AVSA-21 set (together with GSSH-A-18 air sets, MTT-UK-1 handsets) is designed to provide all aircrew (including flight attendants) with internal selective and circular communication; to provide all aircrew (except flight attendants) with external radio communication through onboard radio stations (up to 5 radio stations); to provide service staff with internal telephone communication including all aircrew (through AB 20 blocks) – up to 3 connection points; to receive (wiretap) signals: up to 4 of 12 radio navigation devices (simultaneous wiretap); up to 10 sensors of special signals; emergency receiver signals - up to 2 channels; transmission of analog low-frequency signals to the documentation equipment - up to 3 channels; loudspeaker alert warning passengers and aircrew in the cabin, in the cargo bay and in the cabin (salons) of the aircraft with following levels of nominal electrical power of the sound signal: in the cockpit – up to 5 W; in the cargo bay– up to 5 W; in the cabin of the aircraft – up to 30 W (sound power is distributed to the sound columns, the total nominal power of which is not less than 30 W); batch data exchange containing spoken, control and managerial information, with the control module as part of the BKS IMA platform by AFDX/ETHERNET interface; light indication and audible alarm of « Selective Call» command to the LE cliker.