 in the local environment are high. Susceptible to damage from lightning strikes-Coax may be damaged by lightning strikes. People who live in an area with a lot of lightning strikes must be wary because if that lightning is conducted by a coax, it could very well fry the equipment at the end of it. Microwave Microwave was used during World War II in military applications, and when it was successful in that environment, it was introduced into commercial communica- tions. Microwave was deployed in the PSTN as a replacement for coaxial cable in the late 1940s. Microwave As mentioned earlier, twisted-pain and coax both face limitations because of the frequency spectrum and the manner in which they are deployed. But micro- wave promises to have a much brighter future than twisted-pair or coax. Many locations cannot be cost-effectively cabled by using wires (e.g., the Sahara, the Amazon, places where buildings are on mountaintops, villages separated by val- leys), and this is where microwave can shine. In addition, the microwave spectrum is the workhorse of the wireless world: The vast majority of wireless broadband solutions operate in the microwave spectrum. Note that the discussion of microwave in this chapter focuses on its traditional application in carrier and enterprise private networks, but there are indeed many more systems and applications to discuss, and those are covered at length in Chap- ter 13, "Wireless Communications Basics," and Chapter 16, "Emerging Wireless Applications." This chapter focuses on the general characteristics of microwave and its use in traditional carrier backbones and enterprise private networks. Characteristics of Microwave Microwave is defined as falling in the 1GHz to 100GHz frequency band. But sys- tems today do not operate across this full range of frequencies. In fact, current microwave systems largely operate up to the 50GHz range. At the 60GHz level, we encounter the oxygen layer, where the microwave is absorbed by the surrounding oxygen, and the higher frequencies are severe