![]() system that could support the wide range of digital services available over the PSTN. Beginning in 1981, digital microwave systems began to be deployed in the U.S. ![]() Early systems had a capacity of 2,400 two-way voice circuits, and later systems could support 61,800 two-way circuits. Following the trend of coaxial cable systems, the first microwave links were analog systems. Point-to-point microwave systems generally operate in the frequency ranges of 3.7–4.2 gigahertz or 5.925–6.425 gigahertz some systems operate at 11 or 18 gigahertz. Because microwave systems are line-of-sight media, radio towers must be spaced approximately every 42 km (25 miles) along the route. First employed in 1950, microwave transmission has the advantage of not requiring access to all contiguous land along the path of the system. Long-distance transmission also has been provided by radio link in the form of point-to-point microwave systems.
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