Radiotelefony

The radiotelefony indicates the techniques of communication using the Radioélectricité, i.e. the Hertzian waves to transmit the human voice.
On uses for that of the fixed radioelectric transmitter-receivers and another mobiles (gone up on a vehicle) or transportables ( walkie-talkie ) to dialog, while possibly passing by relay stations.

General information

" parents" radiotelefony are the Wireless telegraphy (TSF), the Radiophonie and the Téléphone.

The last systems of Numerical Mobile telephony (cellphones or cellular GSM, UMTS) just like the preceding analogical generation, are apparatuses of radiotelefony functioning in duplex Full. This article does not cover this subject.

The material and the practices vary according to the uses: Company S private, CB enthusiasts, Radio ham, Air transport and maritime, Organizations first-aid, police force, Armed…

The article thus presents the general principles and conventions, normally used by all.

; Vocabulary

One calls station a person using a broadcasting transmitter.

Principle of the rotation

Apart from mobile telephony and relayed frequencies, the stations emit and receive on the same radio frequency. All the stations taking part in the same network (i.e. which can communicate between them) are on the same frequency. This has three consequences:

when a station emits, all the other stations hear its message (see the section selective Calling further);
if two stations emit at the same time, their messages are recovered (interfere);
a station cannot emit and listen at the same time.

The principle of the rotation is thus applied: each station speaks in turn, a station speaks only when the station which speaks finished its message. To indicate that one finished his message, one uses the term

“Speak! ”, “Answer! ”, “With you! ”.

The radio operator transmitters are equipped with a button of emission, sometimes called pedal of rotation: if the button is slackened, the station listens, if it is inserted, the station emits.

When a message is long, it is advisable to stop the emission intermittently (in general between each sentence), in order to leave the possibility to a station of stopping the message to pass an urgent message. The radio operator transmitters are sometimes provided besides with a limiting device of emission, an alarm which starts when the button of emission remains inserted too a long time (for example more than one minute). This also makes it possible to warn the user if this one presses accidentally on the button without will to emit (transmitting said “of carrying”).

To avoid confusions, the dialog is exclusively made between two stations, even if all the others can listen. When the dialog is finished, one of the stations states the message

“Finished! ”

thus indicating to the other stations that the frequency is released. By convention, it is the station which called the first which finishes: since it is it which had a message to deliver or a question to pose, it is it which knows if the dialog can stop.

When a station cannot answer a request in the immediate future, it states:

“Have patience! ”

It thus announces to the station which makes the request that it received it, and that it is committed recontacting it as soon as it with the answer. It also announces to the other stations that the frequency is free.

---- Note : a radio antenna receives signals on broad wavebands, but the receiving device selects a particular frequency or at least a very narrow waveband. The frequencies of emission and listening are canned and are called channels , channel 1 corresponds to a frequency ƒ₁, channel 2 at a frequency ƒ₂… The correspondence channel-frequency depends on the programming of the apparatus and thus of the licenses had by the user. ----

In English, the rotation is marked by the word ““over””, and the end of the message by the expression ““over and out””.

Code

Contrary to the telegraphic telephone, it is not possible to transmit a message towards only one station: all the stations on the frequency hear the message. To identify the transmitter and the recipient of a message, each station has an indicative . This code can be:

a number attached to the broadcasting transmitter (see the section selective Calling );
a code attached to the user, as for the Radio ham S;
a code of function, i.e. indicating the function of the user in the network;
a name indicating the hierarchical position of the user, it is the code of authority.

A message always begin with

“ indicative from the indicative recipient of from the transmitter , speak! ”

it what the station recipient answers:

“Transmit! ”

indicating thus that it is with listening; in the event of failure to reply, the transmitter will try to transmit a little later, or tries to join another station to check that its transmitter works well and to see whether this other station manages to join the recipient. The making of contact can possibly comprise information on the nature of the message or its urgency. For example in France, when a Ambulance of the firemen whose code is “VSAV1” (Véhicule of help and assistance to the victims) contacts the service urgently medical Samu of the department (for example Samu 17, Charente-Maritime) to transmit to the assessment victim dealt with, the exchange begins by:

VSAV1: “Samu17 of VSAV1 for assessment, speak! ”

SAMU17: “Transmit VSAV1! ”

…

If the station is occupied and cannot answer in the immediate future, it announces it by the message

“Have patience! ”

(cf section Principle of the rotation).

