[MLB-WIRELESS] Free Space Optics Info

Nicholas Hobbs Nickhobbs at optusnet.com.au
Mon Mar 30 14:51:43 AEDT 2015

I run a number ( 6 pairs) of FSO links to connect a number of our sites together. (5 at 1Gbit and 1 @ 100Mbit), some of which have given continuous service for 6+ years. Shortest link is approx 60m, longest is about 600m

They are generally very reliable, however;

In practice most people who sell and have to support them will not recommend them over 1km unless they are very high end multi beam self aligning units. Most units will have a documented db loss over distance and weather conditions. I switch to the 70/80Ghz microwave for runs over 800m and under 3km. 18 and 38Ghz for longer hauls.

They require very stable mounts at both ends, think 75 or 100mm diameter pole, under 500mm long onto a very solid wall/floor mount. Both transmitting optic beams needs to line up with a receiver only 25-40mm wide at the other end.

Whilst they may advertise 'no interference', fog, heavy rain, condensation on the lenses if they have no heaters and spiders (seriously) all present challenges. 
I have a pair of links that are 200m runs and get taken out by the fog in winter and they are located in inner Melbourne about 30-40metres in the air.
I have one link with intermittent issues due to building movement/expansion/contraction. 40mm is a very small target when it's up to 1km away and doesn't give much margin for movement.
Super short distances can be problematic as the units can potentially overload the receiving sensor.
East-West links can be problematic due to sun glare in the morning/evening. As the sun tracks North/south in the skyline through the seasons, it may only be an occasional problem, and really tricky to troubleshoot unless you're there when it happens! (Had this happen too. My one East/Westish link drops at around 4.30pm for 5mins for 2-3 days in May when everything lines up perfectly!

So in short they're really good sub 1km as long as you know the conditions they are in and the units can't move an inch (literally).

For super short distance 10Gb, it's probably more economical to just haul the fibre.

My 2c


> On 30 Mar 2015, at 2:04 pm, Todd Hubers <todd.hubers at gmail.com> wrote:
> Well that is even more limited than what I recalled from our Geelong meet discussion.
> If that's the case, an FSO 1 gbps link over 2.5km is quite good then. I have a feeling the upper limit is very affected by fog (and other weather factors), and that a backup link is needed if full speed needs to be maintained the occasional time the fog is too thick. This puts it close to par with RF IMO (subject to price comparison). If used in a pure mesh network there may be one RF route and another FSO route, rather than having the redundancy on the link, it would be more economical to have redundant routes (given enough nodes).
> Cheap short p2p (in the order of ~100m [and up to 1km] distance) LED FSO links would possibly beat RF links. I'm considering setting one up, pointing across the road. Such links are keyed very simply suffering little, if any, MODEM delay. Much research has been (apparently 100s of Millions of USD) spent on developing long distance and fast FSO with no success. Lightpointe is one such effort, but bundles RF redundancy (= fail). It would be great if FSO research could focus on the 50-100m range to reach ~10Gbps speeds cheaply - maybe a good candidate for crowd funding one a successful prototype is completed.
> Would be a nice hack day - building an LED system that could reach at least 100Mbps.
> Todd
>> On Mon, Mar 30, 2015 at 12:34 PM, <mw at freenet.net.au> wrote:
>> 24GHz Airfiber @ full speed is good for about 1km if you stay within the ridiculously low ACMA regulations ;)
>> There is a 5GHz model, though, that claims similar data rates which can push a bit further, but you need a lot of free spectrum ;)
>> Cheers! 
>> Mike.
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