The card and the reader unit communicate with each other through 125 kHz radio frequency fields (13.56 MHz for the contactless smartcard cards) by a process called resonant energy transfer. Passive cards have three components which are sealed inside the plastic: an antenna consisting of a coil of wire, a capacitor, and an integrated circuit (IC) which contains the user's ID number in specific formats and no other data. The reader has its own antenna, which continuously transmits a short range radio frequency field.