Also see Enhanced Specialized Mobile Radio (ESMR).
Specialized Mobile Radio (SMR) is any two-way radio system in which two or more mobile/portable wireless transceivers are linked by a single repeater. The repeater is elevated above average terrain; this maximizes the area of coverage. Operating frequencies are in the VHF (very-high-frequency) or UHF (ultra-high-frequency) range, that is, between approximately 30 MHz and 3 GHz.
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In some ways, an SMR system is like a cellular telephone network. But there are important differences. An SMR system is simpler than a cellular telephone network. There is only one repeater in a SMR system, and it links only the mobile/portable units for that system, not to other repeaters. In SMR, the range of each individual mobile/portable transceiver is greater than the range of a cell phone set. But total system coverage is usually far more limited than that of a cellular network, because there is no linking among repeaters.
SMR systems use channel pairs. Each transceiver has a transmit frequency and a receive frequency. These frequencies differ by a fixed amount, called the offset. The transmit and receive frequencies are in the same band, that is, relatively close to each other in the radio spectrum. The transmit and receive frequencies of each mobile or portable transceiver in a system are all identical.
An SMR system uses half-duplex communication and a PTT (push-to-talk) mode. Neither party can hear the other while transmitting. An example of half-duplex operation is a radio conversation between two people using simple walkie-talkies. SMR is used by taxi dispatchers, parcel delivery companies, fire departments, paramedic squads, police departments, and amateur radio operators.