What is Group calling?

In conventional FM two-way radio systems, the most common form of selective calling is CTCSS, which is based on a sub-sudible tone. One implementation of this system is by Motorola and is called Private Line, or PL. 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, as many as 50 sets of users 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 user groups. Only one local user group 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 and 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 monitor the channel (i.e. 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.