There are PPM-Pulse Position Modulation also known as FM or Frequency Modulation and PCM-Pulse Code Modulation transmission modulations available. If you go with a PCM unit, you can use PCM only receivers, but most TX's have the PPM or FM as a standard with the PCM as an option. The PCM modulation allows for most TX's to enable a "Failsafe" mode, which allows you to tell the receiver what to do if interference is received, it allows you to set all of the servos/channels to a pre-determined position.