Patent ID: 7209706

Claim:
A communication apparatus for sending and receiving data by electromagnetic waves, comprising: electromagnetic wave generating means for forming a radio frequency (RF) field by generating electromagnetic waves; modulating means for sending data at one of a plurality of transmission rates by modulating electromagnetic waves; demodulating means for demodulating electromagnetic waves so as to acquire data sent from other apparatus at one of the plurality of transmission; and a detector configured to detect a radio frequency field within a first time period before the communication apparatus initiates a communication with other communication apparatus and within a second time period before the communication apparatus responds to a request of said other communication apparatus, the first time period is defined by T IDT +nxT RFW , where T IDT is a first initial delay time, n is a first random number, and T RFW is a first radio frequency waiting time, and said communication apparatus generates a first radio frequency when the radio frequency field is not detected by the detector within the first time period, the second time period is defined by T ADT +n′xT RFW , where T ADT is a second active delay time, n′ is a second random number, and T IDT is larger than T ADT and said communication apparatus generates a second radio frequency when the radio frequency field is not detected by the detector within the second time period, wherein data for requesting an identification (ID) to identify the other apparatus is sent, the ID sent from the other apparatus in reply to the request for the ID is acquired, and the ID sent from the other apparatus includes a random number generated in the other apparatus; data that includes the ID of the other apparatus is sent as the data for the other apparatuses after acquiring the ID of the other apparatus, and data for requesting the ID is sent again if the ID of the other apparatus has not been properly acquired.