Patent ID: 6275476
Filing Date: 2001-08-14
Classification: G06K,G06Q,G07B,H04L,H04W

Abstract:
A system comprising:an interrogator configured to communicate to a selected one or more of a number of communications devices;a plurality of communications devices, respective devices being configured to store unique identification numbers respectively having a first predetermined number of bits, respective devices being further configured to respectively store a second predetermined number of bits to be used for respective random values, respective devices being configured to select random values independently of random values selected by the other devices;the interrogator being configured to define a group of random values, the group being a subset of all possible values the devices could select, and to transmit a command requesting devices having random values corresponding to any of the random values in the defined group to respond;devices respectively being configured to, in response to receiving the command, determine if their chosen random values fall within the defined group and, if so, send a reply to the interrogator within a randomly selected time slot of a number of possible time slots, in accordance with an Aloha technique; and, if not, not send a reply, wherein sending a reply to the interrogator comprises transmitting both the random value of the device sending the reply and the unique identification number of the device sending the reply, wherein the time slot randomly selected by a device is selected using a random number different from the random value of that device, and wherein the number of possible slots varies from one specified group to another;the interrogator being configured to determine if a collision occurred between devices that sent a reply and, if so, to define a new, smaller, specified group; andthe interrogator being configured to send a command individually addressed to a device after communicating with a device without a collision.