Communications systems for radio frequency identification (RFID)

Abstract

A method of system for establishing wireless communications between an interrogator and individual ones of multiple wireless identification devices, the method comprising utilizing a tree search method to establish communications without collision between the interrogator and individual ones of the multiple wireless identification devices, a search tree being defined for the tree search method, the tree having multiple levels respectively representing subgroups of the multiple wireless identification devices, the method further comprising starting the tree search at a selectable level of the search tree. A communications system comprising an interrogator, and a plurality of wireless identification devices configured to communicate with the interrogator in a wireless fashion, the respective wireless identification devices having a unique identification number, the interrogator being configured to employ a tree search technique to determine the unique identification numbers of the different wireless identification devices so as to be able to establish communications between the interrogator and individual ones of the multiple wireless identification devices without collision by multiple wireless identification devices attempting to respond to the interrogator at the same time, wherein the interrogator is configured to start the tree search at a selectable level of the searched tree. The interrogator transmits a first request comprising a bit string comprising a plurality of bits for use in selecting one or more wireless identification devices to participate in a slotted anticollision algorithm. Wireless identification devices in a field of the interrogator use the bit string to determine whether or not they are selected for participation. Those wireless identification devices that are selected for participation respond to the interrogator in a slot with a certain probability in accordance with the slotted anticollision algorithm.

1. A method of establishing wireless communications between an interrogator and individual ones of multiple wireless identification devices, the wireless identification devices having respective identification numbers and being addressable by specifying identification numbers with any one of multiple possible degrees of precision, the method comprising utilizing a tree search in an arbitration scheme to determine a degree of precision necessary to establish one-on-one communications between the interrogator and individual ones of the multiple wireless identification devices, a search tree being defined for the tree search method, the tree having multiple selectable levels respectively representing subgroups of the multiple wireless identification devices, the level at which a tree search starts being variable the method further comprising starting the tree search at any selectable level of the search tree. 
     
     
       2. A method in accordance with  claim 1  and further comprising determining the maximum possible number of wireless identification devices that could communicate with the interrogator, and selecting a level of the search tree based on the determined maximum possible number of wireless identification devices that could communicate with the interrogator. 
     
     
       3. A method in accordance with  claim 2  and further comprising starting the tree search at a level determined by taking the base two logarithm of the determined maximum possible number, wherein the level of the tree containing all subgroups is considered level zero, and lower levels are numbered consecutively. 
     
     
       4. A method in accordance with  claim 2  and further comprising starting the tree search at a level determined by taking the base two logarithm of the determined maximum possible number, wherein the level of the tree containing all subgroups is considered level zero, and lower levels are numbered consecutively, and wherein the maximum number of devices in a subgroup in one level is half of the maximum number of devices in the next higher level. 
     
     
       5. A method in accordance with  claim 2  and further comprising starting the tree search at a level determined by taking the base two logarithm of the power of two nearest the determined maximum possible number, wherein the level of the tree containing all subgroups is considered level zero, and lower levels are numbered consecutively, and wherein the maximum number of devices in subgroup in one level is half of the maximum number of devices in the next higher level. 
     
     
       6. A method in accordance with  claim 1  wherein the wireless identification device comprises an integrated circuit including a receiver, a modulator, and a microprocessor in communication with the receiver and modulator. 
     
     
       7. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices, the method comprising:
 establishing for respective devices unique identification numbers respectively having a first predetermined number of bits;    establishing a second predetermined number of bits to be used for random values;    causing the devices to select random values, wherein respective devices choose random values independently of random values selected by the other devices;    determining the maximum number of devices potentially capable of responding to the interrogator;    transmitting a command from the interrogator requesting devices having random values within a specified group of random values to respond, by using a subset of the second predetermined number of bits, the specified group being chosen in response to the determined maximum number;    receiving the command at multiple devices, devices receiving the command respectively determining if the random value chosen by the device falls within the specified group and, if so, sending a reply to the interrogator; and    determining using the interrogator if a collision occurred between devices that sent a reply and, if so, creating a new, smaller, specified group.    
     
     
       8. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices in accordance with  claim 7  wherein sending a reply to the interrogator comprises transmitting the unique identification number of the device sending the reply. 
     
     
       9. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices in accordance with  claim 7  wherein sending a reply to the interrogator comprises transmitting the random value of the device sending the reply. 
     
     
       10. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices in accordance with  claim 7  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. 
     
