#tree-wifi

[tree-wifi]

WiLDNeT

Abstract WiFi-based Long Distance (WiLD) networks with links as long as 50–100 km have the potential to provide connectivity at substantially lower costs than traditional approaches. However, real-world deployments of such networks yield very poor end-to-end performance. First, the current 802.11 MAC protocol has fundamental shortcomings when used over long distances. Second, WiLD networks can exhibit high and variable loss characteristics, thereby severely limiting end-to-end throughput. This paper describes the design, implementation and evaluation of WiLDNet, a system that overcomes these two problems and provides enhanced end-to-end performance in WiLD networks. To address the protocol shortcomings, WiLDNet makes several essential changes to the 802.11 MAC protocol, but continues to exploit standard (low-cost) WiFi network cards. To better handle losses and improve link utilization, WiLDNet uses an adaptive loss-recovery mechanism using FEC and bulk acknowledgments. Based on a real-world deployment, WiLDNet provides a 2–5 fold improvement in TCP/UDP throughput (along with significantly reduced loss rates) in comparison to the best throughput achievable by conventional 802.11. 

WiFi-based Long Distance (WiLD) networks [8, 9, 23] are emerging as a low-cost connectivity solution and are increasingly being deployed in developing regions. The primary cost gains arise from the use of low-cost and low-power single-board computers and high-volume low-cost off-the-shelf 802.11 wireless cards using unlicensed spectrum. The nodes are also lightweight and don’t need expensive towers [6]. These networks are very different from the short-range multi-hop urban mesh networks [5]. Unlike mesh networks, which use omnidirectional antennas to cater to short ranges (less than 1–2 km at most), WiLD networks are comprised of point-topoint wireless links that use high-gain directional antennas (e.g. 24 dBi, 8◦ beam-width) with line of sight (LOS) over long distances (10–100 km).

[tree-wifi]

Helical coil coupled to a live tree to provide a radiating antenna - 1970 Patent

The HEMAC comprises in effect an air-cored toroidal coil. Alternately it may be described as a helical antenna with its axis bent into a circular shape or a loop. In addition, it differs from a conventional helical antenna in that its dimensions are all very much smaller than the wavelength of its operating frequency. The present device may function as an antenna for directly radiating energy from a transmitter to free space or directly picking up and applying signals to a receiver, or it may function as a coupler for coupling a transmitter or receiver to a larger radiator. In this latter function the HEMAC has been found to be very effective in coupling a high frequency (HF) radio set to a natural radiator comprising a forest. The I-IEMAC coupler was wrapped around a tree trunk. The applied frequency was such that the space between the forest floor or ground and the overhead foliage comprising the interleaved crowns of the trees was approximately the required dimension to form a sort of waveguide or cavity for directing the energy from the HEMAC through the treetops into the airspace above. The conductivity of the foliage and the earth is of course much less than that of the metals of which conventional waveguides are constructed and hence the forest energized by the HEMAC is more analogous to a leaky" waveguide. The energy which leaks from the treetops is propagated through the air and can be received by remote conventional antennas in the open, or by a similar HEMAC receiving system in a remote forest or jungle. The energy which is directed through the forest may be received by a remote HEMAC system. The forest may be converted to an omnidirectional or directional radiator, depending on the location of the HEMAC therein.

The conductivity of the loops will depend on several factors. Assuming the trees are in bloom, the sap thereof carries moisture and nutrients from the ground 42 via the root systems 31 and trunks to the leaves or foliage comprising the crowns 33. The sap flows in the outer ring of the tree, just under the bark. The inner layers or rings of the trunk are relatively dry and of relatively low conductivity compared to the sap-bearing outer ring. The conductivity of the tree crowns will depend on the density of the foliage and its moisture and sap content. The conductivity of the forest or jungle floor or ground 41 will depend on the moisture content and the character of the soil, for example, dry, sandy soil would be less conductive than moist soil containing decayed organic matter. However, even in sandy soil, the interleaved root systems provide a measure of conductivity. The HEMAC 23 surrounding one of the tree trunks comprises in effect the primary of a transformer, with each of the loops 39, 41, 43 and 45 comprising secondaries, through which RF currents will flow. If the RF (Resonant Frequency) is in the lower part of the high frequency band where the wavelengths are of the order 60-80 meters, the quarter wavelength is l5-20 meters and this figure is equal to the distance between the forest floor 41 and the underside of the tree crowns for many typical forests. This distance is indicated as H in FIG. 6. Thus, this space under the tree crowns can act as a giant resonant cavity or waveguide.

[tree-wifi]

Receiving and cracking open the TP-Link router. The top panel is snapped in place with two snap fittings which can be unlocked by pushing a knife under the edge.