Communication system for communicating with tags

• US 5,686,902 A
• Filed: 04/23/1996
• Issued: 11/11/1997
• Est. Priority Date: 06/15/1990
• Status: Expired due to Term

• Current Assignee: Savi Technology Incorporated
• Original Assignee: Texas Instruments, Inc.
• Inventors: Reis, Robert Steven, Verma, Vikram, Mihovilovic, Domingo Antonio, Bertrand, Peter Scott, Stevens, Richard Kenneth
• Primary Examiner(s): ZIMMERMAN, BRIAN A
• Application Number:
  US08/636,665
• Time in Patent Office: 567 Days
• Field of Search: 340/825.54, 340/825.49, 340/825.15, 342/457
• US Class Current: 340/10.2
• CPC Class Codes: G01S 13/74 Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems
• CPC Class Codes: G01S 13/751 wherein the responder or reflector radiates a coded signal
• CPC Class Codes: G01S 13/765 with exchange of information between interrogator and responder
• CPC Class Codes: G01S 13/767 Responders; Transponders teaching or practice apparatus for gun-aiming or gun-laying using reflecting targets or active targets F41G3/26
• CPC Class Codes: G01S 13/825 with exchange of information between interrogator and responder
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer
• CPC Class Codes: G06K 7/10059 transponder driven
• CPC Class Codes: G06K 7/10346 the antenna being of the far field type, e.g. HF types or dipoles

Abstract

A communication system for identifying, locating, tracking and communicating for other purposes with large numbers of tags in a time and energy efficient manner. The tags are associated with items located in a communication region that is interrogated by an interrogator on a one-to-many basis using broadcast commands, on a one-to-one basis using directed commands or on a combination basis using both methods. Identification occurs through organized transmission and reception of signals between the tags and the interrogator. A batch collection protocol uses a combined one-to-many and one-to-one communication system that effectively resolves communication contentions. The batch collection protocol employs a plurality of collection periods which communicate with large or unknown numbers of portable tags.

Claims

1. A communication system for communicating in a communication region comprising,

interrogator including,

interrogator memory for storing interrogator code including a broadcast command and a directed command and including bandwidth code for controlling the bandwidth of said collection periods,

interrogator processor means connected for accessing said interrogator memory and for processing said interrogator code to sequence said broadcast command and said directed command,

interrogator transceiver means connected to said interrogator processor means for sending said commands and receiving said responses in said communication region over said common communication channel including sending said broadcast command and said directed command a plurality of times,

each of said tags having a unique tag ID and each including,

a tag transceiver for sending said responses to and receiving said commands from said interrogator transceiver means over said common communication channel,

a tag memory for storing information including tag code, a tag processor, connected to said tag transceiver to receive said broadcast command and said directed command said plurality of times, for processing said tag code to send one of said responses to said interrogator through said tag transceiver for one or more of said plurality of times, said processor including selection means for selecting one of said tag communication periods when one of said responses is to be sent.

2. The communication system of claim 1 wherein, said broadcast command requests tag ID'"'"'S from all tags,

for each tag, said tag processor sends a tag ID with said response upon receipt of said broadcast command,

for each tag for which said interrogator receives a tag ID, said interrogator processor sends said directed command to acknowledge that said tag ID has been received.

3. The communication system of claim 1 wherein each of said tags includes a controllable power means for providing power to said tag transceiver in a low-power state for a sleep mode and in a normal-power state for an awake mode where said tag processor controls said power means to select said lower power state or said normal-power state.

4. The communication system of claim 3 wherein,

said broadcast command requests all tags but one specific tag to go to said sleep mode,

for each tag except said one specific tag, said tag processor upon receipt of said broadcast command causes said tag to go to said sleep mode,

for said specific tag, said interrogator processor sends said directed command to command an action by said specific tag,

said tag processor for said specific tag upon receipt of said directed command sends said response including the tag ID to said interrogator.

5. The communication system of claim 3 wherein said interrogator includes wake-up means for sending a wake-up signal to said tags and wherein each of said tags includes wake-up detection means connected to said tag transceiver and responsive to said wake-up signal from said interrogator for providing a signal to said tag processor to cause switching to said normal-power state.

6. The communication system of claim 3 wherein said tag transceiver in each of said tags includes,

a radio frequency transmitter,

a radio frequency receiver,

an antenna connected to said transmitter and receiver.

7. The communication system of claim 2 wherein said interrogator collects the tag ID'"'"'s from tags in the communication region using communications between said interrogators and said tags during said plurality of collection periods, said interrogator having in said interrogator code,

synchronizing code for sending synchronizing signals to said tags to initiate each of said collection periods,

each said tag having means for inhibiting sending a tag ID after receipt of said directed command.

