit, tracing what
looks like a figure-8 from the ground (and going beyond the location
that fixed antennas are aimed at). Then it slowly starts to drift
east or west (depending on where it is) out of its 'orbital slot'.
The orbital perturbations are caused by small gravitational anomalies.
If the earth was a perfect sphere (or even a perfect oblate spheroid),
there wouldn't be any problem. There are two reasonably stable points
in geosynchronous orbit: a kind of a metastable point over north
america, and the global stable point over India. Completely dead
geosynch satellites drift towards one of those, most towards India.
In the very long term (how many thousands of years?), of course,
complete orbital decay will eventually set in. Meanwhile, over the
next few centuries, expect to see a collection of dead satellite over
India.
When 'turned off', usually the things go into standby mode, awaiting a
coded control signal from earth. Incidently, the telemetry and
control transceivers have much less gain than the regular broadcast
transponders. To talk with a bired in standby mode, you need a very
large, steerable antenna ('class A', at least). There's probably only
a dozen or so of these in the world.
If Western Union wants to keep their bird as an in-orbit spare,
they'll just juke it slightly away from its orbital slot (to make room
for another active satellite, not necessarily theirs), and expend some
station keeping fuel once or twice a year to keep it from drifting too
far east (I think it would be east, might be west). Then, in a real
emergency (a new satellite goes out, and there's a delay until a
ground spare can be launched, for instance), they can always
reactivate it.
I supose they could give it away, but whoever took over ownership had
better have a ready collection of high-gain 4/6 GHz antennas and
associated paraphenalia ready. Also, they'd have to pay someone (WU?)
to control the bird, monitor telemetry to check what's happening, etc.
And, perhaps most importantly, they must have authorizations to use
whatever orbital/frequencies slot they want to use up there. It's
getting pretty crowded up there (one reason why SBS whent to higher,
suboptimal frequency bands), and the slots are negotiated years in
advance, and involve international as well as national-level
bargaining.
Bob Cunningham <cunningh@nosc-cc>
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End of TELECOM Digest
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