A RF data communication device operates in at least two states. In a first state, the device transmits a response to an RF transmission if specific data therein represents an inquiry as to the presence of one or more RF data communication devices and if specific data therein corresponds to a class maintained...http://www.google.com.tw/patents/US8045929?utm_source=gb-gplus-share專利 US8045929 - Determining presence of radio frequency communication device
