re: Mother Earth Mother Board: A Submarine Cable Laying Odyssey
Jay, the mean time between failures and the mean time to repair metrics
(MTBF/MTTR) of submarine cables are very ambitious. Perhaps more
so than what their actual life expectancies will prove to be at some
point, as obsolescing goes, today.
But they need to be aggressive due to the severe consequences that
would ensue should failures arise, regardless of when they cease to be
economically viable. They are measured in terms of years, and I'm not
exactly sure what the precise numbers are for the newest cables, but they
are near-surrealistic by normal standards for overland systems.
The SDL article to which you are replying, upstream, spells out some of
those numbers.
Many of the questions you or anyone else might have concerning the
laying of deep ocean cables can be found in an excellent primer which
was written in 1996 in anticipation of the laying of FLAG, or Fiber Link
Around the Globe, in Wired Magazine. It's an epoch, so make sure you
have the time to read it in one sitting, since you may not be able to put it
down, if this sort of thing spins your prop. It's titled:
"Mother Earth Mother Board" and was written by Neal Stephenson.
It starts with:
"The hacker tourist ventures forth across the wide and wondrous
meatspace of three continents, chronicling the laying of the longest wire
on Earth...", which can be found at:
wired.com
Some specific answers to the questions you posed upstream are on page 6 of this article, at:
wired.com
Page 6 appears below. Again, keep in mind the time of the writing, 1996:
------snip begins:
So the bad news is that the capacity of
modern undersea cables like FLAG isn't very
impressive by Internet standards, but the
slightly better news is that such cables are
much better than what we have
now. Here's how they work: Signals are
transmitted down the fiber as modulated
laser light with a wavelength of 1,558
nanometers (nm), which is in the infrared range. These signals
begin to fade after they have traveled a certain distance, so it's
necessary to build amplifiers into the cable every so often. In
the case of FLAG, the spacing of these amplifiers ranges from 45
to 85 kilometers. They work on a strikingly simple and elegant
principle. Each amplifier contains an approximately
10-meter-long piece of special fiber that has been doped with
erbium ions, making it capable of functioning as a laser medium.
A separate semiconductor laser built into the amplifier generates
powerful light at 1,480 nm - close to the same frequency as the
signal beam, but not close enough to interfere with it. This light,
directed into the doped fiber, pumps the electrons orbiting
around those erbium ions up to a higher energy level.
The signal coming down the FLAG cable passes through the
doped fiber and causes it to lase, i.e., the excited electrons
drop back down to a lower energy level, emitting light that is
coherent with the incoming signal - which is to say that it is an
exact copy of the incoming signal, except more powerful.
The amplifiers need power - up to 10,000 volts DC, at 0.9
amperes. Since public 10,000-volt outlets are few and far
between on the bottom of the ocean, this power must be
delivered down the same cable that carries the fibers. The
cable, therefore, consists of an inner core of four optical fibers,
coated with plastic jackets of different colors so that the people
at opposite ends can tell which is which, plus a thin copper wire
that is used for test purposes. The total thickness of these
elements taken together is comparable to a pencil lead; they are
contained within a transparent plastic tube. Surrounding this
tube is a sheath consisting of three steel segments designed so
that they interlock and form a circular jacket. Around that is a
layer of about 20 steel "strength wires" - each perhaps 2 mm in
diameter - that wrap around the core in a steep helix. Around
the strength wires goes a copper tube that serves as the
conductor for the 10,000-volt power feed. Only one conductor
is needed because the ocean serves as the ground wire. This
tube also is watertight and so performs the additional function
of protecting the cable's innards. It then is surrounded by
polyethylene insulation to a total thickness of about an inch. To
protect it from the rigors of shipment and laying, the entire
cable is clothed in good old-fashioned tarred jute, although jute
nowadays is made from plastic, not hemp.
This suffices for the deep-sea portions of the cable. In shallower
waters, additional layers of protection are laid on, beginning with
a steel antishark jacket. As the shore is approached, various
other layers of steel armoring wires are added.
This more or less describes how all submarine cables are being
made as of 1996. Only a few companies in the world know how
to make cables like this: AT&T Submarine Systems
International (AT&T-SSI) in the US, Alcatel in France, and
KDD Submarine Cable Systems (KDD-SCS) in Japan, among
others. AT&T-SSI and KDD-SCS frequently work together on
large projects and are responsible for FLAG. Alcatel, in classic
French fasion, likes to go it alone.
This basic technology will, by the end of the century, be
carrying most of the information between continents.
Copper-based coaxial cable systems are still in operation in
many places around the world, but all of them will have reached
the end of their practical lifetimes within a few years. Even if
they still function, they are not worth the trouble it takes to
operate them. TPC-1 (Trans Pacific Cable #1), which connected
Japan to Guam and hence to the United States in 1964, is still in
perfect working order, but so commercially worthless that it has
been turned over to a team at Tokyo University, which is using
it to carry out seismic research. The capacity of such cables is
so tiny that modern fiber cables could absorb all of their traffic
with barely a hiccup if the right switches and routers were in
place. Likewise, satellites have failed to match some of the
latest leaps in fiber capacity and can no longer compete with
submarine cables, at least until such time as low-flying
constellations such as Iridium and Teledesic begin operating.
---------end snip
Iridium (and Teledesic), indeed. Like I stated above, keep in mind the date
of this writing: 1996.
Best Regards, Frank Coluccio
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