     
       11. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices in accordance with  claim 7  wherein, after receiving a reply without collision from a device, the interrogator sends a command individually addressed to that device. 
     
     
       12. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices, the method comprising:
 causing the devices to select random values for use as arbitration numbers, wherein respective devices choose random values independently of random values selected by the other devices, the devices being addressable by specifying arbitration numbers with any one of multiple possible degrees of precision;    transmitting a command from the interrogator requesting devices having random values within a specified group of a plurality of possible groups of random values to respond, the specified group being less than the entire set of random values, the plurality of possible groups being organized in a binary tree defined by a plurality of nodes at respective levels, wherein the size of groups of random values decrease in size by half with each node descended, wherein the specified group is below a node on the tree selected based on the maximum number of devices capable of communicating with the interrogator;    receiving the command at multiple devices, devices receiving the command respectively determining if the random value chosen by the device falls within the specified group and, if so, sending a reply to the interrogator; and, if not, not sending a reply; and    determining using the interrogator if a collision occurred between devices that sent a reply and, if so, creating a new, smaller, specified group by descending in the tree.    
     
     
       13. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices in accordance with  claim 12  and further including establishing a predetermined number of bits to be used for the random values. 
     
     
       14. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices in accordance with  claim 13  wherein the predetermined number of bits to be used for the random values comprises an integer multiple of eight. 
     
     
       15. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices in accordance with  claim 13  wherein devices sending a reply to the interrogator do so within a randomly selected time slot of a number of slots. 
     
     
       16. A method of addressing messages from an interrogator to a selected one or more of a number of RFID devices, the method comprising:
 establishing for respective devices a predetermined number of bits to be used for random values, the predetermined number being a multiple of sixteen;    causing the devices to select random values, wherein respective devices choose random values independently of random values selected by the other devices;    transmitting a command from the interrogator requesting devices having random values within a specified group of a plurality of possible groups of random values to respond, the specified group being equal to or less than the entire set of random values, the plurality of possible groups being organized in a binary tree defined by a plurality of nodes at respective levels, wherein the maximum size of groups of random values decrease in size by half with each node descended, wherein the specified group is below a node on a level of the tree selected based on the maximum number of devices known to be capable of communicating with the interrogator;    receiving the command at multiple devices, devices receiving the command respectively determining if the random value chosen by the device falls within the specified group and, only if so, sending a reply to the interrogator, 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;    using the interrogator to determine if a collision occurred between devices that sent a reply and, if so, creating a new, smaller, specified group using a level of the tree different from the level used in the interrogator transmitting, the interrogator transmitting a command requesting devices having random values within the new specified group of random values to respond; and    if a reply without collision is received from a device, the interrogator subsequently sending a command individually addressed to that device.    
     
     
       17. A method of addressing messages from an interrogator to a selected one or more of a number of RFID devices in accordance with  claim 16  and further comprising determining the maximum possible number of wireless identification devices that could communicate with the interrogator. 
     
     
       18. A method of addressing messages from an interrogator to a selected one or more of a number of RFID devices in accordance with  claim 16  wherein selecting the level of the tree comprises taking the base two logarithm of the determined maximum possible number, wherein a level of the tree containing all subgroups is considered level zero, and lower levels are numbered consecutively. 
     
     
       19. A method of addressing messages from an interrogator to a selected one or more of a number of RFID devices in accordance with  claim 16  wherein selecting the level of the tree comprises taking the base two logarithm of the determined maximum possible number, wherein a level of the tree containing all subgroups is considered level zero, and lower levels are numbered consecutively, and wherein the maximum number of devices in a subgroup in one level is half of the maximum number of devices in the next higher level. 
     
     
       20. A method of addressing messages from an interrogator to a selected one or more of a number of RFID devices in accordance with  claim 16  wherein selecting the level of the tree comprises taking the base two logarithm of the power of two nearest the determined maximum possible number, wherein the level of the tree containing all subgroups is considered level zero, and lower levels are numbered consecutively, and wherein the maximum number of devices in a subgroup in one level is half of the maximum number of devices in the next higher level. 
     
     
       21. A method of addressing messages from an interrogator to a selected one or more of a number of RFID devices in accordance with  claim 16  wherein the wireless identification device comprises an integrated circuit including a receiver, a modulator, and a microprocessor in communication with the receiver and modulator. 
     
