204 Urban trees are a possible obstruction to the Wi-Fi signal, and the potential for trees to interfere with microwave signals has been recognized by RF engineers. Perras and Bouchard (2002) evaluated fading of RF signals that passed through tree canopies and found that mean attenuation was 21.8 dB for a signal pass- ing through a 20 m (65 ft) span of foliated broadleaf canopies (Acer and Malus) and 12.6 dB when the signal passed through a 25 m (82 ft) deep conifer canopy (Picea spp.). Notably, signal attenuation was considerably lower (14.9 dB) when the broad- leaf trees were defoliated. Regarding the effect of foliage size and tree architecture, the authors concluded that greatest signal attenuation occurred “when the size of the obstructions in the foliated path and the wavelength of the signal passing through (≈12 cm at 2.5 GHz) are similar in size” (p. 271). Dalley and colleagues (1999) evaluated the effect of moisture in trees, and calculated that a wet tree attenuated the signal considerably more (loss of 18 dB) than a dry tree (loss of 11 dB, both at 3.5 GHz - a somewhat higher frequency than used in municipal Wi-Fi). What both of the above studies indicated to RF engineering professionals (Dobkin, pers. comm.) was the need to increase the density of APs in an outdoor network to compensate for the pres- ence of trees (which reduce the maximum usable computer-to-AP distances from any one AP). However, both of these studies used a purpose-built experimental setup. They did not evaluate an actual- ly-operating wireless network or attempt to simulate the perspec- tive of a typical computer user with a wireless-equipped laptop. The purpose of this study was to evaluate how urban trees affect the performance of a municipal WLAN network operat- ing with the 802.11b Wi-Fi protocol. A focus was placed on the computer-to-AP component of the network (the one most likely to be affected by trees) and evaluated the degree of interference of a tree in relation to its characteristics (canopy size, leaf size, leaf presence, leaf type). A simple setup was used (laptop com- puter with a wireless-network card) to simulate the experience of a user trying to connect to the internet using a municipal Wi- Fi network from a public space, such as a sidewalk or a park. MATERIALS AND METHODS The study was carried out in the City of Mountain View, on the San Francisco Peninsula (Northern California, U.S.; 37°25'19"N, 122°5'4"W, 32 m (105 ft) elevation; 72,000 residents). The city climate is mediterranean, with the average high temperatures ranging from 14–26°C (58–79°F), and the average lows from 4–13°C (39–57°F). Most of the yearly rainfall [average: 40 cm (15.71 in)] occurs during the winter months (November–March), while at least two sum- mer months (July, August) are typically, completely dry. In August 2006, a municipal Wi-Fi known as “Google-Fi” was established in the city, operated by the Google Corpora- tion (which is headquartered in Mountain View). Users can con- nect (without charge, as of 2009) to Google-Fi through any of approximately 400 APs mounted on 10.5 m tall (34.5 ft) light- posts throughout the city (a coverage map can be found online: http://wifi.google.com/city/mv/apmap.html). The APs (Metro- Mesh 5320 wireless routers) are made by Tropos Networks Corp. (Sunnyvale, California) specifically for outdoor use and can op- erate in either 2.4 GHz or 5 GHz frequency bands. The network was primarily intended for outdoors access; for indoor use, a “bridge” device is recommended as the network was not set up ©2009 International Society of Arboriculture Laćan and McBride: City Trees and Municipal Wi-Fi Networks to provide enough signal strength to pass through building mate- rials. However, the signal-diminishing effect of urban trees was taken into account when the network was planned, as is typically done for outdoor Wi-Fi networks (Blais and Kruse, pers. comm.). To simulate conditions that might be experienced by a typical user, a PC laptop computer was used (Gateway MX3210; Gate- way Inc., Irvine, California, U.S.) with a Windows XP Home op- erating system (Microsoft Corp., Redmond, Washington, U.S.), and equipped with an 802.11b add-on wireless card (Belkin ConnectPlus 128 in the PC-card slot; Belkin International Inc., Compton, California, U.S.). Note that in some respects (limited wireless-protocol and frequency capabilities, older Wi-Fi hard- ware and software) the equipment for this study represented one of the least-capable options available in 2007. That is, most Wi- Fi users have a better-performing computer and wireless card, and these results represent a “worst-case” of the WLAN sys- tem performance. Measurements were taken of signal strength, RF noise level, and signal-to-noise ratio (SNR) using Network Stumbler software (v. 0.4.0, by Marius Milner), which can de- tect individual APs and record their signal parameters. The data was then transferred to SigmaStat 3.1 and SigmaPlot (Systat Software, Richmond, California, U.S.) for analysis and plotting. To evaluate how urban trees and their characteristics af- fect municipal Wi-Fi signals, the laptop was positioned so that trees—varying in number, species, size, and leaf characteristics —were located between the computer and the AP (Figure 1). Records were taken of the signal, noise, and SNR for approxi- mately five minutes (about 300 data points at 60 measurements per minute) and called this the “tree” condition (tree blocking the line of sight to the AP). The laptop was then moved so that there was a clear line of sight (LOS) to the AP (but the distance remained the same) and again recorded signal strength for five minutes to obtain measurements under the “clear” condition. The difference between the average signal (∆Signal) recorded under the clear condition and that recorded under the tree con- dition represented the attenuation caused by the tree. All mea- surements were taken during daylight hours, dry weather, and in calm wind. Measurements on foliated trees were carried out in late May 2007, and were repeated on the same trees in win- ter (when deciduous trees were defoliated), in February 2008. Figure 1. Schematic of the experimental setup (plan view).
July 2009
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