military@cbnews.att.com (William B. Thacker) (06/04/90)
From: att!jsugate!nak
A bit more light on the fiber optic debate!
[this is from memory] Many years ago (1980 or so),
the Air Force [Boeing may have been in on this] studied using fiber optic
data cable for all instrumentation data feeds. Even with the added
problems of zillions of D/A and mux hardware, the final result was:
On a 747 or C5 class airplane, 2 TONS of copper can can away. The
wiring bundles on these airplanes goes from something as big around as
a person to a handful of fiber cables.
[many advantages, all true, of fiber deleted]
>[mod.note: As a balancing factor, I think fiber optics are more difficult
>to repair. - Bill ]
Compared to what? If you compare it to a phone line, absolutely. I have
fixed a phone line with a paperclip when some idiot chopped it. But if you
compare it to a highspeed coax cable that carries say, radar data, then
it's not so simple. For example, the Hughes radar system circa 1982, uses
twin coax cables with Manchester encoding and no single points of failure.
A single coax run will not take lightly to a noisy coax splice (think
Ethernet and you get the idea). There are two just in case something bad
happens to one. The data rates on that coax are pretty high. They don't
push the limit of state of the art (by design) but they are still out of
the realm of simple phone lines.
I speculate that the EMI in a battle damaged Naval warship is going to be
pretty fierce. To fix the coax, you will need a bit of time and some tools,
and there is no substitute for having done it at least once right. Given
a bit of time, tools, and perhaps some experience, fiber optic cables can
be field spliced and put back into service. As long as the designer stays
well back from the limits, splices can be easily tolerated in a fiber link.
So yes they are more difficult to repair than simple analog or power lines,
but they are not much worse than digital data cables. Fixing the intercom
to the engine room is easy, getting the radar back to the CIC is harder.
Neil Kirby
...att!jsugate!nak