RFID readers operate contactless with magnetic or electromagnetic coupling between the reader and the RFID card. The RFID tag and the RFID reader communicate with each other via the Contactless Chipcard Interface( CCI). Depending on the RFID frequency used, the distance between the RFID card and reader can be a few centimeters and up to ten meters
The field strength of the RFID reader depends on whether it is an active or passive concept. For passive and semi-passive RFID cards and radio tags, the transmitting field strength is about a thousand times higher than for active RFID tags, because passive tags generate the supply energy from the carrier signal of the RFID reader.
Passive RFID tags differ from semi-passive ones in that the passive ones derive the operating voltage from the inductive or electromagnetic field, whereas the semi-passive ones have a small battery that powers the volatile memories. Active RFID transponders have a battery and also a transmitter for data transfer.
To read data from an RFID tag, the RFID reader generates a high-frequency electromagnetic field at the appropriate RFID frequency. The RFID tag, which is placed in the vicinity of the alternating field, receives the carrier signal and derives a voltage from it, which is used to charge the battery of the passive and semi-passive tags. This voltage forms the supply voltage of the microprocessor. The signal transmissionfrom the tag is carried out by a kind of load modulation of the RF field from the reader, in which the microprocessor switches components on and off, thus influencing the alternating field. Depending on the frequency, the power generation and load modulation are done with inductive or electromagnetic coupling.
Communication in RFID
Communication between RFID reader and contactless smart card can be duplex, half-duplex and sequential. In half-duplex mode, energy transmission and data transmission between the RFID reader and the transponder take place alternately. Energy is transmitted for 60 ms as soon as a transponder is in the receiving range of the reader. In general, the operating mode has a direct influence on the range. ISO/ IEC have specified the physical and data properties of the transmission path between the RFID reader and a contactless smart card in various standards. For example, ISO 18092 specifies the transmission standard and ISO 14443 the physical and data properties of the transmission link between the RFID card or PICC card and the RFID reader, the proximity coupling device( PCD), and ISO 18000 describes the protocols used on the contactless chipcard interface (CCI). For example, the Logical Link Control Protocol( LLCP).
In the case of RFID devices, there are also combined read/write devices with which RFID tags can be read and written.