og. Unlike FSO and 60GHz wireless, VF systems are very insensitive to common weather effects; they are affected only by heavy rain. At 0.5 mile (1 km), roughly 4 inches (100 mm) per hour of rain would be needed to cause interruption of a VF link. Based on statistical weather patterns, at least in the United States, VF systems can offer 99.9% weather availability at approximately 1.75 miles (3 km). WMANs In comparison, FSO, which is very sensitive to fog, dust, snow, and small occlu- sions, is 99.9% available to only about 1,000 feet (300 m); 60GHz wireless is very sensitive to oxygen in the air and, based on statistical weather information, is effec- tive to only about 0.5 mile (800 m) with 99.9% availability. As is the case for microwave links, the range-versus-weather availability performance of MMW links is calculated based on rain rate statistics for different regions. MMW systems have several other problems. First, free space loss increases with the square of the frequency, and losses are much higher in the MMW range. Second, multipath losses can be quite high. Finally, security is an important con- sideration. The best practice for security when using MMW is to prevent sniffing between the links. In other words, line-of-sight links should be positioned so no one can tap into the beam. In addition, some vendors offer multiple encryption levels. Because VF is not currently standards based, it is important to evaluate claims of interoperability carefully. VF technology deployed as point-to-point links and using ultra-high radio frequencies supports multi-Gbps transmission of data, voice, and video, as well as streaming HDTV, enabling it to support implementation of triple-play and quadruple-play services. In addition to support of high-bandwidth applications, VF promises to make it easy for IT managers to interconnect LANs among build- ings because the transceivers can be set up in a day. There are also many additional applications of VF, including fiber (backbone) POP access, network diversity (pro- viding re