N.B. : This operating process corresponds to the analogical radioelectric networks of the French firemen in band 85.500 to 86.9500 MHz, resulting from the reform of the transmissions of the years 1990 (OBNT of the Civil security approved by the DSIC of the Ministry for the Interior, old DTI). With the law of modernization of the civil security of 2004, these networks and their operating processes are in the course of reform to integrate digital technologies following the example networks of the gendarmerie (RUBY into 78 and 77 MHz) and national police force (ACROPOL in 380 and 390 MHz). This numerical reform, baptized ANTARES, must lead the public services which concourrent with the missions of civil security (police force, gendarmerie, departmental services of fire and help, samu) to lay out in the band frequency harmonized with the European plan (380-385 MHz coupled to 390-395 MHz according to Decision CER/DEC/(96) 01 of the ERO), an operational system interopérable for daily aid operations and of crisis situations.

Selective calling

Certain radio operator transmitters are provided with a function of selective calling. Each station has a single number on the network; when a user composes this number on the keyboard of his broadcasting transmitter, that sends a train of notes by radio which makes “biper” the station of the recipient, in order to draw its attention.

When the network is charged (many of messages is exchanged on the frequency) or when it is necessary to be discrete (for example to avoid disturbing the people in the neighborhoods), a station can be put in selective listening: the radio is then takes care of it and activates itself only if it is bipée. If the radio is assembled on a vehicle, that can start the sound signalling Hooter (horn) to inform the user who would have moved away from his vehicle.

The trains of notes can be also used to activate automatic actions on an automated station. For example, the agents of the SNCF start the starting messages of the trains since the quay thanks to their portable transmitter. On certain networks, the standard messages can be replaced by the sending of a code which is then deciphered and is posted in light on the screen of computer of the direct station, for example for the networks mobile of intervention, the messages of the type:

“available, return to the center” (the intervention is finished, the vehicle returns towards its center but can be diverted for a new intervention),
“inalienable” (the crew is temporarily inalienable),
“arrived on the spot” (the vehicle has just arrived on the place of intervention)
…

In the same order of idea, when a driver of Train sees a person on a way or in the vicinity, it sends a radio signal to the central station of regulation, which causes the automatic stop of all the trains in the zone.

In certain standards, like PMR446 (transmitting of weak range and usable without license), the apparatuses are permanently in mode listens selective. A button makes it possible to indicate the code in function, called CTSS (for continuous ton squelch system ). When one presses on the button of emission, the transmitter emits the code automatically (inaudible) CTSS chosen, and only the apparatuses regulated on the same CTSS are activated. This makes it possible to divide the frequency into sub-networks, each station receiving only the messages of the sub-network to which it belongs. There are however risks of “collision”: if two stations out of two different CTSS emit simultaneously (they cannot know that the other emits), their messages interfere.

Clearness of the transmissions

The transmissions are frequently disturbed by parasites. It is thus necessary upon the departure speaking clearly, slowly, with detachment, not too extremely nor too close to the apparatus (risk of saturation).

The important short words are replaced by less ambiguous words:

“yes” → “affirmative”
“not” → “negative”

What avoids receiving " … atif"

When it is necessary to spell a word (for example a proper name), one uses a particular phonetic alphabet, the Alphabet radio operator; indeed, one can easily confuse the sounds “Be”, “vé” and “EP”, the letter B, V and P are thus stated respectively “Cheer”, “Victor” and “Dad”. In the same way, to transmit the numbers, one uses the enumeration radio when the transmission is bad.

The receiving station shows reception of the message by the “received” standard term or “received”. In the event of accident of transmission, it can ask to repeat whole or part of the message:

“Repeat after “man”, speak! ”

The transmitting station can require of the receiving station to reformulate the message in order to check that this one passed well, by giving the order “Collate! ”

When a station takes a network (i.e. the user lights his transmitter), or after an technical incident, the station carries out a radio operator test to assess the quality of the transmissions; the receiving station then indicates the way in which it received the message:

is by an appreciation quantified, from “five out of five” if the transmission is perfect with “one on five” if the transmission is very bad;
is by two adjectives,

one appreciating the force (sound volume) message, “extremely”, “rather extremely”, “weak” or “very weak”,
the other appreciating the legibility (absence of parasites), “clearly”, “readable”, “deformed” or “with interferences”.

