[Beowulf] propagation velocity in cables

Hearns, John john.hearns at mclaren.com
Thu Sep 6 04:16:12 PDT 2012

Yes, there are laser comm systems, but generally used only for short distances (<10km) like between two buildings. Advantage is that you don't need much power, it's really wide bandwidth (Gbps is no problem) and it's difficult to snoop on (beams are very narrow with minimal spillover).  OTOH, you also have to have active pointing, typically.. the building sway in the wind is enough to break the link.  And, of course rain is a problem.

Fog is also a problem!
Living in Old London Town, it does tend to get foggy on winter days.

I have used laser links in two situations between buildings - once when working at Guys Hospital my co-workers were temporarily
Moved to a building next door, and a laser was used to link them to the network.
Secondly when at Framestore in Soho, we had a laser link running above Oxford Street to our building on the other side.
As you say, worked fine till there was a foggy day!

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