Extending Municipal Wi-Fi Mesh Indoors
March 30, 2007
Muni Wi-Fi providers will likely find angry users if they don't make it clear early on that indoor usage is going to require extra equipment.
Ive been assisting municipalities such as Philadelphia and San Francisco with the deployment of municipal Wi-Fi networks. These networks offer a means for supporting wireless meter reading and public safety applications as well as public Internet access. In most cases, the city also offers Wi-Fi Internet access to homes and businesses, either free of charge or through an Internet service provider leasing use of the network.
Most municipalities require the network to have Wi-Fi signal coverage in at least 90% of outdoor areas and 70% inside homes and businesses. For indoor places, the coverage is generally required just inside the exterior wall of the facility on the first and second floors. In order to meet overall coverage requirements, the trend is to install Wi-Fi mesh nodes on city-owned assets, such as street light poles and traffic lights. This generally offers good coverage outside throughout the city, but indoor coverage is generally not sufficient without careful planning.
Indoor signal propagation suffers because the exterior walls of the facilities offer significant attenuation. In addition, the transmit power of user devices operating inside the buildings is usually relatively weak. The transmit power of the mesh nodes is much higher, but this only affects the downlink communications path from the mesh node to the user device. This doesnt do much good when the user devices have much lower transmit power. In this case, the ability for the communications link to operate is based on the weaker uplink signal, which causes limited range.
Because of varying wall attenuation and relatively weak transmit power of user devices operating inside the facilities, signal propagation indoors is not reliable and very difficult to verify. You cant depend on getting inside an appropriate sample of these facilities to test signal propagation and tweak the positioning of the mesh nodes to improve coverage. Most of the indoor locations are private homes and businesses, and a large number of them are not likely to let you in to do testing.
As a result, when designing a municipal Wi-Fi network, I highly recommend specifying the use of indoor customer premise equipment (CPE)
A CPE installs inside the facility near one of the exterior walls, and associates via Wi-Fi to the nearest mesh node. It also provides a separate Wi-Fi or Ethernet connection inside the facility. If its Wi-Fi, then users will likely have much greater indoor Wi-Fi coverage than what the municipal network requirements specify. Each indoor location will have the equivalent of a Wi-Fi router installed inside the building. If its Ethernet, then users will need to connect a laptop or PC directly to the CPE device or plug in an external Wi-Fi router or access point. Popular CPE devices are made by PePLink and Ruckus Wireless.
Something to keep in mind is to properly set user expectations regarding indoor coverage of a wireless municipal network and the need for CPE. Many potential users are accustomed to using Wi-Fi hotspots, where signal coverage is good without CPE devices. A municipality can circumvent frustrated users by educating users early on during the deployment of the network. Users need to realize that without indoor CPE, they wont have reliable Wi-Fi signal coverage.
Jim Geier is an independent consultant and founder of Wireless-Nets, Ltd (www.wireless-nets.com), a consulting firm assisting municipalities, enterprises, hospitals, airports and equipment providers with the development and deployment of wireless networks.