Long-Distance WiFi
Increasing Range

High-gain antennas andprotocol hacking

Specially-shaped antennas can be used to increase the range ofa

Wi-Fi transmission without a drastic increase of transmission power.Parabolic high-gain antennas allow transmitting a narrow beam overdistances of several kilometers. Alternatively, a low-cost approach canbe used to increase the range of a Wi-Fi transmission using very simpleenhanced antennas while keeping standard hardware (see "www.usbwifi.orcon.net.nz/").

The standard 802.11protocol stacks can also be modified to make them more suitable forlong distance, point-to-point usage, at the risk of breakinginteroperability with other Wi-Fi devices and suffering interferencefrom transmitters located near the antenna. These approaches are usedby the TIER project (see "Rethinking Wireless in the Developing World").

Obstacles tolong-range Wi-Fi

Methods that stretch the range of a Wi-Fi connection may alsomake it fragile and volatile, due to mundane problems including:

 Phones

Many cordless phones in the US and Canada use the 2.4GHzfrequency,the same frequency at which Wi-Fi standards b, g and n operate. Thiscan cause a significant decrease in speed, or sometimes the totalblocking of the Wi-Fi signal when a conversation on the phone takesplace. There are several ways to avoid this though, some simple, andsome more complicated.

• Buy/Use wired phones.
• Buy 5.8GHz or 900MHz phones, commonly available today.
• Use VoIP/WiFi phones; these share the WiFi base stationsand participate in the WiFi contention protocols.
• Test several different Wi-Fi channels to avoid the phonechannels.

The last will sometimes not be successful, as numerouscordless phones use a feature called Digital Spread Spectrum.This technology was designed to ward off eavesdroppers, but the phonewill change channels at random, leaving no Wi-Fi channel safe fromphone interference.

 Car alarms

Certain car manufacturers use the 2.4GHz frequency for their caralarminternal movement sensors. These devices broadcast on 2.45GHz (betweenchannels 8 and 9) at a strength of 500mW. Because of channel overlap,this will cause problems for channels 6 and 11 which are commonly useddefault channels for Wi-Fi connections. Because the signal istransmitted as a continuous tone, it causes particular problems forWi-Fi traffic. This can be clearly seen with spectrum analysers.

Landscape interference

Obstacles are among the biggest problems when setting up along-range Wi-Fi. Trees and forests degrade the microwavesignal, and rolling hills make it difficult to establish Line-of-sight propagation.

In a city, buildings will impact integrity, speed andconnectivity.Steel frames partly reflect radio signals, and concrete or plasterwalls absorb microwave signals significantly, but sheet metal in wallsor roofs may efficiently reflect Wi-Fi signals,causing an almost total loss of signal.