Wednesday, April 25, 2007

Optical link hacking

I saw this article on The Register and thought it was worth commenting on. Basically they've found that you can bend a fibre and get enough signal out of the bend to reconstruct the data stream. I can only imagine that this has been tried on mono-mode cable only (mono-mode transceivers are a lot more robust when it comes to dB loss). If you've ever ended-off a fibre you know how fragile the glass is - having to open up a loose-tube cable, scrape off the coloured ident paint, attach an optical coupler and bend the fibre just enough to get leakage but not have it break would be a real feat! I know in military applications they de-tune the sender to the point where the receiver has less than 3dBs of tolerance - any mucking about with the cable makes the link fall over and an engineer investigates. That's why the MOD ban UDP traffic on their fibre networks - they know the 3-way TCP handshake requires a link that hasn't been interfered with.
I started thinking about whether you could try this with multi-mode fibre - it's a lot thicker (62.5 microns as opposed to typ. 9 microns) and so physically is a lot easier to manipulate. However, after spending last weekend measuring the response of multi-mode cables that had been only mildly abused I realised that where you can tolerate 10dBs of loss on a mono-mode fibre (even at 10 gigabits per sec) you rapidly run into trouble at even half that data-rate with multi-mode fibre.
So I think I'll consign this story to the category of you might be able to do it in the lab but the practicalities are too troublesome.

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