Re: Submarine Cable Breaks, Splicing, DWDM
Hi Dennis,
"Reminds me of items in the GBLX prospectus about undersea cables and the risks caused by "fishing" trawlers... "
Now you are bringing me back in time. But I've kept up. Fishing trawlers are still a nuisance, but the problem has been somewhat reduced due to the trenching and burying techniques that are now used when placing undersea cables on the continental shelf. But those fishing nets still wreak havoc, to be sure.
It was far more problematic during the cold war (for reasons you could probably deduce on your own), especially prior to the advent of the "sea plows" that are now in common use. At one time (don't know if this is still a common practice) AT&T and the international record carriers (RCA, ITT and WUI, at the time... today the number of carriers would number into the dozens) would maintain a fleet of small aircraft. These single engine craft would routinely shower fishing fleets, who were not always within their rightful lanes, with leaflets strongly advising them to avoid fishing in the cable lanes and in areas next to the 'cable heads' which were on shore.
I'm somewhat curious about the technology used to repair these cables - splicing a 700-odd fiber cable can't be that trivial, especially when it's an undersea cable!"
Submarine cables of the transoceanic class rarely have more then a dozen or so strands to their construction. This seemingly low fiber count has to do with a wide range of parameters and pricing considerations, not the least of which is the onerous powering of deep sea repeaters which is a requirement of each lit strand, from the land based cable stations. Of course, another factor is the gain factor (efficiencies) of dense wavelength division multiplexing (DWDM) techniques which are now in wide use.
Through DWDM they are able to create dozens of additional "virtual strands" per each physical strand. And, at the same cost of powering an individual strand in the deep sea section - which, you might recall, was one of the biggest challenges and cost considerations of the entire undersea pull.
It doesn't make sense, in this case, to lay more strands. But it does make eminent sense to provision the most gainful DWDM.
How are the severed ends of the cable found in deep water? A land based operator can tell to within a very precise distance where a break has occurred through a variety of means. Usually, through some optical reflectometry method which by now is most likely incorporated in transponders within the deep sea repeaters, and telemetered back to shore [or to crew on board ship... I would imagine that in some cases it is first sent to shore and then sent back via satellite to the crew operator aboard ship... talk about a round about way of getting there].
The transponder works by "pinging" and waiting for a measured [calibrated] response from the surface of the break: Broken or severed fiber ends reflect power back to the transponder source.
In reality, this only takes a couple of hundred microseconds. The end product of this is a screen based presentation which shows within meters where the break has occurred.
[If my memory serves me correctly, the metal electrification member which is used for powering [which is at the center core of the cable] also puts out a periodic hig freq sounding of its own, but I'd have to check on this to be sure. If I am mistaken about this I shall retract it.]
Whether divers are used or not depends on where the break takes place, and how deep the water it is. Assuming that it is in deep water, once the break is located, one end is fished out of the water and attached to a flotation device. Then the crew goes for the other end which receives the same treatment. When both ends have been secured, if they cannot be brought together for a direct splice (as is usually the case), a new fiber section is spliced at both broken ends, serving as a bridge between the two.
The worst kind of subsea fiber break, ironically, is the one that occurs on the continental shelf, where the fiber is buried.
The problem here comes into play when divers go down and attempt to uncover the earth from the cable, using blowers. You guessed it. The blowers tend to turn up the top layer on the ocean floor, causing zero visibility for hours on end. It could be a long and laborious task just identifying where the cable ends are located.
I have always been fascinated by submarine cable operations. Thanks for bringing this question to the board.
Regards, Frank Coluccio |