     
       22. A method of addressing messages from an interrogator to a selected one or more of a number of RFID devices in accordance with  claim 16  and further comprising, after the interrogator transmits a command requesting devices having random values within the new specified group of random values to respond, determining, using devices receiving the command, if their chosen random values fall within the new smaller specified group and, if so, sending a reply to the interrogator. 
     
     
       23. A method of addressing messages from an interrogator to a selected one or more of a number of RFID devices in accordance with  claim 22  and further comprising, after the interrogator transmits a command requesting devices having random values within the new specified group of random values to respond, determining if a collision occurred between devices that sent a reply and, if so, creating a new specified group and repeating the transmitting of the command requesting devices having random values within a specified group of random values to respond using different specified groups until all of the devices within communications range are identified. 
     
     
       24. A communications system comprising an interrogator, and a plurality of wireless identification devices configured to communicate with the interrogator in a wireless fashion, the wireless identification devices having respective identification numbers, the interrogator being configured to employ a tree search in a search tree having multiple selectable levels, to determine the identification numbers of the different wireless identification devices with sufficient precision so as to be able to establish one-on-one communications between the interrogator and individual ones of the multiple wireless identification devices, wherein the interrogator is configured to start the tree search at any selectable level of the search tree. 
     
     
       25. A communications system in accordance with  claim 24  wherein the tree search is a binary tree search. 
     
     
       26. A communications system in accordance with  claim 24  wherein the wireless identification device comprises an integrated circuit including a receiver, a modulator, and a microprocessor in communication with the receiver and modulator. 
     
     
       27. A system comprising:
 an interrogator;    a number of communications devices capable of wireless communications with the interrogator;    means for establishing a predetermined number of bits to be used as random numbers, and for causing respective devices to select random numbers respectively having the predetermined number of bits;    means for inputting a predetermined number indicative of the maximum number of devices possibly capable of communicating with the receiver;    means for causing the interrogator to transmit a command requesting devices having random values within a specified group of random values to respond, the specified group being chosen in response to the inputted predetermined number;    means for causing devices receiving the command to determine if their chosen random values fall within the specified group and, if so, send a reply to the interrogator; and    means for causing the interrogator to determine if a collision occurred between devices that sent a reply and, if so, create a new, smaller, specified group.    
     
     
       28. A system in accordance with  claim 27  wherein sending a reply to the interrogator comprises transmitting the random value of the device sending the reply. 
     
     
       29. A system in accordance with  claim 27  wherein the interrogator further includes means for, after receiving a reply without collision from a device, sending a command individually addressed to that device. 
     
     
       30. 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;    the devices being configured to select random values, wherein respective devices choose random values independently of random values selected by the other devices, different sized groups of devices being addressable by specifying random values with differing levels of precision;    the interrogator being configured to transmit a command requesting devices having random values within a specified group of a plurality of possible groups of random values to respond, the specified group being less than the entire set of random values, the plurality of possible groups being organized in a binary tree defined by a plurality of nodes at respective levels, wherein the size of groups of random values decrease in size by half with each node descended, wherein the specified group is below a node on the tree selected based on a predetermined maximum number of devices capable of communicating with the interrogator;    devices receiving the command being configured to respectively determine if their chosen random values fall within the specified group and, if so, send a reply to the interrogator; and, if not, not send a reply; and    the interrogator being configured to determine if a collision occurred between devices that sent a reply and, if so, create a new, smaller, specified group by descending in the tree.    
     
     
       31. A system in accordance with  claim 30  wherein the random values respectively have a predetermined number of bits. 
     
     
       32. A system in accordance with  claim 30  wherein respective devices are configured to store unique identification numbers of a predetermined number of bits. 
     
     
       33. A system in accordance with  claim 30  wherein respective devices are configured to store unique identification numbers of sixteen bits. 
     
