DTX : in case no useful speech is to be sent on the radio interface, the burst(s) is (are) sent almost empty –> no speech = no usage of the radio resource. This radio resource cannot be used by someone else though. Only benefit is a lower usage of the frequency.
Power control : in case the receiver measures a strong signal from the emitter, then the emitter decreases its TX Power.
In case the receiver measures a weak signal, the emitter increases its TX Power (if possible).
The goal is for the receiver to measure an average signal, because
1) if the signal is too strong –> emitter might interfer with other emitted signals (high frequency usage = higher interference probability)
2) if the signal is too weak, okay, it’s good to avoid interfering other signals, but the receiver might loose the connection.
other benefits of PC and DTX : less battery consumption and less electromagnetic waves traveling in your brain.