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