     
       34. A system comprising:
 an interrogator configured to communicate to a selected one or more of a number of RFID devices;    a plurality of RFID devices, respective devices being configured to store unique identification numbers respectively having a first predetermined number of bits, respective devices being further configured to store a second predetermined number of bits to be used for random values, respective devices being configured to select random values independently of random values selected by the other devices;    the interrogator being configured to transmit an identify command requesting a response from devices having random values within a specified group of a plurality of possible groups or random values, the specified group being less than or equal to the entire set of random values, the plurality of possible groups being organized in a binary tree defined by a plurality of nodes at respective levels, wherein the maximum size of groups of random values decrease in size by half with each node descended, wherein the specified group is below a node on a level of the tree selected based on the maximum number of devices known to be capable of communicating with the interrogator;    devices receiving the command respectively being configured to determine if their chosen random values fall within the specified group and, only if so, send a reply to the interrogator, 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;    the interrogator being configured to determine if a collision occurred between devices that sent a reply and, if so, create a new, smaller, specified group using a level of the tree different from the level used in previously transmitting an identify command, the interrogator transmitting an identify command requesting devices having random values within the new specified group of random values to respond; and    the interrogator being configured to send a command individually addressed to a device after communicating with a device without a collision.    
     
     
       35. A system in accordance with  claim 34  wherein the interrogator is configured to input and store the predetermined number. 
     
     
       36. A system in accordance with  claim 34  wherein the devices are configured to respectively determine if their chosen random values fall within a specified group and, if so, send a reply, upon receiving respective identify commands. 
     
     
       37. A system in accordance with  claim 34  wherein the interrogator is configured to determine if a collision occurred between devices that sent a reply in response to respective identify commands and, if so, create further new specified groups and repeat the transmitting of the identify command requesting devices having random values within a specified group of random values to respond using different specified groups until all responding devices are identified. 
     
     
       38. A system, comprising:
   an interrogator comprising:      one or more antennas,        a transmitter and a receiver coupled to the one or more antennas, and        a controller coupled to the transmitter and the receiver, the controller to cause the transmitter to transmit, via the one or more antennas, an initial command to select one or more radio frequency identification  ( RFID )  devices from among a potential plurality of RFID devices that are within communications range, the initial command specifying a bit string having multiple bits; and          the one or more RFID devices, each respective device of the one or more RFID devices including:      memory to store a respective identification code,        a receiver to receive the initial command from the interrogator,        a circuit to compare the bit string specified in the initial command against corresponding bits stored in the respective device to determine whether the respective device is a member of a population of RFID devices selected according to the initial command, and if the respective device is a member of the population, pick a respective random value from a range of values to determine a respective slot, and        a transmitter to communicate a respective reply, including at least a portion of an identifier of the respective device, to the interrogator in accordance with the respective slot.        
     
     
       39. The system of  claim 38 , wherein the interrogator is configured to send an acknowledge command in response to the interrogator receiving the respective reply from the respective device and in response to the interrogator determining the respective reply to be collision- free; and the respective reply includes the bit string.    
     
     
       40. The system of  claim 38 , wherein the respective device is to communicate a respective identification code to identify a person with whom the respective device is associated.  
     
     
       41. The system of  claim 38 , wherein the interrogator is to further access the respective device individually by sending the identifier to the respective device after receiving the identifier from the respective device.  
     
     
       42. The system of  claim 38 , wherein the interrogator is to indicate the range of values.  
     
     
       43. The system of  claim 38 , wherein the identifier comprises a random number generated by the respective device, and the random number is sixteen bits in length.  
     
     
       44. A system comprising:
   an interrogator having a transmitter to send an initial command to select one or more radio frequency identification  ( RFID )  devices, the initial command specifying a bit string having multiple bits; and        an RFID device comprising:      a device memory to store a plurality of bits,        a receiver to receive the initial command,        a circuit to, in response to the initial command, determine whether the RFID device is selected via a comparison between the bit string and the plurality of bits stored in the memory and, if the RFID device is determined to be selected, communicate a response in accordance with a slotted anticollision algorithm, and        a transmitter to communicate one or more identifiers of the RFID device to the interrogator in accordance with the slotted anticollision algorithm, wherein in accordance with the slotted anticollision algorithm the transmitter is to communicate the one or more identifiers in a time slot with a certain probability.        
     
     
       45. The system of  claim 44 , wherein the one or more identifiers comprise a sixteen bit random number and the interrogator is configured to send an acknowledge command to the RFID device if the interrogator receives the random number without a collision error.  
     
     
       46. The system of  claim 44 , wherein the RFID device is further configured to communicate an identification code to the interrogator to identify a person with whom the RFID device is associated.  
     
     
       47. The system of  claim 44 , wherein the interrogator is further configured to individually address the RFID device using an access command, wherein the access command includes a random number to be included in the one or more identifiers.  
     
     
       48. The system of  claim 44 , wherein the one or more identifiers comprise both a random number dynamically generated by the RFID device and a static number programmed into the RFID device.  
     
