These tones have many names:
Tones are often attached to a repeater's uplink frequency to control access or limit noise/interference. In the commercial radio applications and invented by Motorola, the term is known as PL, which was short for Motorola Private Line. In the amateur radio community, the non-propriety term of Continuous Tone-Coded Squelch System, or CTCSS, if often used.
These tones were originally analog only (see chart below). Later, a digital method was also introduced known as Digital Coded Squelch, or DCS. These methods are only used on analog systems. Other methods for controlling access are available for different transmission modes and are discussed on their respective pages.
The tone system can also be applied to the downlink tone to limit interference and noise on the user/client radio. There are many sources of noise that can break the radio's set squelch threshold, even when the repeater is not transmitting. Although the radio will still internally “hear” the noise, it will not open the squelch until the proper tone has been received.
To reduce confusion as to which tone is being referenced in terms of a paired frequency set (duplex with separate input and output frequencies, as repeaters typically use, the uplink tone refers to the tone that the user radio transmits with the uplink frequency and is required to activate (key) the repeater. This is the tone that is programmed into the user radio (HT, mobile, base) that you transmit to the repeater on.
Confusion sometimes occurs when it is not known from which radio, the user's or the repeater's, that is being requested. Usually, it seems to get turned around when a repeater owner enters that data. Many repeater coordination councils request the repeater's tones in terms of the repeater's transmit and receive tones, which are opposite from the user radio's on a duplex system.
Repeaterbook enters and displays data from the perspective of the remote (user) radio.
Despite the propriety term by Motorola “Private Line”, these tones offer no privacy. Some repeater owners decline to publish their tones in an effort to deter users from operating their repeaters. However, most modern amateur radios include a tone scan function and modern scanners can locate a tone in less than a second. If all else fails, a user could simply try each tone until one successfully activated the repeater.
All tones are in Hz.