Specialized Mobile Radio

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Specialized Mobile Radio (SMR) may be an analog or digital trunked two-way radio system, operated by a service in the VHF, 220, UHF, 700, 800 or 900 MHz bands. Some systems with advanced features are referred to as an Enhanced Specialized Mobile Radio (ESMR). [1] Specialized Mobile Radio is a term defined in US Federal Communications Commission (FCC) regulations. The term is of US regulatory origin but may be used in other regions to describe similar commercial systems which offer a radio communications service to businesses.

In telecommunications, trunking is a method for a system to provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of this include telephone systems and the two-way radios commonly used by police agencies. Trunking, in the form of link aggregation and VLAN tagging, has been applied in computer networking as well.

Two-way radio a radio that can do both transmit and receive a signal (a transceiver), unlike a broadcast receiver which only receives content; allows the operator to have a conversation with other similar radios operating on the same radio frequency (channel)

A two-way radio is a radio that can both transmit and receive a signal, unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver designed for bidirectional person-to-person voice communication with other users with similar radios using the same radio frequency (channel). Two-way radios are available in mobile, stationary base and hand-held portable configurations. Hand-held two-way radios are often called walkie-talkies, handie-talkies or hand-helds.

Federal Communications Commission independent agency of the United States government

The Federal Communications Commission (FCC) is an independent agency of the United States government created by statute to regulate interstate communications by radio, television, wire, satellite, and cable. The FCC serves the public in the areas of broadband access, fair competition, radio frequency use, media responsibility, public safety, and homeland security.

Contents

History

SMRs were created when the Federal Communications Commission began to license business and commercial 800 MHz two-way radio systems in the late 1970s. [2]

Compatibility and purpose

Any company, such as a taxi service, towing service, or construction company, may use an SMR service. These concerns may rent radios from the SMR operator or may buy compatible radios. SMR systems use differing protocols, frequency ranges, and modulation schemes: not every radio is compatible with every SMR system.

These systems generally consist of one or more repeaters used to maintain communications between a dispatch fleet of mobile or hand-held walkie talkie radios. One- to five-channel systems may be conventional two-way radio repeaters. More than five channel systems must be trunked.

Dispatch is a procedure for assigning employees (workers) or vehicles to customers. Industries that dispatch include taxicabs, couriers, emergency services, as well as home and commercial services such as maid services, plumbing, HVAC, pest control and electricians.

Mobile radio

Mobile radio or mobiles refer to wireless communications systems and devices which are based on radio frequencies(using commonly UHF or VHF frequencies), and where the path of communications is movable on either end. There are a variety of views about what constitutes mobile equipment. For US licensing purposes, mobiles may include hand-carried,, equipment. An obsolete term is radiophone.

Radio repeater

A radio repeater is a combination of a radio receiver and a radio transmitter that receives a signal and retransmits it, so that two-way radio signals can cover longer distances. A repeater sited at a high elevation can allow two mobile stations, otherwise out of line-of-sight propagation range of each other, to communicate. Repeaters are found in professional, commercial, and government mobile radio systems and also in amateur radio.

Fees

The radio system is operated by a commercial service. Paying a fee allows users to utilize the radio system backbone, increasing their range. Some SMR systems offer telephone interconnect. This allows telephone calls to be made from the mobile radio or walkie talkie. Some systems may also offer selective calling, allowing customers to communicate with individual radios or segments of their entire radio fleet.

Users are charged a fee for some combination of:

Conventional two-way radio SMR system

In conventional systems, units are separated into groups by selective calling. Radios are usually segmented into customers with each customer on a separate selective calling code. Systems may use continuous tone-coded squelch system for this purpose. A disadvantage is that conventional systems require the user to disable selective calling, (to monitor,) in order to see if another company is talking before using the channel. Conventional systems are usually analog FM.

In a conventional, analog two-way radio system, a standard radio has noise squelch or carrier squelch, which allows a radio to receive all transmissions. Selective calling is used to address a subset of all two-way radios on a single radio frequency channel. Where more than one user is on the same channel, selective calling can address a subset of all receivers or can direct a call to a single radio. Selective calling features fit into two major categories — individual calling and group calling. Individual calls generally have longer time-constants: it takes more air-time to call an individual radio unit than to call a large group of radios.

Block diagram of a five channel trunked system. Trunked 5ch central control.svg
Block diagram of a five channel trunked system.

Trunked SMR system

In trunked systems, units are separated into virtual groups by the embedded radio system logic. Channel assignments are controlled by a computer and software. These virtual groups are often referred to as "fleet" and "subfleet" or "talk groups". There is no monitor function on trunked systems, but users may get busy signals or cryptic overload messages on their radio's display if an attempt is made to transmit while all channels are busy. [3]

A service operating an SMR system must maintain a certain number of users for each licensed channel. This is called loading. The service must show adequate loading before licenses for more channels can be granted.

Quality of service

In SMR systems, many factors affect the quality of service. This may include the capacity of the radio system versus the number of radio units using the system. A talkative user community may cause busy signals while a disciplined, professional group of SMR users may keep channels idle.

Some systems experience seasonal peaks. For example, a trunked system in an agricultural area used by crop harvesting crews may overload and experience busy signals during peaks in harvesting. A system used by a ski resort may overload on a holiday weekend. Disaster planning experts like to say that trunked system users are likely to hear busy signals for the first time when a disaster occurs. A major flood or earthquake is likely to generate a level of call traffic that loads the system to capacity. [4]

Agriculture Cultivation of plants and animals to provide useful products

Agriculture is the science and art of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to live in cities. The history of agriculture began thousands of years ago. After gathering wild grains beginning at least 105,000 years ago, nascent farmers began to plant them around 11,500 years ago. Pigs, sheep and cattle were domesticated over 10,000 years ago. Plants were independently cultivated in at least 11 regions of the world. Industrial agriculture based on large-scale monoculture in the twentieth century came to dominate agricultural output, though about 2 billion people still depended on subsistence agriculture into the twenty-first.

Siting of the system affects coverage: the system antennas should overlook the desired coverage area. Federal Communications Commission regulations require systems using the same radio frequency to be spaced 70 miles from one another or do an engineering study to confirm there will be no interference from closer spacing.

Maintenance practices also can affect quality. In some systems, the operator may remove one channel of a trunked system from service to perform repairs. In a heavily used system, this could cause busy signals.

Engineering documentation for these systems suggests another evaluation would be delivered audio quality. This is a measurement of the sound quality over all of the involved pieces of communications equipment from end to end. The first clue this may be an issue is that radio users with normal hearing have trouble understanding others when speaking over the radio.

See also

Notes

  1. ↑ See External link: Federal Communications Commission definition of SMR below.
  2. ↑ The date is unclear. The FCC web site has a page under the Wireless Telecommunication Bureau defining Specialized Mobile Radio Service. It claims licenses were first issued in 1979. Another publication suggests licenses were first issued in 1976. See: "3.6.1 Overview," Statewide Radio Interoperability Needs Assessment, (Phoenix, Arizona: Macro Corporation and The State of Arizona, 2004) pp. 30.)
  3. ↑ Some Nextel phones give the message, 'Please try again' when all channels are busy.
  4. ↑ SMR systems are not designed and built to the same standards as government trunked systems used to manage emergencies. There are many documents addressing this issue. One article discussing this topic is, "Intra- and Inter-County Coordination and Interoperability: Existing Cooperative Agreements," Contra Costa County Public Safety Mobile Radio Master Plan, Final Report, June 18, 2002, (Fairfax, Virginia: Federal Engineering, Inc., 2002) pp. 25.
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