dundant access), enterprise campus connectivity, LAN extension, local loop, MANs, WAN access, local exchange bypass, storage access (including storage area networks [SANs] and network attached storage [NAS]), wireless backhaul (3G, 4G, WiMax, and Wi-Fi), and high-definition video. The two principal companies producing VF products, GigaBeam (www.gigabeam. com) and Loea (www.loeacom.com), support duplex rates of at least 1.5Gbps, with promised future enhancements reaching 10Gbps. Distances offering 99.999% reli- ability are short, roughly 1 to 1.25 miles (1.5 to 2 km), which means many pairs of radios are needed even for moderate distances. Effective reach can be extended to 3 miles (5 km), pending availability requirements; reliability at the longer distance is only 99.9%. VF technology is based on arrays of low-noise amplifiers developed to let mili- tary aircraft see through fog. These amplifiers receive very subtle signals created by the electromagnetic radiation coming off objects. At the center of a VF network are transceivers that resemble satellite TV dishes (see Figure 15.3). The transceivers must have line of sight, but they work through windows, SO they can be placed inside buildings. The system offers 99.999% weather reliability, which translates to only 5 minutes of downtime annually. Laser-based communications (such as FSO), on the other hand, historically have 99.9% weather availability over 750 feet (230 m). Chapter 15 WMANs, WLANs, and WPANs Wi-Fi Backhaul Cell Tower Backhaul POP or Fiber Hotel WiMax WiMax Point-to-Point Backhaul Access Points Mobile Phone, PDA, etc. WiMax Point-to-Multipoint Wi-Fi Hotspots Figure 15.3 An example of a VF network The key benefits associated with VF include reduced costs (by lowering or eliminating fiber deployment and access charges), reduced risk (by increasing net- work resiliency through diverse access paths), and reduced time-to-market (by reducing network backlog and deployment time). WLANs The world of WLANs is truly an exciting area, with major activity world