     
       49. The system of  claim 44 , wherein the transmitter is to further communicate the plurality of bits, along with the one or more identifiers, to the interrogator in accordance with the slotted anticollision algorithm.  
     
     
       50. A system, comprising:
   a radio frequency identification  ( RFID )  device disposed in a communication field of an interrogator, the RFID device including:      a receiver to receive a first command from the interrogator after the RFID device is disposed in the field and before the interrogator transmits any other command, the first command specifying a bit string comprising two or more bits,        a circuit to, in response to the first command, use the bit string to determine if the RFID device is selected for participation in a slotted anticollision algorithm, and        a transmitter to communicate one or more responses to the interrogator in accordance with the slotted anticollision algorithm if the RFID device is determined to be selected for participation in the slotted anticollision algorithm, wherein the one or more responses include at least a portion of a first identifier and at least a portion of a second identifier stored in the RFID device.        
     
     
       51. The system of  claim 50 , wherein the first identifier comprises a random number dynamically generated by the RFID device for the interrogator to use to individually address the RFID device, and the second identifier is a static number stored in the RFID device.  
     
     
       52. The system of  claim 51 , wherein the receiver is to further receive from the interrogator an indication of a number of slots from which the RFID device is to randomly select a slot in which to communicate the one or more responses in accordance with the slotted anticollision algorithm.  
     
     
       53. The system of  claim 51 , wherein the receiver is to further receive from the interrogator an indication of a change in a number of slots in accordance with the slotted anticollision algorithm.  
     
     
       54. The system of  claim 51 , wherein the RFID device is configured for use in a wireless payment application and further comprises memory storing an identification code to identify a person to be charged for payment.  
     
     
       55. The system of  claim 51 , wherein the random number is sixteen bits long and the transmitter is to communicate the bit string back to the interrogator with the one or more responses.  
     
     
       56. The system of  claim 51 , wherein the RFID device is configured to pick a random value from a range of values and to communicate the one or more responses with a probability corresponding to the random value in accordance with the slotted anticollision algorithm, wherein the first command is to indicate the range of values.  
     
     
       57. The system of  claim 56 , wherein the receiver is to further receive a second command comprising an indication of a change in the range of values.  
     
     
       58. The system of  claim 51 , wherein the transmitter is to communicate the bit string back to the interrogator with the one or more responses.  
     
     
       59. The system of  claim 50 , wherein the RFID device is configured to receive a signal from the interrogator, after the interrogator sends the first command to the RFID device and before the RFID device communicates the one or more responses to the interrogator, wherein the signal indicates the RFID device when to communicate the identifier to the interrogator.  
     
     
       60. The system of  claim 50 , wherein the RFID device communicates the one or more responses via modulating an radio frequency ( RF )  field provided by the interrogator.    
     
     
       61. The system of  claim 50 , wherein the transmitter is to communicate the bit string back to the interrogator with the one or more responses.  
     
     
       62. The system of  claim 50 , wherein the RFID device is configured for use in a wireless payment system and further comprises memory storing an identification code to identify a person to be charged for payment.  
     
     
       63. The system of  claim 50 , wherein the RFID device is configured to randomly pick an integer from a number of integers and to communicate the one or more responses in a first slot with a probability corresponding to the randomly picked integer in accordance with the slotted anticollision algorithm, wherein the first command is to indicate the number of integers.  
     
     
       64. The system of  claim 63 , wherein the receiver is to further receive from the interrogator a second command, the second command to indicate a different number of integers for the RFID device to use in accordance with the slotted anticollision algorithm.  
     
     
       65. A system comprising:
   an interrogator including:      at least one antenna to provide a radio frequency  ( RF )  field, a plurality of radio frequency identification  ( RFID )  devices to modulate the RF field to transmit responses to the interrogator;        a transmitter to send a first signal after the plurality of RFID devices are disposed in the field and before any of the plurality of RFID devices transmit responses to the interrogator, the first signal including a bit string comprising multiple bits; and        a receiver to receive responses to the first signal in accordance with a slotted anticollision algorithm;          wherein the plurality of RFID devices include:      a first RFID device configured to store a first set of bits,        receive the first signal and determine if the first RFID device is selected by the interrogator, including comparing the bit string with the first set of bits, and if so,      pick a first random integer from a variable range of random integers and associate the first random integer with a first time slot in accordance with the anticollision algorithm, and        modulate the RF field to communicate a first identification code of the first RFID device during the first time slot; and            a second RFID device to      store a second set of bits,        receive the first signal and determine if the second device is selected by the interrogator, including comparing the bit string with the second set of bits, and if so,      pick a second random integer from a variable range of random integers and associate the second random integer with a second time slot in accordance with the anticollision algorithm, and        modulate the RF field to communicate a second identification code of the second RFID device during the second time slot.          
     
