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A radiotelephone (or radiophone), abbreviated RT, is a
radio communication Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a trans ...
system for conducting a
conversation Conversation is interactive communication between two or more people. The development of conversational skills and etiquette is an important part of socialization. The development of conversational skills in a new language is a frequent focus ...
; radiotelephony means
telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...
by radio. It is in contrast to ''
radiotelegraphy Wireless telegraphy or radiotelegraphy is transmission of text messages by radio waves, analogous to electrical telegraphy using cables. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for t ...
'', which is radio transmission of
telegram Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas ...
s (messages), or ''
television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertising, ...
'', transmission of moving pictures and sound. The term is related to
radio broadcasting Radio broadcasting is transmission of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based radio ...
, which transmit audio one way to listeners. Radiotelephony refers specifically to
two-way radio A two-way radio is a radio that can both transmit and receive radio waves (a transceiver), unlike a radio broadcasting, broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and radio receiver, receive ...
systems for bidirectional person-to-person voice communication between separated users, such as
CB radio Citizens band radio (also known as CB radio), used in many countries, is a land mobile radio system, a system allowing short-distance person-to-many persons bidirectional voice communication among individuals, using two way radios operating on ...
or marine radio. In spite of the name, radiotelephony systems are not necessarily connected to or have anything to do with the
telephone network A telephone network is a telecommunications network that connects telephones, which allows telephone calls between two or more parties, as well as newer features such as fax and internet. The idea was revolutionized in the 1920s, as more and mor ...
, and in some radio services, including GMRS, interconnection is prohibited.


Design


Mode of emission

The word ''phone'' has a long precedent beginning with early US wired voice systems. The term means ''voice'' as opposed to telegraph or
Morse code Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one ...
. This would include systems fitting into the category of two-way radio or one-way voice broadcasts such as coastal maritime weather. The term is still popular in the
amateur radio Amateur radio, also known as ham radio, is the use of the radio frequency spectrum for purposes of non-commercial exchange of messages, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency commu ...
community and in US
Federal Communications Commission The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdicti ...
regulations.


Modes of operation

A standard
landline A landline (land line, land-line, main line, home phone, fixed-line, and wireline) is a telephone connection that uses metal wires or optical fiber telephone line for transmission, as distinguished from a mobile cellular network, which us ...
telephone allows both users to talk and listen simultaneously; effectively there are two open
communication channel A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for informa ...
s between the two end-to-end users of the system. In a radiotelephone system, this form of working, known as
full-duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
, requires a radio system to simultaneously transmit and receive on two separate frequencies, which both wastes
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
and presents some technical challenges. It is, however, the most comfortable method of voice communication for users, and it is currently used in cell phones and was used in the former IMTS. The most common method of working for radiotelephones is
half-duplex A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
, operation, which allows one person to talk and the other to listen alternately. If a single frequency is used, both parties take turns to transmit on it, known as simplex. Dual-frequency working or duplex splits the communication into two separate frequencies, but only one is used to transmit at a time with the other frequency dedicated to receiving. The user presses a special switch on the transmitter when they wish to talk—this is called the "press-to-talk" switch or PTT. It is usually fitted on the side of the microphone or other obvious position. Users may use a procedural code-word such as "over" to signal that they have finished transmitting.


Features

Radiotelephones may operate at any
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
where they are licensed to do so, though typically they are used in the various bands between 60 and 900 MHz ( 25 and 960 MHz in the United States). They may use simple
modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the '' carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informat ...
schemes such as AM or FM, or more complex techniques such as digital coding,
spread spectrum In telecommunication and radio communication, spread-spectrum techniques are methods by which a signal (e.g., an electrical, electromagnetic, or acoustic signal) generated with a particular bandwidth is deliberately spread in the frequency d ...
, and so on. Licensing terms for a given band will usually specify the type of modulation to be used. For example,
airband Airband or aircraft band is the name for a group of frequencies in the VHF radio spectrum allocated to radio communication in civil aviation, sometimes also referred to as ''VHF'', or phonetically as ''"Victor"''. Different sections of the band ...
radiotelephones used for air to ground communication between pilots and controllers operates in the
VHF Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF ...
band from 118.0 to 136.975 MHz, using amplitude modulation. Radiotelephone receivers are usually designed to a very high standard, and are usually of the double-conversion superhet design. Likewise, transmitters are carefully designed to avoid unwanted interference and feature power outputs from a few tens of milliwatts to perhaps 50
watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
s for a mobile unit, up to a couple of hundred watts for a
base station Base station (or base radio station) is – according to the International Telecommunication Union's (ITU) Radio Regulations (RR) – a "land station in the land mobile service." The term is used in the context of mobile telephony, wireless c ...
. Multiple channels are often provided using a
frequency synthesizer A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in many modern devices such as radio receivers, televisions, mobile telephones, radiotelephon ...
. Receivers usually feature a
squelch In telecommunications, squelch is a circuit function that acts to suppress the audio (or video) output of a receiver in the absence of a strong input signal. Essentially, squelch is a specialized type of noise gate designed to suppress weak s ...
circuit to cut off the
audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum * Digital audio, representation of sou ...
output from the receiver when there is no
transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...
to listen to. This is in contrast to
broadcast Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum (radio waves), in a one-to-many model. Broadcasting began wi ...
receivers, which often dispense with this.


