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Microwave radio systems, which operate at frequencies over 1 GHz, have been around since the 1940s. Use of most of this spectrum requires a license by the FCC, but key blocks of spectrum--from 2.4 to 2.4835 GHz, from 5.150 to 5.35 GHz, and from 5.725 to 5.85 GHz--have been allocated by the FCC for unlicensed operation. Vendors producing systems operating in these frequency bands must employ protocols that minimize potential interference, and systems must be certified by the FCC before they can be sold.

The rules governing radio operation in the unlicensed ISM and UNII bands are complex, and some of these bands, especially 2.4 GHz, have become heavily used, thus increasing the potential for interference, particularly in densely populated areas. While most interference-related problems can be mitigated, it's not surprising that many companies seek to minimize their exposure by implementing systems that operate in the 5-GHz bands. These bands less congested and more spectrum is available, which results in better performance and more implementation flexibility. However, many point-to-point products that run in the 2.4-GHz band are based on WiFi WLAN chipsets, making these products significantly less expensive than 5-GHz offerings.

Although most of today's 5-GHz systems are based on custom radio implementations, it is likely that future products will leverage the cost-economies of commodity 802.11a chipsets. Standards are critical for WLAN products, but not so important for point-to-point systems because interoperability is not a critical requirement. In fact, a lack of interoperability can be a benefit to the extent that proprietary radio designs and modulation schemes add to system security.

Finally, it's worth noting that wireless point-to-point systems aren't always based on radios. Second-generation FSO (free space optical) systems based on lasers and LEDs are an option for some applications. These systems provide significantly greater bandwidth than radio systems, but are vulnerable to adverse weather conditions, particularly fog. Vendors are working to overcome these vulnerabilities, but for now, applications are limited to short-distance connections and environments where high-availability is not required or redundant systems make sense.