     
       66. The system of  claim 65 , wherein the first identification code includes a first random number generated by the first RFID device; and the second identification code includes a second random number generated by the second RFID device.  
     
     
       67. The system of  claim 66 , wherein the interrogator is configured to receive the first random number from the first RFID device during a period of time associated with the first slot and, in response thereto, to send a first acknowledge signal to acknowledge the first RFID device; and the interrogator is configured to receive the second random number from the second RFID device during a period of time associated with the second slot and, in response thereto, to send a second acknowledge signal to acknowledge the second RFID device.  
     
     
       68. The system of  claim 66 , wherein after receiving the first random number and the first identification code from the first RFID device, the interrogator is to send a command that includes the first random number to individually identify the first RFID device.  
     
     
       69. The system of  claim 68 , wherein the interrogator is configured to receive a first identifier from the first RFID device to identify a person with whom the first RFID device is associated and to receive a second identifier from the second RFID device to identify a person with whom the second RFID device is associated.  
     
     
       70. The system of  claim 69 , wherein the interrogator is configured to send a second signal after sending the first signal; and wherein the first RFID device is configured to communicate the first identification code in response to receiving the second signal.  
     
     
       71. The system of  claim 65 , wherein the interrogator is configured to send a first acknowledge signal to acknowledge the first RFID device and a second acknowledge signal to acknowledge the second RFID device.  
     
     
       72. A system comprising:
   an interrogator including:      an antenna to provide a radio frequency  ( RF )  field to interrogate;        a transmitter to send an initial command to identify radio frequency identification  ( RFID )  devices disposed in the field, the initial command to be sent after the RFID devices are disposed in the field and before any of the RFID devices communicate any responses to the interrogator, the initial command to include a first specifying a plurality of bit values to select one or more of the RFID devices to participate in a slotted anticollision algorithm; and        a receiver to receive responses to the initial command in accordance with the slotted anticollision algorithm; and          an RFID device disposed in the RF field of the interrogator, the RFID device comprising:      a receiver to wirelessly receive the initial command;        a random number generator to randomly select an integer value from a range of integer values in accordance with the slotted anticollision algorithm, the range to be adjustable and to be indicated to the RFID device by the interrogator; and        a transmitter to modulate the RF field to communicate a one or more responses to the interrogator based at least in part on whether the plurality of bit values received from the interrogator identify the RFID device for response, wherein the one or more responses are to include a first identifier and are to be communicated in accordance with the randomly selected integer value in accordance with the slotted anticollision algorithm.        
     
     
       73. The system of  claim 72 , wherein the RFID device is configured to communicate at least a portion of an identification code to the interrogator, wherein the identification code identifies a person with whom the RFID device is associated.  
     
     
       74. The system of  claim 72 , wherein the one or more responses are to further include a second identifier to be communicated in accordance with the randomly selected integer value in accordance with the slotted anticollision algorithm.  
     
     
       75. The system of  claim 74 , wherein the first identifier comprises a random number generated by the RFID device; and the second identifier comprises a static code programmed into the RFID device.  
     
     
       76. The system of  claim 72 , wherein the RFID device is to compare the plurality of bit values to at least a portion of a number stored in the RFID device to determine whether the plurality of bit values received from the interrogator identify the RFID device for response, wherein the one or more responses to the interrogator are to include the number as a second identifier from the RFID device; and wherein the interrogator is configured to send a subsequent command specifically addressed to the RFID device using the identifier.  
     
     
       77. The system of  claim 76 , wherein the RFID device is to communicate the one or more responses in a first slot in accordance with the slotted anticollision algorithm with a first probability corresponding to the integer value.  
     
     
       78. The system of  claim 72 , wherein the range is to be indicated to the RFID device by the initial command, and a different range is to be indicated to the RFID device by a subsequent command, wherein the subsequent command includes a field re- specifying the plurality of bit values to select one or more of the RFID devices.