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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 on a channel. Selective calling is used to address a subset of all two-way radios on a single radio frequency channel. more...

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Where more than one user is on the same channel, (co-channel users,) 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.

Selective calling is akin to the use of a lock on a door. A radio with carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct code. Selective calling systems can overlap; a radio may have CTCSS (group call) and DTMF individual calling.

Selective calling prevents you from hearing others on a shared channel. It does not eliminate interference from co-channel users. If another user is talking and you try to talk at the same time, your signal will be affected by the other party using the channel.

Some selective calling systems experience falsing. In other words, the decoder activates when a valid signal is not present. Falsing may come from a maintenance problem or poor engineering.

Group calling

In conventional US FM two-way radio systems, the most common form of selective calling is CTCSS. Radios made by nearly any manufacturer will work acceptably with existing systems using CTCSS. The system allows groups of radios to remain muted while other users are talking on the channel. In business and industrial systems, two companies could share the same channel without having to listen to calls for each other's staffs. In government systems, users can avoid having to hear users outside their own agency. (Government channels are usually separated by distance between users. Only one local user is assigned to a channel.)

In uses where missed calls are allowable, selective calling can also hide the presence of interfering signals such as receiver-produced intermodulation. Receivers with poor specifications — such as scanners or low-cost mobile radios — cannot reject the unwanted signals on nearby channels in urban environments. The interference will still be present. It will still degrade system performance but by using selective calling the user will not have to hear the noises produced by receiving the interference.

In the US, Federal Communications Commission rules require users of selective calling to switch to carrier squelch before transmitting. In other words, the user must monitor (listen) to make sure the channel is not in use by someone on another selective calling code before transmitting. To enforce this rule, base stations often have a monitor switch on the microphone. The push-to-talk button is split into two segments. One segment turns the selective calling off. The other segment of the button transmits. A mechanical interlock prevents the transmit button from being pressed until the monitor button is down. This is called, "compulsory monitor before transmit." In mobile radios, microphones are stored in a hang-up box. When the microphone is pulled out of the hang-up, the radio reverts to carrier squelch, (the selective calling feature is disabled). The user automatically monitors -- verifies no one else is using the channel -- by pulling the mike out of the hang-up box. Hand-held radios sometimes have LED indicators that show when the channel is in use.

Read more at Wikipedia.org