8. The communication system of claim 7 wherein said interrogator establishes said collection periods as collection periods CP(1), CP(2), . . . , CP(p), . . . , CP(P) having respectively listen periods LP(1), LP(2), . . . , LP(p), . . . , LP(P) and having respectively acknowledge periods AP(1), AP(2), . . . , AP(p), . . . , AP(P).

9. The communication system of claim 8 wherein said bandwidth code sets the bandwidth of each collection period by setting the duration of the respective listen period.

10. The communication system of claim 9 wherein for one or more subsequent listen periods after the first listen period LP(1), the listen periods are shorter in duration than the listen period LP(1).

11. The communication system of claim 10 wherein said first listen period LP(1) and said one or more subsequent listen periods each have durations that permit receipt of separate transmissions of tag IDs from a plurality of tags.

12. The communication system of claims 11 wherein for a plurality of said one or more listen periods, each successive listen period is approximately one-half the duration of the preceding listen period.

13. The communication system of claim 9 wherein for said listen periods, each of said tags has response means for distributing the tag ID to a particular response time during the respective listen period.

14. The communication system of claim 13 wherein said particular response time is determined for each said tag by a hash table algorithm using the tag ID for said tag.

15. The communication system of claim 13 wherein said response means includes means to randomly determine said particular response time for each said tag.

16. The communication system of claim 2 wherein said interrogator collects the tag ID'"'"'s from tags in the communication region using communications between said interrogators and said tags during said plurality of collection periods, said interrogator having in said interrogator code,

synchronizing code for sending synchronizing signals to said tags to initiate each of said collection periods,

each said tag having means for inhibiting sending a tag ID after receipt of said directed command and for sending a tag ID only once for each collection period.

17. The communication system of claim 1 wherein said interrogator transceiver includes,

a radio frequency transmitter,

a radio frequency receiver,

an antenna connected to said transmitter and receiver.

18. The communication system of claim 1 wherein said interrogator transceiver means includes a plurality of transceivers each including,

an antenna,

a radio frequency transmitter connected to said interrogator processor to provide output signals to said antenna for transmission to said tags,

a radio frequency receiver connected to said antenna to receive signals from said tags to provide input signals to said interrogator processor.

19. The communication system of claim 1 wherein said interrogator transceiver means includes a plurality of spatially diverse transceivers each including,

an antenna having a polarization,

a radio frequency transmitter connected to said interrogator processor to provide output signals to said antenna for transmission to said tags,

a radio frequency receiver connected to said antenna to receive signals from said tags to provide input signals to said interrogator processor, wherein the polarization for an antenna of one of said transceivers is different from the polarization of an antenna of another one of said transceivers.

20. The communication system of claim 1 wherein said interrogator transceiver means includes,

a plurality of transceivers where a particular one of said transceivers is selectable to be active for said transceiver means, each transceiver including,

an antenna,

a radio frequency transmitter connected to said interrogator processor to provide output signals to said antenna for transmission to said tags,

a radio frequency receiver connected to said antenna to receive signals from said tags to provide input signals to said interrogator processor,

wherein the antenna from one or more of said transceivers is polarization diverse and spatially diverse from the antenna of another one of said transceivers,

signal strength detection means for detecting the signal strength of the signals received from said tags and for providing a signal strength indication to said interrogator processor from each of said receivers,

a switch under control of said interrogator processor for selecting, for each of said tags, a transceiver having the highest signal strength as said particular one of said transceivers.

21. The communication system of claim 1 wherein each of said tags includes a battery power unit.

22. The communication system of claim 1 wherein said interrogator includes an interface unit for providing communications relating to tags from and to a computer.

23. The communication system of claim 1 wherein one or more of said tags is connected to a unit for communicating information between said interrogator and said unit.

24. The communication system of claim 1 wherein said tags have sleep and awake modes and said interrogator collects messages from said tags in the communication region using communications between said interrogators and said tags during a collection period,

said interrogator having in said interrogator code,

synchronizing code for sending a synchronizing signal to said tags to initiate said collection period,

each said tag having in said tag code,

mode code for controlling the sleep mode and awake mode of the tag processor,

sensing code for sensing said synchronizing signal if said tag is in said awake mode,

send code for sending said response including a tag message to said interrogator in response to said synchronizing signal from said interrogator if said tag is in the awake mode.

25. The communication system of claim 24 wherein said tag processor is interruptible in response to an interrupt signal and wherein said system includes a unit connected to said tag for providing said interrupt signal for signaling the presence of said tag message and wherein said mode code is responsive to said interrupt signal to cause said tag to be in the awake mode.

