WiFi Long Shots

What you need to know to successfully design and build a wifi long shot. RF Calculation. Knowledge about the Fresnel Zone. Polarisation of electromagnetic waves. Tricks to avoid interference. Timeout problems of 802.11abg and how to deal with them.

Slightly more comprehensive description: Wifi is a really inexpensive way to build datalinks up to 120 kilometer distance. Such use of Wifi is already relatively widespread in developing countries that lack communication infrastructure. The world wide web is so far not truly for everyone - may 1 billion people use it and about 5 billions don't. It maybe unavailable, too expensive or both. Indeed, Information needs to be free - that also means that there must be cheap ways to share it. So far WiFi is the cheap way to go where no infrastructure exists - although that may change with WiMax in the future. Even if you live in a developed country it is fun or may be necessary to build your own (community) network beyond the range a few hundred meters.

During the last months I was designing Wifi-links (long shots) in Bangladesh - mostly used for inexpensive calls with Voice over Internet Protocol since most people are illiterate there. The use of the network for applications beside speech is slowly increasing.

If you are interested to build your own metropolitan area network - or even bigger - with inexpensive technology for a few bucks - then this presentation may be interesting. A brief list of the topics:

Link calculation - propagation attentuation, transmit power, receiver sensitivity, antenna gain, additional losses, curvation of earth, fresnel zone, polarisation of electromagnetic waves.

Recommended hardware - Access Points, WiFi Cards, Antennae a.s.o.

Important Do's and Dont's.

Propagation delay and 802.11.

Tricks to minimize interference.

How to do long shots and stay in legal limits if you are scared. (You won't be able to achieve 120 km, though.)