A simple handheld radiotelephone is sometimes called a "walkie-talkie". CB Radio is also a very similar technology. While in recent years the cell phone has largely superseded radiotelephones for the average user, they are still widely used in many more specialist applications, for example police communications, emergency services, taxi services, private mobile radio networks (PMR).
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2 Technology 3 Privacy and selective calling |
A standard land line based telephone allows both users to talk and listen simultaneously; effectively there are two open channels between the two end-to-end users of the system. In a radiotelephone system, this form of working, known as full duplex, is unusual. That's because it would require a radio system to simultaneously transmit and receive on two separate channels, which is both a waste of bandwidth, and presents some technical challenges. It is however, the most comfortable method of voice communication for users, and is used in cell phones.
The most common method of working for radiotelephones is single or dual-frequency simplex operation, which allows one person to talk and the other to listen alternately. If a single channel is used, both ends take turns to transmit on it. An eavesdropper would hear both sides of the conversation. Dual frequency working splits the communication into two separate channels, but only one is used to transmit at a time. The end users have the same experience as single frequency simplex, but an eavesdropper would only hear one side of the conversation.
A halfway house system called half duplex allows one end to transmit and receive simultaneously, but the other to talk and listen alternately.
The user presses a special switch on the transmitter when she wishes 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 special code-word such as "over" to signal that they have finished transmitting, or it may follow from the conversation.
Radiotelephones may operate at any frequency where they are licensed to do so, though typically they are used in the various bands between 60 MHz and 900 MHz. They may use simple modulation schemes such as AM or FM, or more complex techniques such as digital coding, spread spectrum, and so on. Licensing terms for a given band will usually specify the type of modulation to be used. For example, airband radiotelephones used for air to ground communication between pilots and controllers operates in the VHF band from 110 MHz to 130 MHz approximately, 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 watts for a mobile unit, up to a couple of hundred watts for a base station. Multiple channels are often provided using a Frequency synthesiser.
Receivers usually features a squelch circuit to cut off the audio output from the receiver when there is no transmission to listen to - this is in contrast to broadcast receivers which often dispense with this.
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 assist with directing messages to the correct recipients and avoiding irrelevant traffic on the network 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, his 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 subaudio 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 her unique address to the base, so they 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.Modes of operation
Technology
Privacy and selective calling