26. The communication system of claim 25 wherein said interrogator processor after receipt of said tag message sends an acknowledge signal as a directed command to said tag.

27. The communication system of claim 26 wherein said acknowledge signal causes said tag to be in the sleep mode.

28. The communication system of claim 24 wherein said tag is connected to a unit providing said tag message and said interrogator processor receives said tag message from said unit when said tag is in the awake mode.

29. The communication system of claim 28 wherein said interrogator transceiver means includes,

a plurality of transceivers where a particular one of said transceivers is selectable to be active for said transceiver means, each transceiver including,

an antenna,

a radio frequency transmitter connected to said interrogator processor to provide output signals to said antenna for transmission to said tags,

a radio frequency receiver connected to said antenna to receive signals from said tags to provide input signals to said interrogator processor,

wherein the antenna from one or more of said transceivers is polarization diverse and spatially diverse from the antenna of another one of said transceivers,

signal strength detection means for detecting the signal strength of the signals received from said tags and for providing a signal strength indication to said interrogator processor from each of said receivers,

a switch under control of said interrogator processor for selecting for each of said tags a transceiver having the highest signal strength as said particular one of said transceivers.

30. The communication system of claim 24 wherein said tag processor is operable in a low-power state during said sleep mode and is operable in a normal-power state during said awake mode and is operable to be set in said normal-power state in response to a reset signal and wherein said tag processor includes a reset input for resetting tag processor operation and includes a unit connected to said reset input for providing said reset signal.

31. The communication system of claim 1 wherein said tags have sleep and awake modes and said interrogator collects messages from said tags in the communication region using communications between said interrogators and said tags during a collection period,

said interrogator having in said interrogator code,

wake-up code for sending a wake-up signal to said tags,

synchronizing code for sending a synchronizing signal to said tags to initiate a collection period,

each said tag having a controllable power gate for providing power to said tag transceiver in a low-power state and in a normal-power state where said tag processor controls said power gate to select said normal-power state in response to said wake-up signal, and each said tag having in said tag code,

code for controlling the sleep mode in the low-power state,

code for sensing said synchronizing signal if said tag is in said awake mode,

code for sending a message as part of a response to said interrogator in response to a synchronizing signal from said interrogator if said tag is in the awake mode.

32. A communication system for efficiently communicating with a large number of tags in a communication region formed by a plurality of cells where the location of the tags in the region may not be known and where tags may be removed from or introduced into the region comprising,

a plurality of interrogators, one for each cell, for communicating with said tags in said plurality of cells, each cell having a corresponding interrogator, said tags and said corresponding interrogator communicating over a common communication channel with a sequence including a plurality of collection periods, said collection periods including interrogator communication periods for said interrogator to send commands over said communication channel to said tags and including tag communication periods for said tags to send responses over said communication channel to said corresponding interrogator, where said responses from different tags collide if tag communication periods for said different tags coincide and where said responses from said different tags do not collide if the tag communication periods for said different tags do not coincide, each of said interrogators including,

interrogator memory for storing interrogator code including a broadcast command and a directed command and including bandwidth code for controlling the bandwidth of said collection periods,

interrogator processor means connected for accessing said interrogator memory and for processing said interrogator code to send said broadcast command and to send said directed command,

interrogator transceiver means connected to said interrogator processor means for sending said commands and receiving said responses tags within the interrogator cell over said common communication channel, said communications including sending said broadcast command and said directed command a plurality of times,

each of said tags located in one or more of said cells within said communication region, each of said plurality of tags including,

a tag transceiver for sending and receiving communications to and from said interrogator transceiver means over said common communication channel,

a tag memory for storing information including tag code,

a tag processor, connected to said tag transceiver, for processing said tag code to receive said broadcast command and said directed command said plurality of times, and to send a response to said interrogator for one or more of said plurality of times,

a computer connecting to each of said interrogators for controlling the operation of said interrogators.

33. The communication system of claim 32 wherein said cells are not contiguous.

34. The communication system of claim 32 wherein said cells are overlapping.

35. The communication system of claim 32 wherein each tag has a unique tag ID.

36. The communication system of claim 32 includes controllable power means for providing power to said tag transceiver in a low-power state and in a normal-power state where said tag processor controls said power means to select said normal-power state or said low-power state.

37. The communication system of claim 36 wherein said interrogator includes wake-up means for sending a wake-up signal to said tags and wherein each of said tags includes wake-up detection means connected to said tag transceiver and responsive to said wake-up signal from said interrogator for providing a signal to said tag processor to cause switching to said normal-power state.

38. The communication system of claim 32 wherein said tag transceiver in each of said tags includes,

a radio frequency transmitter,

a radio frequency receiver,

an antenna connected to said transmitter and receiver.

