~1.2km WiFi link

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eXDee

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#62826 15-Jun-2010 17:46

Just had a random thought about doing a potential 1.1 to 1.5km link (exact distance unknown)

How important is line of sight? There aren't any massive obstacles like hills or big office buildings in the way, it'd be from rooftop to rooftop, though there are a few trees and a couple of buildings in the way (though the buildings are roughly the same height or not too much higher i think).

How powerful will the antennas have to be at either end? Is it even possible with a few obstacles in the way?
I took a few photos but the trees somewhat block the view so they arent much use.

I assume 5.8ghz Wireless N would help in terms of minimizing interference?

This is just an idea atm, wondering if its feasable at all.

chriswiggins

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#342038 15-Jun-2010 18:31

5.8ghz is a really good choice. Not many people use this frequency (apart from the odd home phone etc) so you are minimising interferance.

One problem though: trees are probably the worst possible thing to send wifi through. Don't ask me how, (because I'm not sure of the absolute technicalities

) but I do know that it will perform pretty horribly.

~1.2km is not actually at all that far to send wifi provided you have good antenna's etc. May I recommend you look at the Ubiquiti Wireless gear (if you havn't already got equipment): www.ubnt.com

But as I said concerning the trees, YMMV but in my experience trees don't play nice with wifi. Especially 5.8ghz, as it doesn't bend as easily as 2.4ghz. I am going to attempt a link in the next few weeks sending wifi through (actually over) a small hill. Not going to try with 5.8ghz but I may be lucky with 2.4.

Hope that has been some help to you,
Cheers
Chris

nickb800

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#342045 15-Jun-2010 18:43

From what i have read (ive never done this sort of thing) i think the fresnel zone is an important consideration for wireless transmission.
http://en.wikipedia.org/wiki/Fresnel_zone

Basically those buildings might be a problem if they are the same height as the points you're linking, because they may protrude into the fresnel zone

These guys have a the sort of gear you might need
http://www.gowifi.co.nz/

PANiCnz

990 posts

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#342049 15-Jun-2010 18:48

I reckon it pretty doable, might just need to get some mast's to get above the tree's. Surely the tree aren't that big that house+mast < tree?

The only problem with wifi links is they can soon start to get relatively expensive, you need 2 x AP's, 2 x water proof enclosures, 2 x Pigtails, 2 x antenna's it soon adds up. Did notice the other day on TradeMe a guy selling external enterprise AP's for ~$100ish.

Also need to think about getting power up to the AP's, PoE is a good option but if you want proper injectors it can get expensive.

eXDee

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#342084 15-Jun-2010 20:34

EDIT: UGH double post, i hate the edit limit around here. Can a mod please delete the above post?

If it can work, cost isnt a few issue if its only several hundred as this could be very useful if i can get a 100mbit+ link. If its too much higher though it'll become quite unfeasible.

In terms of the height of trees, i'd have to have a pretty tall mast, something thats not going to happen. Plus most of the trees are out of my control
How many trees do there have to be in the path before it starts severely impacting on the link? I think im going through ~3-5 ish in the first 100m and then its clear the rest of the way.

I'll try and get some photos up of the situation, but this samsung 12.2MP camera takes photos which are very disappointing quality compared to what i expected.

Basically aside from trees and easily clearing the roofs of a few houses houses, the signals mainly going straight through an industrial area, but the buildings arent too high, just very large (as one would expect in such an area). Now that i think of it, i think they may pose a bigger problem than i thought especially when considering the fresnel zone.

Originally before i got up on the roof i thought i had an 'obstructed' line of site but its looking a bit worse than expected.

When you say getting power up to the AP's, should they be up with the antenna rather than running cable up to it with the AP inside?

Saying all of that that, this is still very much in the 'theory' stage, and theres a high chance it won't happen (unless a bunch of things fall into place). But still considering it anyhow, and its something that i'm very interested in the concept of even for future use.

I realised 5.8 would be less tolerent of obstacles, thoughts on that vs 2.4 interference? The area isnt heavily congested with wifi, and i imagine the majority of the link path doesnt have too much interference from 2.4 due to being industrial rather than residential. Thats making guesses though.

raytaylor

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#342123 15-Jun-2010 23:11

Unless you can see the blue sky on the other side when looking through the tree in summer, it usually wont work.
You also wont get a 100mbit connection. Trees absorb the signal, it doesnt pass through very well. It can sometimes bounce around in the tree and sometimes get a scattered signal through but its usually low quality. 5.8ghz is harder to get through treesthan 2.4ghz.

54mbit (perfect signal) means the auctual TCP throughput is only 22mbit as half of it is wireless protocol overhead.

Go to www.gowifi.co.nz and look at the Ubiquiti nanostations (not locos). These in a low noise urban environment will go up to 5km. They are designed to mount directly to your TV antenna or a mast, have built in 10dbi antennas, and use power over ethernet.

- You just mount the nanostation on the mast,
- Get outdoor UV sunlight rated ethernet cable and run it down the building to a power plug
- connect the power over ethernet injector to the wall socket
- plug your computer or network hub using a cat5 cable into the poe injector
- program the nanostation using the built in web interface

I would say a realistic speed you will get in an urban environment is 10 to 14mbit with a pair of nanostations. If you look at the Nanostation M, it may be a bit faster but if your going through trees, one of the antenna polarisations will be more scattered than the other so it may not work any better.

When i am setting up links, line of sight is REQUIRED for anythign over 400m.

Also if you do go with the seperate antenna and AP option, make sure the antenna cable is as short as possible, which means running power and ethernet up to the roof. A wifi signal can be completley lost in as little as 10m of the proper wifi antenna cable - its best to get it into the air sooner than later.

When you say using wireless N - you need multiple antennas for that to work. The nanostation M's use a 2 chain N signal by having two built in antennas. One is horizontally aligned and the other is vertical. Avoid using off the shelf units that use multiple channels with 2 or 3 antennas. With an increase in channel width, means an increase in interference.

With regard to the fresnel zone - over 1.2km its probably only about 1.5m wide at the middle so that isnt going to be much of a concern. I worry about fresnel only on long links of 20km+ where there may be a hill that we can see directly over - but it might intrude a little too much.

Also to note in an industrial area, wifi, cordless phones, wireless security cameras, and heaps of other equipment are on 2.4ghz that could interfere - so its not just a matter of scanning for wireless AP's and thinking that the area is clean.
I use a spectrum analyser to find out about interference.

Ray Taylor

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eXDee

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#342153 16-Jun-2010 01:46

Hmmm this is making it sound more and more unfeasible. I thought much of the tree problem could be overcome with high powered antennae.

raytaylor

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#342413 16-Jun-2010 18:04

When you raise the power of the antenna, you are not really raising power - but instead you are focusing power.
By focusing more power at the tree, you will cook it a little faster. Over short distances, like 300m, i find that a wider angle antenna (lower power around 10dbi or 60degrees) gets through better. But at your distance it probably wouldnt work.

Ray Taylor

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