DCS is a digital squelch scheme sent by a transmitter to control the un-muting of the receiving radio's speaker. Like CTCSS it is designed to work in the "so called" sub-audible portion of the communications audio band below 300 Hz.
Instead of sending an analog tone like CTCSS does, DCS signals are digital square wave signals where the square wave signal represents '1' and '0' data bits. There are a total of 23 bits sent, and these bits are decoded by the receiving radio.
There is a loose standard set of 83 DCS codes used by most of two-way radio manufacturers .