Can someone please guide me about "iCAT's"

[pushycat]

Can anyone guide me about the operation and the frequency used by the iCAT's ( Intelligent Computerised Anti theft) system used in modern cars? In fact the company mentions in the user manual about an immobiliser keys used and the car woudnt start with a key made by a local key maker, but nothing was found about its  operation in detail. Browsing through the net i found that those keys are have an RFID chip embedded which communicates with the ECU (Electronic control unit) fitted in the engine which then matches the code of the key with that in the ECU and only a valid code will crank the engine. Actaully i happened to scan a pair of keys with 125Khz and 13.5 Mhz RFID readers but coudnt read anything. Do anyone know about how the keys communicate with the ECU and what frequency is used?