Denti,
It is a "simple" software mod in the digital radio on the aircraft, only applies to 3 Rockwell Collins models. Basically the aircraft radio will not transmit unless it senses no other radios are transmitting (that is CSMA). The ground station sends a heartbeat message to each aircraft every 2 minutes, if the aircraft fails to respond to 3 of these, the network locks the aircraft out (in ATC talk it is known as a provider abort). The cone head medical diagnosis of this is call VHF digital radio deafness, of VDR deafness for short.