* Posts by Terranesia

4 publicly visible posts • joined 5 Mar 2012

Half of Milky Way's mass found in million-Kelvin gas cloud

Terranesia

Thanks for this. A refreshing change from some bloke being reported to have had some problem with some Apple thing (and I'm not in the Apple camp).

Submarine cables get simpler, faster

Terranesia
Windows

Attenuation is a problem

Hi Alan Brown, Rustident Spaceniak

Attenuation is the problem that limits how long the cable can be. Multiple vendors now offer the DSP that means the dispersion of a cable to be compensated (equalised) allowing thousands of kilometers with no inline dispersion compensation. Loss on the other hand limits you at best to several hundreds of kilometers before an optical amplifier is needed if you wanted to do something like a cable link from an island to a mainland. Crossing something like the Atlantic requires an optical amplifier every 50km or so due to noise considerations. There's no way around it.

Since the Atlantic was mentioned - if you assume something like 6500km of a cable route between Cornwall and New York and a state of the art loss of about 0.15dB/km you still have 1000dB of attenuation which is 100 orders of magnitude. To convert from optical to electrical I need to detect a signal of about 10 microWatts in NY. Therefore I would need to launch something like 10 to the power of 95 Watts to overcome the fibre attenuation. Give that our sun is outputting about about 10 to the power of 26 Watts you can see how big the attenuation problem is and how the DSP can't address that problem. We can probably argue about an order of magnitude or two in my above hand waving, but we're not going to find the missing 70 or so orders.

If we are just confining ourselves to shorter island hopping systems, then the cable is on the continental shelf and and still needs plenty of steel armouring to protect against drag fishing nets, anchors and so forth. This completely dominates the weight of the cable compared to any in-line repeater housings, so again the article is misleading.

Terranesia
Facepalm

There's nothing new here

And it's misleading. The big problem to go any significant distance is attenuation and that's a fundamental problem that doesn't go away with a fancy product name :-)

Wireless breakthrough: one frequency, multiple signals

Terranesia
Windows

In the late 90s when the bubble was expanding a Californian start-up called SilkRoad made all sorts of outlandish claims (that amounted to nothing) for optical fibre transmission and when I read this I was reminded of the similarity in language used. That's all I can say about this latest news as I have no deep understanding of radio.

The SilkRoad brochure was hilarious and included a history of optics/physics with a lineage that basically went from Da Vinci to Newton to Einstein to the founder of SilkRoad.

http://www.pcmag.com/encyclopedia_term/0,2542,t=SilkRoad&i=51365,00.asp

Despite what PC mag says, the prototype systems were not impressive and 15 or so years later fibre optics is busy plundering existing radio techniques to increase bandwidth and there's not even a hint that there was anything more to the SilkRoad hype than encouraging investors.