[Beowulf] propagation velocity in cables

James Cownie jcownie at cantab.net
Thu Sep 6 09:44:26 PDT 2012

On 6 Sep 2012, at 12:16, Hearns, John wrote:

> 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!

You may have been breaking the law! Back in the 70s I believe that he GPO had a monopoly on "mechanical or electrical means of telecommunication". (The Cambridge folk who had an IR laser link from New Museum site to the Engineering department joked about this at the time, and the fact that if read broadly it meant you couldn't talk without breaking the law :-)).

-- Jim
James Cownie <jcownie at cantab.net>

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