Also known as Trunk Mobile Radio (TMR), SMR was introduced in the 1960s. Mobile radio has its roots early in the century, however.The United States Army Signal Corps mounted early spark transmitters in vehicles in 1904 and experimented with air-to-ground communications in 1908.The Detroit, Michigan (United States), Police Department placed the first experimental one-way mobile radio dispatch system into service in 1921, operating with the call letters KOP. The first twoway mobile system was installed by the Bayonne, New Jersey (United States), Police Department in the early 1930s. In 1946,AT&T was granted the first commercial license for two-way mobile service.That first system in St. Louis, Missouri (United States), was based on frequency modulation (FM) and employed a centralized antenna with a range of 50 miles.The system served not only to interconnect mobile phones, but also provided connection to the public switched telephone network (PSTN).The early systems were relatively inexpensive and proved very convenient, and were soon oversubscribed. In 1976, for example, service in the New York metropolitan area consisted of 20 channels supporting 543 subscribers out of a total population of approximately 20 million.There was a waiting list of approximately 3,700. In the 1960s, SMR was made commercially available, marketed as Improved Mobile Telephone Service (IMTS). SMR made better use of FM bandwidth through narrowband communications involving smaller frequency channels and enabled users to manually search multiple frequency channels. Shortly thereafter, intelligent mobile sets were developed that searched channels automatically. SMR involves omni-directional transmit/receive antennas placed on a radio tower positioned on the highest possible point in a geographic area for maximum line of sight (LOS) and transmitting at the maximum allowable power level for maximum geographic coverage in what is termed a macrocell configuration.Although some SMR systems support full duplex (FDX) communications, many are only half-duplex (HDX), which supports transmission in only one direction at a time and, therefore, requires that the parties take turns talking.The talker must depress a key or button on the microphone to talk and must release it to listen. This procedure is commonly known as the push-to-talk (PTT) protocol. SMR largely has been supplanted by cellular service offerings, although it remains widely used in dispatch and fleet applications such as police, fire, and emergency vehicles, as well as taxi and utility fleets. Enhanced Switched Mobile Radio (ESMR) is a technology developed by Nextel Communications and Geotek Communications for the development of a voice and data, cellular-like network using legacy SMR networks operating in the 800 MHz and 900 MHz range. Nextel acquired and linked a large number of SMR networks throughout the United States, in February 1999 acquired the 191 900-MHz licenses of the bankrupt Geotek, and later added the 1.5 GHz band to the mix.Time division multiple access (TDMA) divides each frequency channel into multiple time slots to support multiple conversations. ESMR also supports call handoff so mobile users can maintain connectivity as they travel from cell to cell. The network offers data throughput of 7.2 kbps, with coverage, including most major metropolitan areas in the United States. Nextel terminal equipment supports integrated voice, data, paging, and Internet access. See also antenna, band, channel, FDX, FM, frequency, HDX, Internet, LOS, macrocell, paging system, PSTN, PTT, TDMA, and time slot.
(Specialized Mobile Radio) The communications services used by police, ambulances, taxicabs, trucks and other delivery vehicles. Throughout the U.S., approximately 3,000 independent operators are licensed by the FCC to offer this service, which provides always-on communications similar to a walkie-talkie, but over a larger geographic area. Nextel Communications is known in this field for having acquired a large number of SMR operators and turning them into a nationwide system. See Nextel.