Here for example a typical radio operator test enters the station having the code “Alpha” and that having the code “Cheer”:

a: Cheer of Alpha for radio operator test, speak!

b: extremely Received and clear Alpha, speak!

a: Received extremely and clearly also, finished!

The radio waves are subjected to many parasites and interferences, which creates a background noise (breath). To avoid to have breath permanently, transmitters are provided with filter called squelch (literally “gargouillis”), which cuts the loudspeaker normally when the signal is weak, i.e. when nobody speaks and that there is only noise. When two transmitters are distant, the signals are weak and the filter activates the loudspeaker only intermittently, the message arrives chopped. It is then necessary to decontaminate the filter ( squelch ) and to make repeat the message to hear it in entirety; the signal probably weak and will be deformed.

Free network and directed network

A free network is a network in which all the stations can communicate the ones with the others.

A network directed is a network in which the communications can be made only with one station called direct station ; one can call the direct station, to answer a call of the direct station, but two stations cannot communicate directly without opinion of the direct station.

In general, one puts oneself in directed network when it is necessary to manage a big number of transmissions, or when because of the layout of the premises, certain stations cannot join, a central station is then charged to collect all the messages and of the retransmettre. The networks of the firemen and Samu are directed.

The direct station can intimate the order to cease transmitting to a given station (“Silence Alpha! ”), or at all the stations (“At all the stations, except urgency, silence! ”); this takes place usually when the direct station treats a priority call. Once the urgently treated situation, the station again authorizes the transmissions (“Silence suspended! ”).

This rule of silence is all the more important as for the wide area networks and without relay, it is frequent that the stations at the ends of the device do not get along between them. Thus, the direct station can hear all the stations and vice versa , but it may be that two stations emit at the same time without the knowledge because they do not get along mutually.

In English, one uses the French terms:

“silence” (often retranscribed “seelonce phonetically”) to impose silence;
“silence finished” (often retranscribed “seelonce feenee”) to raise silence.

Optical range and relay

The transmitters in general function in mode known as carried optical : they emit on same the frequencies that they listen, and the communication is not possible that if two antennas are “in sight” (i.e. too not moved away and without obstacle). It is the most flexible mode and most economic, it does not require any infrastructure.

When it is necessary to transmit in a reliable way at long distances, one uses automatic relays : they are fixed stations without operator whose antennas are located in height. The transmitters (portable, mobile or fixed) emit on the frequency ƒ₁ and listen to the frequency ƒ₂; two transmitters cannot thus communicate together directly. By sending a particular selective calling, that activates the relay; this one receives the messages on the frequency ƒ₁ and the retransmet on the frequency ƒ₂, thus, all the stations can hear the transmitted messages.

Sometimes, the signal starting the relay is emitted automatically when the pedal of emission is inserted.

---- Note : the radios receive the signals of the frequencies ƒ₁ and ƒ₂, but only the signals emitted on ƒ₂ are selected; the stations with listening thus receive the message simultaneously on two frequencies, but only that coming from the relay is restored. ----

Radiotelefony and fixed telephones

When a relay is used, it is possible to inter-connect the network of radiotelefony with the network of fixed telephony (and thus the mobile telephone network) via the relay. This system is used much by the companies of Taxi. When a person calls the phone number since her fixed telephone, that starts the relay and all that the person says is emitted on the waves; the radio communications are also rocked on the network of fixed telephone. For the person on the telephone, the communication is done in a usual way, like it not being in general with the fact of conventions of radiotelefony, the radio station thus sticks to speak normally, and only when it feels that the person on the telephone finished speaking.

See too

Radio ham
VHF ( very high frequencies )
BLU (single edge line)
Receiving radio operator
Citizen-band

External bonds

National agency of the frequencies (France)
File on the radio operator procedures of the Secourisme-pratique.com site (document pdf, 2p, 20 KB)

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