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“Naked” WiFi World Record If, like many WiFi users, you’re struggling to get garden-variety, unamplified WiFi signals to reach the other end of your house (despite the manufacturers’ 100 or 200m claims!) you might be interested to know that a group in Venezuela has raised the WiFi distance bar a tad – without using amplifiers or other cheats! by Ermanno Pietrosemoli With the sleet stopped and the fog lifted, the 2.7m dish on top of Pico del Aguila stands ready for its part in the world record. The fine rope seen coming from the centre of the dish was used to help aim it towards El Baul, 280km away. 14  Silicon Chip siliconchip.com.au The Radio Link window showing the 280km path parameters overlaying the map of Venezuela showing the world-record distance. The two sites stayed in contact via cell phones. There has been a longer 2.4GHz link but it used amplifiers and a stratospheric balloon! T he world record for 802.11 data transmission (WiFi) is (at time of going to press) 310km. However this record was set by the Swedish Space Agency using 6W amplifiers and a stratospheric balloon. Using “naked” out-of-the-box equipment (albeit into dish antennas) and a ground-to-ground link, US radio amateurs achieved a distance of some 200km back in 2005. Thanks to a favorable topography – high mountains with plains in between – Venezuelans have already achieved some long-range WiFi links, such as the 70km between Pico Espejo and Canagua and a trial 100km link between Maracaibo and Machiques in Zulia State. But the record books have been rewritten by a group in Venezuela with a distance of some 280km. To put it in an Australian perspective, that’s roughly the distance from Sydney to Canberra! siliconchip.com.au Here’s how they did it. To break the record, the first step was to find a clear link path, ideally between two elevated areas with no peaks or other obstructions in between. Many sites in the Guayana region were looked at – even though the famous “tepuys” (tall mesas with steep walls) looked promising there were always obstacles in the middle ground. Attention was then turned to the mighty Andes mountains whose steep slopes, rising abruptly from the plains, looked adequate to the task. First examined was Pico Espejo (at 4765m above sea level) and several sites in the Amazonas State but there were again obstacles in the path. Using the free software Radio Mobile, (available at www.cplus.org/ rmw/english1.html), there appeared to be no obstruction along the whole 300km path between Pico Espejo and the town of El Baul, in Cojedes State. However, Pico Espejo can only be reached by cable car, making the transportation of a 2.4m dish rather difficult. An alternative mountain, Pico del Aguila, has road access to the summit, so this peak was chosen instead. Several sites were considered possible on and around this peak. Antennas While it would have been easiest to use 30dBi gain commercial antennas the group decided instead to recycle parabolic reflectors formerly used for satellite service, replacing the feed with a 2.4GHz one. The concept was proved with an 80cm dish but the gain was way too low, so they next tried an offset-fed 2.4m reflector. This offered ample gain, albeit with some difficulties in the aiming of the February 2007  15 Accordingly, it was not difficult to persuade Dr Sandro Radiciella, the head of the Aeronomy and Radio Propagation Laboratory at ICTP, to support Carlo Fonda’s trip in early April to Venezuela in order to participate in the experiment. Back home, Ermanno noticed a 2.75m parabolic centre-fed mesh antenna at the home of a neighbour, who graciously lent it for the experiment. Action Plan All smiles after the new world record was confirmed and the dish dismantled. The author of this article, Ermanno Pietrosemoli, is at the left in this photo, alongside Javier Triviño. 3.5° beam. The 22.5° offset also meant that the dish appeared to be pointing downwards when it was horizontally aligned. Several tests were performed with cantennas as feeds and also using a 12dBi Yagi. They were able to establish a link with the Base Station at Aguada but efforts to measure the gain of the setup using Netstumbler were not successful. There was too much fluctuation on the received power values of live traffic. Evidently, for a meaningful measurement of the gain, a signal generator and spectrum analyser were required and would also be needed for the field trip in order to align the antennas properly. While waiting for the required equipment, they looked for an antenna to be used at the other end, as well as for a pointing system better suited to the narrow radio beam. In February 2006, Ermanno travelled to Triest to partake in an annual wireless training event. While there he mentioned the project to colleague Carlo Fonda who was thrilled and eager to participate. The collaboration between the Latin