A microwave is a short radio wave that varies from 1 millimeter to 30 centimeters in length. Because microwaves can pass through the ionosphere, which blocks or reflects longer radio waves, microwaves are well suited for long-distance, satellite, and space communications and for control of navigation.
Much of the microwave technology in use today for point-to-point communications was derived from radar developed during World War II. Initially, these systems carried multiplexed speech signals over common carrier and military communications networks; but today they are used to handle all types of information—voice, data, facsimile, and video—in either an analog or digital format.
The first microwave transmission occurred in 1933, when European engineers succeeded in communicating reliably across the English Channel—a distance of about 12 miles (20 km). In 1947, the first commercial microwave network in the United States came online. Built by Bell Laboratories, this was a New York-to-Boston system consisting of ten relay stations carrying television signals and multiplexed voice conversations.
A year later, New York was linked to San Francisco via 109 microwave relay stations. By the 1950s, transcontinental microwave networks were routinely handling over 2000 voice channels on hops averaging 25 miles (41.5 km). By the 1970s, not a single telephone call, television show, telegram, or data message crossed the country without spending some time on a microwave link.
Over the years, microwave systems have matured to the point that they have become major components of the nation's public switched network and essential mechanisms which private organizations use to satisfy internal communications requirements and to monitor their primary infrastructure. Microwave systems can even exceed the 99.85 percent reliability standard set by the telephone companies for their phone lines.
There are basically two microwave network configurations: point-to-point and point-to-multipoint. Point-to-point microwave products and systems meet a wide variety of low and medium density communications requirements. These range from simple links to more complex extended networks, such as:
§ T1/E1 and sub-T1/E1 data links
§ Ethernet/token ring LAN extensions
§ Low-density digital backbone for wide area mobile radio and paging services
§ PBX/OPX/FXO/FXS voice, fax, and data extensions
§ Facility-to-facility bulk data transfer
Point-to-multipoint microwave systems provide communications between a central command and control site and remote data units. A typical radio communications system provides connections between the master control point and the many remote data collection and control sites. In the United States, this type of radio system must have a minimum of four remote locations. Repeater configurations are also possible. The basic equipment requirements for this type of system include:
§ Antennas: for the master, an omni-directional antenna; for the remotes, a highly directional antenna, typically a yagi, aimed at the master station's location.
§ Tower (or other structure, such as a mast): to support the antenna and transmission line.
§ Transmission line: a low-loss coaxial cable connecting the antenna and the radio.
§ Master station radio: interfaces with the central computer. It transmits and receives data from the remote radio sites and can request diagnostic information from remote transceivers. The master radio can also serve as a repeater.
§ Remote radio transceiver: interfaces to the remote data unit; receives and transmits to the master radio.
§ Personal computer: can be connected to the master station's diagnostic system either directly, or remotely, for control and collection of diagnostic information from master and remote radios.
As the nation's cellular and personal communications systems continue to grow, point-to-multipoint microwave will provide backhaul and backbone links, enabling these wireless systems to serve less populated areas on a more economical basis.
Subscriber and base station antennas for fixed wireless local loop (WLL) systems are used to extend the service areas of existing fiber and wireline communications. The antennas offer a way to replace outdated equipment with advanced telecommunications technology without major reconstruction in urban areas.
Today's point-to-multipoint technology also enables private users to employ microwave frequencies to operate and control equipment at remote sites, such as switches and valves associated with the operation of oil and gas pipelines, to gather data related to services, control traffic signals, and to obtain toll data from moving vehicles, as well as other monitoring functions.
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