Wireless link frequency choice (900Mhz vs 5.8Ghz) for 2-3km distance
Solution 1:
3km is approaching the limit of 5.8GHz equipment with reasonable size antennas and typical WiFi radios. With some of the best equipment you can only get ten times that distance with some caveats.
900MHz can easily go 3km, and much much farther. 900MHz is used for a wide variety of devices for this reason, so there's a good level of background noise. This might be a problem, especially if your neighbor has a 900MHz phone or similar (not a popular frequency for phones these days, but people hold on to technology for a long time too). I would avoid this frequency unless you're in the middle of nowhere (which you aren't). The 2.4GHz spectrum has worse problems with this, our microwave at work blasts 2.4GHz (I'm sure it violates FCC something or other, but nobody really cares as we have 5GHz WiFi).
Solution 2:
Chris S's answer is incorrect in several areas.
3km is not approaching the limit of 5.8GHz. Similarly, that's not as big of a distance as for 2.4GHz wifi either. Chris says 10X, I say 100X the distance is possible. The longest range 5.8GHz wifi that I've heard of is 304KM with a 1.2M hand made antenna (See: Long Range Wifi). I believe it went over water so there were not any thing in the way of the signal. It used Ubiquiti radios. I don't know if it was reliable, but to get a connection and send data over that distance is nothing short of amazing.
I used to work for a wisp carrier and had wifi radios about twice the size of your hand easily going 10KM with good line of site. I personally had wifi on my house going 8KM without any issues. Amazingly, they were only 500mW.
We were using 5GHz radios and a mixture of Ubiquiti and Microtik hardware. They work almost flawlessly.
While it is true that with lower frequencies you need bigger antennas. You'll notice that your home wifi doesn't have large antennas. Neither do 5GHz ones.
In reality, 2.4GHz should perform better, but 5GHz in my experience works just as well. In theory, 5GHz could be affected by rain fade, but I didn't find that even over the 8KM link.
As for 2.4GHz and other devices like microwaves and cordless phones inside the office, it doesn't even factor in. The reason it doesn't factor in is that you'll be using directional antennas. Better still, if the antenna is on an iron roof you will be shielded from all that noise coming from inside the office/house.
Having said all of that, I will say that it's not perfect for every situation. You need:
- Line of site between the two buildings. Factoring in the freznel zone (good open clearance)
- A suitable place to mount the antenna to achieve point 1.
- Hopefully not much wifi traffic facing your directional antennas (no gaurantees there, but it should be workable) even in a reasonably crowded wifi space.
Freznel zone looks like this:
By the way, I've had trees in the way of the freznel zone and in some cases right in the middle and it still worked, although it does affect the performance a lot. Install your antennas as high as practical/possible.
You can still get interference issues. Usually because of other devices transmitting in your direction at the same frequency. So it's not perfect. Directional antennas help a lot with that.
I can recommend the 5.8GHz radios simply because I know they work well. And yes, provide good bandwidth! From memory I used a Ubiquity Bullet M5 with a patch antenna (another brand). The other end was a less directional antenna feeding multiple clients but used a similar Microtik radio.
I don't know of anyone who uses 900MHz. But I do see you can buy them at around 3X the cost than 2.4 or 5.8. Stick away from it, it seems uncommon. In several countries the 900MHz spectrum is quite crowded anyway.
Update: You've been looking at the airmax antennas and the 900MHz radios. While I think performance should be great. I think you may be spending much more than you need to... perhaps a little overkill!? 3KM is not a long distance. I'd try some of these if you can.
http://www.gowifi.co.nz/antennas/5-ghz/directional/5.8-ghz-27-dbi-cast-reflector-grid-antenna.html combined with a Bullet M5.
Update2: You can also mitigate interference issues using polarized antennas. Here's a good write up that explains the concept. From memory, we used horizontal polarization. As I recall sometimes if I set the client radio to vertical polarization I could see maybe 12 AP's. But when setting it to horizontal I only saw 3 or 4 and half of them were ours.