American Networking School (EsLaRed) and the Abdus Salam International Centre for Theorethical Physics (ICTP) in the wireless field goes back to 1992, when the first Net16  Silicon Chip working School was held in Mérida with ICTP support. Since then, several activities in which members of both institutions have participated have taken place, notably the yearly training in wireless networking at ICTP and the ones dedicated to computer networks in general organised by EsLaRed in several countries of Latin America. Once satisfied with the existence of a suitable path, the group looked at the equipment needed to achieve the goal. Well-known Orinoco 802.11 (WiFi) cards have been used for a number of years. They are robust and trustworthy, sporting an output power of 15dBm and receive threshold of -84dBm. The free space loss at 282km is 149dB, so to compensate 30dBi antennas would be needed at both ends and even that would leave very little margin for other losses. On the other hand, the popular Linksys WRT54G wireless router runs Linux and the Open Source community has written several firmware versions that allow for a complete customisation of every transmission parameter. In particular, OpenWRT firmware allows for the adjustment of the acknowledgment time of the MAC Conditions atop the 4100m Pico del Aguila were not particularly pleasant. Here the team led by Carlo Fonda assembles the 2.7m mesh antenna in the middle of driving sleet and fog. siliconchip.com.au Transporting a 2.7m dish on the roof of a large 4WD is not without its perils . . . beware of low-flying trees! This was the dish taken to the top of Pico del Aguila. layer, as well as the output power. Another firmware, DD-WRT has a GUI interface and a very convenient site survey utility. Furthermore, the Linksys can be located closer to the antenna than a laptop, so they decided to go with a pair of these boxes, one as an AP (access point) and the other as a client. The WRT54G can operate at 100mW with good linearity and can even be pushed up to 200mW – but at this value non-linearity is very severe and spurious signals are generated, so this is discouraged. Although this is consumer-grade equipment and quite inexpensive, after years of using them the group felt confident that they could serve the purpose, provided a spare set was kept handy. By setting the output power at 100mW (20dBm), the Linksys still had a 5dB advantage compared with the Orinoco, so a pair of them was used. previously surveyed area at Pico del Aguila in a truck carrying the 2.7m mesh antenna. The Aguila team was able to install and point the mesh antenna before the fog and sleet began, as is common at this altitude of 4100m above sea level. Power for the signal generator was supplied from the truck by means of a 12 V DC to 120V AC inverter. Communication between the two sites was maintained via cell (mobile) phones – both ends have cell phone towers. At 11am in El Baul they were able to observe a -82dBm signal at the agreed 2450MHz frequency on the spectrum analyser. To be sure, they asked Carlo to switch off the signal and indeed the trace on the spectrum analyser showed only noise, confirming that they were really seeing a signal that originated some 280km away. After again turning on the signal generator they performed a fine pointing in elevation and azimuth at both ends. When they were satisfied that had attained the maximum received signal, Carlo removed the signal generator and replaced it with a Linksys WRT54G wireless router configured as an Access Point, while Javier connected another WRT54G at the opposite end, configured as a client. At once, they started receiving “beacons” but the ping packets did not get through. This was expected, since the propagation time of the radio wave over a 300km link is 1ms and therefore it takes at least 2ms for an acknowledgment to reach the transmitter. Fortunately, Open-WRT firmware allows for tinkering with the ACK timing and after Carlo adjusted for the three orders of magnitude increase in delay above what the standard Wi-Fi link expects they began receiving packets with delays above 5ms. Several .pdf file transfers from Carlo’s to Javier’s laptops were made to prove the world-record-breaking SC link. Doing it! Javier Triviño and Ermanno Pietrosemoli travelled to Baúl with a 4-wheel-drive truck loaded the offset antenna. Early next morning the antenna was installed and pointed in the direction of Pico del Aguila. The Garmin III plus GPS showed a total path length of 279km. At the same time, the other team, comprising Carlo Fonda and Gaya Fior from ICTP, with assitance from Franco Bellarosa, Lourdes Pietrosemoli and José Triviño, rode to the siliconchip.com.au Proving it: screenshot of Javier’s laptop showing details of .PDF file transfer from Carlo’s laptop 280km away, using two WRT54G wireless routers and no amplifiers. Note the ping times as well. February 2007  17