Arboriculture & Urban Forestry 35(4): July 2009 Arboriculture & Urban Forestry 2009. 35(4): 203–210 203 City Trees and Municipal Wi-Fi Networks: Compatibility or Conflict? Igor Lacan and Joe R. McBride ´ Abstract. Conflict between city trees and infrastructure remains a problem in urban forestry. Municipal Wi-Fi, a city- wide wireless computer network, may become a part of urban infrastructure, and because trees can diminish Wi-Fi sig- nals, potential exists for conflict between urban trees and municipal Wi-Fi. This study examines attenuation of Wi-Fi sig- nals in the City of Mountain View, California, U.S. by positioning a wireless-equipped computer so that trees obstructed the line-of-sight (LOS) between the computer and a Wi-Fi access point. Signal attenuation ranged from < 2 dB to 19 dB (mean: 5.6 dB), depending on the number and types of trees present. Although trees significantly attenuated signals, they did not di- minish the average signal strength below -75 dBm (the minimum for a Wi-Fi connection) in any of the tests. A general linear model (r2 = 0.55) indicated that some tree characteristics (tree size, canopy depth, leaf type), but not others (number of trees in LOS, presence of leaves, leaf size, and shape) helped explain variation in signal attenuation. As long as the effect of ur- ban trees is taken into account during planning of Wi-Fi networks, trees should not interfere with municipal Wi-Fi operation. Key Words: Interference; Urban Infrastructure; Wireless Network; Wi-Fi. The sometimes-problematic interaction between urban trees and city infrastructure has been a longstanding difficulty in the practice of urban forestry and remains a perennial topic of ar- boricultural research. The most common example – the “roots/ pavement conflict” (e.g., cracks in pavement around street trees (Wagar and Barker 1983; Lesser 2001) – illustrates the difficul- ties of determining the ultimate causes of the problem (Sydnor et al. 2000), preventing the conflict, and managing it when it occurs (Randrup et al. 2001). Suggestions for resolving tree- infrastructure conflicts (e.g., Costello and Jones 2003) have included (1) changing the design of either the infrastructure or of the tree-planting-setup (e.g., flexible pavements, root bar- riers); (2) changing the tree species to those better “suited” to the infrastructure; or (3) changing tree maintenance practices. The idea of making more substantial modifications to urban in- frastructure to make it more compatible with trees has recently taken hold, as evidenced by the development of structural soils and structural cells (Grabosky and Bassuk 1995; Urban 2008). One new technology that may become a part of future ur- ban infrastructure is a citywide wireless computing network. Today, most such networks are based on the 802.11 standards (Dobkin 2005), also known as the wireless local area network (WLAN) or “Wi-Fi,” and operate on 2.4 GHz (for 802.11b, and 802.11g network protocols) and 5 GHz microwave frequencies (for 802.11a, a less common protocol). In the U.S., city gov- ernments first became interested in municipal 802.11 networks (“municipal Wi-Fi”) in the early 2000s, and despite some recent difficulties with funding such operations (Kim 2008), this tech- nology remains of interest to many cities today (e.g., Oklahoma City 2008). Potential uses for municipal Wi-Fi include con- necting different city offices to one another and to employees in the field. The latter application could be especially valuable as both a standard communication link for nonemergency per- sonnel (e.g. public works crews could access site plans or data- bases on-location) and as a backup for the radio and data trans- mission systems currently used by the police and fire services. The most visible element in a typical municipal Wi-Fi network is a set of wireless routers, also known as “access points” or APs, distributed throughout the city (often installed on light poles) that allow individual users to connect their Wi-Fi equipped laptop or desktop computer to the wireless network. The APs then con- nect either directly, or through one another in what is known as a “mesh network,” to a wired router that is connected to the internet. Positive aspects of municipal Wi-Fi include flexibility (com- pared to wired networks), low cost (compared to other wire- less systems such as cellular data services), potential for rapid deployment, backwards-compatibility (with older laptop and desktop computers), and familiarity to users (Dobkin 2005). In addition, concerns with radio frequency (RF) exposure are less- ened, as the wireless routers’ operating power is low, usually restricted to 1W, or < 1/500 that of a cellular telephone tower (Dobkin 2005; see end of article for a note on signal-measure- ment units). However, this characteristic of Wi-Fi APs – low ra- diated power – also severely restricts the range of the wireless link. The AP-to-user range varies from a theoretical maximum of 3.5 km (2.2 mi) (cf. 70 km (43.5 mi) for cell phones, to a practically-achievable outdoor maximum of less than 200 m (219 yds); indoor ranges can be even shorter (Dobkin 2005). The range restriction occurs because for the wireless link to function, the RF signal must reach the receiver with some minimum of power (usually -75 dBm) after diminishing (attenuating) from (1) passage through air (“free-space loss”) and (2) passage through or refraction around the objects in its path (Dobkin 2005). ©2009 International Society of Arboriculture
July 2009
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