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Large satellite dish used for long-range Wi-Fi connection in Venezuela

Long-range Wi-Fi is used for low-cost, unregulated point-to-point connections, as an alternative to cellular networks or satellite links.

Contents

Introduction

Since the development of the Wi-Fi radio standard, great leaps in the technology have been made. In the area of range Wi-Fi has been pushed to an extreme, and both commercial and residential applications of this Long Range Wi-Fi have cropped up around the world. It has also been used in experimental trials in the developing world to link communities separated by difficult geography with few or no other connectivity options.

Applications

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Business

  • Provide coverage to a large office or business complex or campus.
  • Establish point-to-point link between large skyscrapers or other office buildings.
  • Bring Internet to remote construction sites or research labs.

Residential

  • Bring Internet to a home if regular cable/DSL cannot be hooked up at the location.
  • Bring Internet to a vacation home or cottage on a remote mountain or on a lake.
  • Bring Internet to a yacht or large seafaring vessel.
  • Share a neighborhood Wi-Fi network.

Large-scale deployments

The Technology and Infrastructure for Emerging Regions (TIER) project at University of California at Berkeley, in collaboration with Intel, utilizes a modified Wi-Fi setup to create long-distance point-to-point links for several of its projects in the developing world. This technique, dubbed Wi-Fi over Long Distance (WiLD), is used to connect the Aravind Eye Hospital with several outlying clinics in Tamil Nadu state, India. Distances range from five to over fifteen kilometers (3–10 miles) with stations placed in line of sight of each other. These links allow specialists at the hospital to communicate with nurses and patients at the clinics through video conferencing. If the patient needs further examination or care, a hospital appointment can then be scheduled. Another network in Ghana links the University of Ghana, Legon campus to its remote campuses at the Korle bu Medical School and the City campus; a further extension will feature links up to 80 km (50 mi) apart.

Increasing range in other ways

See also 802.11 non-standard equipment

Specialized Wi-Fi channels

In most standard Wi-Fi routers, the three standards, a, b and g, are enough. But in long-range Wi-Fi, special technologies are used to get the most out of a Wi-Fi connection. The 802.11-2007 standard adds 10 MHz and 5 MHz OFDM modes to the 802.11a standard, and extend the time of cyclic prefix protection from 0.8 µs to 3.2 µs, quadrupling the multipath distortion protection. Some commonly available 802.11a/g chipsets support the OFDM 'half-clocking' and 'quarter-clocking' that is in the 2007 standard, and 4.9 GHz and 5.0 GHz products are available with 10 MHz and 5 MHz channel bandwidths. It is likely that some 802.11n D.20 chipsets will also support 'half-clocking' for use in 10 MHz channel bandwidths, and at double the range of the 802.11n standard.

802.11n (MIMO)

Preliminary 802.11n working became available in many routers in 2008. This technology works by using multiple antennas to target one or more sources to increase speed. In tests, the speed increase was said to only occur over short distances rather than the long range needed for most point to point setups.[1] On the other hand, using dual antennas with orthogonal polarities along with a 2x2 MIMO chipset effectively enable two independent carrier signals to be sent and received along the same long distance path.

Power increase or receiver sensitivity boosting

A rooftop 1 watt WiFi amp, feeding a simple antenna

Another way of adding range uses a power amplifier. Commonly known as "range extender amplifiers" these small devices supply usually around ½ watt of power to the antenna. Such amplifiers may give more than five times the range to an existing network. Every 6 dB gain doubles range. The alternative techniques of selecting a more sensitive WLAN adapter (some are quite "deaf") and more directive antenna should also be considered.

Higher gain antennas and adapter placement

Specially shaped antennas can be used to increase the range of a Wi-Fi transmission without a drastic increase in transmission power. High gain antenna may be of many designs, but all allow transmitting a narrow signal beam over distances of several kilometers, often nulling out nearby interference sources. A popular low-cost home made approach increases WiFi ranges by just placing standard USB WLAN hardware at the focal point of modified parabolic cookware (see "www.usbwifi.orcon.net.nz/rvwifi.jpg", "karlherrick.com/dev/2006/02/10/woktenna/"). Such "WokFi" techniques typically yield gains of 12–15 dB over the bare system—enough for line of sight (LOS) ranges of several kilometers and improvements in marginal locations. N.B. Although often low power, cheap USB WLAN adapters suit site auditing and location of local signal "sweet spots". As USB leads incur none of the losses normally associated with costly microwave coax and SMA fittings, just extending a USB adapter (or AP, etc) up to a window, or away from shielding metal work and vegetation, may dramatically improve the link.(See "www.usbwifi.orcon.net.nz/wifiledge.jpg")

Protocol hacking

The standard IEEE 802.11 protocol stacks can also be modified to make them more suitable for long distance, point-to-point usage, at the risk of breaking interoperability with other Wi-Fi devices and suffering interference from transmitters located near the antenna. These approaches are used by the TIER project (see "Rethinking Wireless in the Developing World").

In addition to power levels it is also important to know how the 802.11 protocol uses acknowledge for each received frame. If acknowledge is not received the frame is re-transmitted. By default the maximum distance between transmitter and receiver is 1 mile (1.6 km). On longer distances the delay will force retransmissions. On some professional equipment such as Cisco Aironet 1200 this parameter can be tuned for optimal throughput.