Privacy and selective calling

Often, on a small network system, there are many mobile units and one main base station. This would be typical for police or taxi services for example. To help direct messages to the correct recipients and avoid irrelevant traffic on the network's being a distraction to other units, a variety of means have been devised to create addressing systems. The crudest and oldest of these is called
CTCSS In telecommunications, Continuous Tone-Coded Squelch System or CTCSS is one type of in-band signaling that is used to reduce the annoyance of listening to other users on a shared two-way radio communication channel. (See squelch.) It is sometimes ...
, or Continuous Tone-Controlled Squelch System. This consists of superimposing a precise very low frequency tone on the audio signal. Only the receiver tuned to this specific tone turns the signal into audio: this receiver shuts off the audio when the tone is not present or is a different frequency. By assigning a unique frequency to each mobile, private channels can be imposed on a public network. However this is only a convenience feature—it does not guarantee privacy. A more commonly used system is called selective calling or
Selcall Selcall (selective calling) is a type of squelch protocol used in radio communications systems, in which transmissions include a brief burst of sequential audio tones. Receivers that are set to respond to the transmitted tone sequence will open ...
. This also uses audio tones, but these are not restricted to sub-audio tones and are sent as a short burst in sequence. The receiver will be programmed to respond only to a unique set of tones in a precise sequence, and only then will it open the audio circuits for open-channel conversation with the base station. This system is much more versatile than CTCSS, as relatively few tones yield a far greater number of "addresses". In addition, special features (such as broadcast modes and emergency overrides) can be designed in, using special addresses set aside for the purpose. A mobile unit can also broadcast a Selcall sequence with its unique address to the base, so the user can know before the call is picked up which unit is calling. In practice many selcall systems also have automatic transponding built in, which allows the base station to "interrogate" a mobile even if the operator is not present. Such transponding systems usually have a status code that the user can set to indicate what they are doing. Features like this, while very simple, are one reason why they are very popular with organisations that need to manage a large number of remote mobile units. Selcall is widely used, though is becoming superseded by much more sophisticated digital systems.


Uses


Conventional telephone use

Mobile radio telephone Mobile radio telephone systems were telephone systems of a wireless type that preceded the modern cellular mobile form of telephony technology. Since they were the predecessors of the first generation of cellular telephones, these systems are s ...
systems such as
Mobile Telephone Service The Mobile Telephone Service (MTS) was a pre- cellular VHF radio system that linked to the Public Switched Telephone Network (PSTN). MTS was the radiotelephone equivalent of land dial phone service. The Mobile Telephone Service was one of the ear ...
and Improved Mobile Telephone Service allowed a mobile unit to have a telephone number allowing access from the general telephone network, although some systems required mobile operators to set up calls to mobile stations. Mobile radio telephone systems before the introduction of
cellular telephone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link while ...
services suffered from few usable channels, heavy congestion, and very high operating costs.


