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, 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 is able to receive the signal: 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. 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 Selcall is widely used, though is becoming superseded by much more sophisticated digital systems.
Conventional telephone use
"Mobile radio telephone systems such as "Mobile Telephone Service 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 services suffered from few usable channels, heavy congestion, and very high operating costs.
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The Marine Radiotelephone Service or HF ship-to-shore operates on "shortwave radio frequencies, using "single-sideband modulation. 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. This service is retained for safety reasons, but in practice has been made obsolete by satellite telephones (particularly "INMARSAT) and "VoIP telephone and email via "satellite internet.
Short wave radio is used because it bounces between the "ionosphere 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), about 3.5 "kHz. In comparison, "AM radio uses about 8 kHz, and "narrowband (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.
Some ships, including almost all military ships, carry "teletypewriters, and use them to communicate over short wave. This is called "marine "radiotelegraphy". The equipment is a shortwave radio transceiver with an attachment that generates and receives "audio tones in order to drive the teletypewiter.
In the United States, since the Communications Act of 1934 the "Federal Communications Commission (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 broadcast stations.) The certificate currently issued is the "general radiotelephone operator license.
- "ASTRA2Connect Maritime Broadband
- "AT&T High Seas Service
- "Car phone
- "Improved Mobile Telephone Service
- "Mobile radio telephone
- "Mobile Telephone Service
- "Two-way radio
- Bruce, Robert V. Bell: Alexander Bell and the Conquest of Solitude. Ithaca, New York: "Cornell University Press, 1990. "ISBN 0-8014-9691-8.
- Carson, Mary Kay (2007). "8". Alexander Graham Bell: Giving Voice To The World. Sterling Biographies. New York: Sterling Publishing Co., Inc. pp. 76–78. "ISBN "978-1-4027-3230-0. "OCLC 182527281.
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