Selcall

Selcall (selective calling) is a type of squelch protocol used in radio communications systems, in which transmissions include a brief burst of sequential audio tones.

Receivers that are set to respond to the transmitted tone sequence will open their squelch, while others will remain muted.

The transmission of a selcall code involves the generation and sequencing of a series of predefined, audible tones.

A typical tone period selection is 40ms, so for a 5-tone sequence this represents a total selcall duration of 5 x 40ms = 200ms.

Typically a mobile radio will be configured to transmit a preconfigured selcall sequence when the user presses the ‘push-to-talk’ (PTT) button, which will automatically identify them to other devices listening on the same frequency on the radio network.

Leading-edge ANI will transmit the selcall sequence as soon as the user presses the PTT button.

Trailing-edge ANI will transmit the selcall sequence as soon as the user releases the PTT button.

Together, ANI and status provide a convenient way to rapidly relay information via the radio network, without the user having to speak.

For example, an ambulance paramedic in the field, having encountered some emergency, can simply press and release the PTT button on their radio to signal their predicament to the base.

The ANI will identify the caller, the status code will indicate the scenario and the base can dispatch assistance as required.