Packet Fragmentation can also be used to improve throughput in noisy/congested situations. Although packet fragmentation is often thought of as something bad, and does indeed add a large overhead, reducing throughput, sometimes it is necessary. For example, in a congested situation, ping times of 30 byte packets can be excellent, whilst ping times of 1450 byte packets can be very poor with high packet loss. Dividing the packet into two, by setting a fragmentation threshold to 750, can vastly improve the throughput. The fragmentation threshold should be a division of the MTU, typically 1500, so should be 750, 500, 375, etc. However, excessive fragmentation can make the problem worse, since the increased overhead will increase congestion.

Obstacles to long-range Wi-Fi

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

Landscape interference

Obstacles are among the biggest problems when setting up a long-range Wi-Fi. Trees and forests degrade the microwave signal, and rolling hills make it difficult to establish line-of-sight propagation.

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

2.4 GHz interference

Microwave ovens in residences dominate the 2.4 GHz band and will cause "meal time perturbations" of the noise floor. There are literally hundreds of other sources of interference that aggregate into a formidable obstacle to enabling long range use in occupied areas: baby monitors, wireless cameras, remote car starters, DECT and residential wireless phones, Bluetooth products to name just a few.

Due to the intended nature of the 2.4 GHz band, there are many users of this band, with as many as 2 or 3 devices per household. By its very nature, "Long Range Wifi" connotes an antenna system which can see many of these devices, which when added together produce a very high noise floor, whereby no single signal is usable, but nonetheless are still received. The aim of a long range system is to produce a system which over-powers these signals and/or uses directional antennas to prevent the receiver "seeing" these devices, thereby reducing the noise floor.

Several of the devices on the market are not legal in the UK. The UK appears to have particularly specific and strict regulations regarding the 2.4 GHz band. In many other countries, anything with 100 mW EIRP is considered "fair game". However, in the UK, there are extremely strict and specific regulations as to what can and cannot be used and sold on 2.4 GHz. The most notable difference in the UK is that video senders can only have a 10 mW EIRP, and must dissipate the transmitted signal across 20 MHz. Full details can be found on the Ofcom website, under IR2030

More information about 2.4 GHz interference can be found on the article Electromagnetic interference at 2.4 GHz, which lists the different types of appliances on 2.4 GHz, and how they interfere with each other.

Notable links

Italy

The longest unamplified Wi-Fi link is a 304 km link[2] achieved by CISAR (Center for Radio Activities) in Italy.

  • link established on 16-06-2007
  • frequency: 5765MHz
  • IEEE 802.11a (Wi-Fi), bandwidth 5MHz
  • Radio: Ubiquiti Networks XR5
  • Wireless routers: MikroTik RouterBOARD with RouterOS, NStreme optimization enabled
  • Length: 304 km (189 mi).
  • Antenna is 120cm Satellite dish prime focus with handmade waveguide. 35dBi estimated

Venezuela

Another notable unamplified Wi-Fi link is a 279 km link[3] achieved by Fundación Escuela Latinoamericana de Redes (Latin American Networking School).

  • Pico del Águila - El Baúl Link.
  • frequency: 2412 MHz
  • link established in 2006
  • IEEE 802.11 (Wi-Fi), channel 1, bandwidth 22 MHz
  • Wireless routers: Linksys WRT54G, OpenWrt firmware at el Águila and DD-WRT firmware at El Baúl.
  • Length: 279 km (173 mi).
  • Parabolic dish antennas were used at both ends, recycled from satellite service.
  • At El Aguila site an aluminum mesh reflector 9 ft (2.74 m) diameter, center-fed, at El Baúl a fiberglass solid reflector, offset-fed, 8 by 9 ft (2.44 by 2.74 m). On both ends the feeds were 12 dBi Yagis.
  • Linksys WRT54g routers fed the antennas with short LMR400 cables, so the effective gain of the complete antenna is estimated at about 30 dBi.
  • This is the largest known range attained with this technology, improving on a previous US record of 125 miles (201 km) achieved last year in U.S. The Swedish space agency attained 310 km (190 mi), but using 6 watt amplifiers to reach an overhead stratospheric balloon.

Peru

Napo's Network, Loreto (March 2007)
Antenna's instalation at Napo, Loreto (March 2007)

In the jungle region of Peru, Loreto, is located the chain multihop WiFi based longest network of the world. This network has been implemented by the Rural Telecommunications Research Group of the Pontificia Universidad Católica del Perú (GTR PUCP). The Wi-Fi chain goes through many small villages. It takes seventeen hops to cover the whole chain. It begins in Cabo Pantoja's Health Post and finish at Iquitos downtown. Its length is about 445 km. The intervention zone was established in the lowland jungle with altitudes elevations under 500 meters above sea level. It is a flat zone, for this reason GTR PUCP to installed towers 80 meters average height, 2.5 tons average weight.

  • It was established in 2007 and it is working until now. GTR PUCP, Regional Government of Loreto and Vicariate San José de Amazonas are working hard together on maintenance the network.
  • Frequency channels used: 1, 6 and 11, 802.11g non-interfered channels
  • Routers alix 2C0 were used alone with the Voyage GTR PUCP (GTR's version of Voyage).
  • Were used antennas of L-com.

References

  1. ^ "Wireless Networks". Radiolabs. 2006-07-14. http://radiolabs.com/Articles/wireless-network.php. Retrieved 2007-01-05. 
  2. ^ Mikrotik Forum
  3. ^ Long Reach LinkPDF (6452 KiB)

http://www.ab9il.net/wlan-projects/EnlaceAguila_Baul_EN.pdf

External links


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