This part of ISO/IEC 18000 defines the air interface for radio frequency identification (RFID) devices operating below 135 kHz. The purpose of this part of ISO/IEC 18000 is to provide a common technical specification for RFID devices that can be used by ISO committees developing RFID application standards. This part of ISO/IEC 18000 is intended to allow for compatibility and to encourage inter-operability of products in the international marketplace. This part of ISO/IEC 18000 defines the physical layer used for communication between the interrogator and the tag and further defines the communications protocol used in the air interface. Two types of tag are defined by this part of ISO/IEC 18000: Type A and Type B, which differ only by their physical layer. Both support the same inventory (anti-collision) and protocol. Type A tags are permanently powered by the interrogator, including during the tag-to-interrogator transmission, and operate at 125 kHz. Type B tags are powered by the interrogator, except during the tag-to-interrogator transmission, and operate at 125 kHz or 134,2 kHz.