An IFF system is a two-channel system that uses the 1030 MHz band for query signals and the 1090 MHz band for response signals. It can be divided into four modes, two for military use and two for military and civilian aircraft:
Mode 1 - This mode has 64 response codes and is used in military air traffic control to determine what type of aircraft it responds to or what type of mission it is in.
Mode 2 – This mode is for military use only; it has 4096 possible response codes that identify the "Queue number" of an aircraft.
Module C - Automatic altitude reporting mode. Returns the current altitude of the aircraft.
Mode 3/A - This is the standard mode of air traffic control. It is used with automatic altitude reporting mode (C mode), to provide identity and positive control to all flying aircraft according to instrument flight rules.
IFF systems can communicate encrypted to avoid identification or imitation by the enemy, or they can respond with clear text if the aircraft operates according to the rules of civil air traffic.