Marine use

The ''Marine Radiotelephone Service'' or ''HF ship-to-shore'' operates on
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
radio frequencies, using
single-sideband modulation In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modul ...
. The usual method is that a ship calls a shore station, and the shore station's marine operator connects the caller to the
public switched telephone network The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telep ...
. This service is retained for safety reasons, but in practice has been made obsolete by satellite telephones (particularly INMARSAT) and
VoIP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
telephone and email via
satellite internet Satellite Internet access is Internet access provided through communication satellites. Modern consumer grade satellite Internet service is typically provided to individual users through geostationary satellites that can offer relatively high d ...
. Short wave radio is used because it bounces between the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
and the ground, giving a modest 1,000 watt transmitter (the standard power) a worldwide range. Most shore stations monitor several frequencies. The frequencies with the longest range are usually near 20 MHz, but the ionospheric weather (propagation) can dramatically change which frequencies work best. Single-sideband (SSB) is used because the short wave bands are crowded with many users, and SSB permits a single voice channel to use a narrower range of radio frequencies (bandwidth) when compared to earlier AM systems. SSB uses about 3.5 kHz, while
AM radio AM broadcasting is radio broadcasting using amplitude modulation (AM) transmissions. It was the first method developed for making audio radio transmissions, and is still used worldwide, primarily for medium wave (also known as "AM band") transmis ...
uses about 8 kHz, and
narrowband Narrowband signals are signals that occupy a narrow range of frequencies or that have a small fractional bandwidth. In the audio spectrum, narrowband sounds are sounds that occupy a narrow range of frequencies. In telephony, narrowband is usua ...
(voice or communication-quality) FM uses 9 kHz. Marine radiotelephony first became common in the 1930s, and was used extensively for communications to ships and aircraft over water. In that time, most long-range aircraft had long-wire antennas that would be let out during a call, and reeled-in afterward. Marine radiotelephony originally used AM mode in the 2-3 MHz region before the transition to SSB and the adoption of various higher frequency bands in addition to the 2 MHz frequencies. One of the most important uses of marine radiotelephony has been to change ships' itineraries, and to perform other business at sea.


Regulations

In the United States, since the Communications Act of 1934 the
Federal Communications Commission The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdicti ...
(FCC) has issued various commercial "radiotelephone operator" licenses and permits to qualified applicants. These allow them to install, service, and maintain voice-only radio transmitter systems for use on ships and aircraft. (Until deregulation in the 1990s they were also required for commercial domestic radio and television broadcast systems. Because of treaty obligations they are still required for engineers of international
shortwave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (100 to 10 me ...
broadcast stations.) The certificate currently issued is the
general radiotelephone operator license The General Radiotelephone Operator License (GROL) is a license granted by the U.S. Federal Communications Commission (FCC) that is required to operate certain radio equipment. It is required for any person who adjusts, maintains, or internally repa ...
.


See also

*
ASTRA2Connect Maritime Broadband SES Broadband for Maritime is a two-way satellite broadband Internet service for use on private boats and commercial ships throughout European waters. SES Broadband for Maritime provides high-speed Internet access (at up to 2 Mbit/s downlink) alo ...
*
AT&T High Seas Service The AT&T High Seas Service was a radiotelephone service that provided ship-to-shore telephone calls, which consisted of stations WOO (transmitter station in Ocean Gate, New Jersey (), receiver station in Manahawkin, New Jersey, United States), WOM ...
*
Car phone A car phone is a mobile radio telephone specifically designed for and fitted into an automobile. This service originated with the Bell System and was first used in St. Louis on June 17, 1946. Overview The original equipment weighed , and the ...
* Improved Mobile Telephone Service * Inmarsat *
Mobile radio telephone Mobile radio telephone systems were telephone systems of a wireless type that preceded the modern cellular mobile form of telephony technology. Since they were the predecessors of the first generation of cellular telephones, these systems are s ...
*
Mobile Telephone Service The Mobile Telephone Service (MTS) was a pre- cellular VHF radio system that linked to the Public Switched Telephone Network (PSTN). MTS was the radiotelephone equivalent of land dial phone service. The Mobile Telephone Service was one of the ear ...
*
Two-way radio A two-way radio is a radio that can both transmit and receive radio waves (a transceiver), unlike a radio broadcasting, broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and radio receiver, receive ...


Notes


References

* Bruce, Robert V. ''Bell: Alexander Bell and the Conquest of Solitude''. Ithaca, New York:
Cornell University Press The Cornell University Press is the university press of Cornell University; currently housed in Sage House, the former residence of Henry William Sage. It was first established in 1869, making it the first university publishing enterprise in ...
, 1990. . *


External links

{{Authority control Maritime communication Mobile telecommunications Telephony equipment Wireless communication systems no:Radiotelefoni pt:Radiotelefonia sv:Radiotelefoni te:రేడియో టెలిఫోను