The ICAO requires all passenger aircraft to be equipped with TCAS - Traffic Collision Avoidance System. It is a last line of defense to avoid collision. When two TCAS-equipped airplanes are on a collision course, the TCAS modules will contact each other and negotiate, then issue corrective actions to their respective pilots - one to ascend, and the other to descend. Responding to a TCAS command is mandatory and overrules ATC instructions.