39. The communication system of claim 32 wherein each of said tags has a unique tag ID and said interrogator collects the tag ID'"'"'s from tags in the communication region using communications between said interrogators and said tags during a plurality of collection periods, said broadcast command requests tag ID'"'"'s from all tags,

for each tag, said tag processor sends a tag ID with said response upon receipt of said broadcast command,

for each tag for which said interrogator receives a tag ID, said interrogator processor sends said directed command to acknowledge that said tag ID has been received, said interrogator having in said interrogator code,

synchronizing code for sending synchronizing signals to said tags to initiate each of said collection periods,

each said tag having means for inhibiting sending a tag ID after receipt of said directed command.

40. The communication system of claim 32 wherein said interrogator transceiver means includes a plurality of transceivers each including,

an antenna,

a radio frequency transmitter connected to said interrogator processor to provide output signals to said antenna for transmission to said tags,

a radio frequency receiver connected to said antenna to receive signals from said tags to provide input signals to said interrogator processor.

41. The communication system of claim 32 wherein said interrogator transceiver means includes a plurality of spatially diverse transceivers each including,

an antenna having a polarization,

a radio frequency transmitter connected to said interrogator processor to provide output signals to said antenna for transmission to said tags,

a radio frequency receiver connected to said antenna to receive signals from said tags to provide input signals to said interrogator processor, wherein the polarization for an antenna of one of said transceivers is different from the polarization of an antenna of another one of said transceivers.

42. The communication system of claim 32 wherein each of said tags includes a battery power unit.

43. The communication system of claim 32 wherein said interrogator includes an interface unit for providing communications relating to tags from and to a computer.

44. The communication system of claim 32 wherein one or more of said tags is connected to one or more units for communicating for each tag information between said interrogator and said one or more units through said tag.

45. The communication system of claim 32 wherein said tags have sleep and awake modes and said interrogator collects messages from said tags in the communication region using communications between said interrogators and said tags during a collection period,

said interrogator having in said interrogator code,

synchronizing code for sending a synchronizing signal to said tags to initiate a collection period,

each said tag having in said tag code,

mode code for controlling the sleep mode and awake mode of the tag processor,

sensing code for sensing said synchronizing signal if said tag is in said awake mode,

send code for sending to said interrogator a response including a tag message in response to a synchronizing signal from said interrogator if said tag is in the awake mode.

46. The communication system of claim 32 wherein said tags have sleep and awake modes and said interrogator collects messages from said tags in the communication region using communications between said interrogators and said tags during a collection period,

said interrogator having in said interrogator code,

wake-up code for sending a wake-up signal to said tags,

synchronizing code for sending a synchronizing signal to said tags to initiate a collection period,

each said tag having a controllable power gate for providing power to said tag transceiver in a low-power state and in a normal-power state where said tag processor controls said power gate to select said normal-power state in response to said wake-up signal, and each said tag having in said tag code,

code for controlling the sleep mode in the low-power state,

code for sensing said synchronizing signal if said tag is in said awake mode,

code for sending a response including a tag message to said interrogator in

response to a synchronizing signal from said interrogator if said tag is in the awake mode.

47. Apparatus for identifying items through transmission and reception of signals comprising:

a plurality of tags, one for each of said items;

an interrogator for communicating with said tags,

said tags and said interrogator communicating over a common communication channel with a communication protocol including a sequence, common to all tags, said sequence including a plurality of collection periods, each collection period having,

interrogator communication periods for said interrogator to send commands over said communication channel to said tags,

a listen period having time slots during which one or more of said plurality of tags can communicate a tag identifying signal to said interrogator, where tag identifying signals from different tags collide if the time slots for said tag identifying signals for said different tags coincide and where tag identifying signals from different tags do not collide if the time slots for said tag identifying signals for said different tags do not coincide,

an acknowledge period during which said interrogator can communicate acknowledge signals to said tags,

said interrogator including,

bandwidth control means for controlling the bandwidth of the collection periods by controlling the duration of the listen periods,

signal sending means, operable for each of said collection periods, for sending a signal to the tags to signal the commencement of the listen period,

receiver means, operable for each of said collection periods, for receiving tag identifying signals during said listen period from a subset of said tags,

acknowledge sending means, operable for each of said collection periods, for sending acknowledge signals during said acknowledge period to said subset of said tags,

each of said tags including,

signal receiving means, operable for one or more of said collection periods, for receiving the signal from the signal sending means so as to identify the start of the listen period,

identification sending means, operable for one or more of said collection periods, for sending a tag identifying signal during the listen period,

acknowledge receiving means, operable for one or more of said collection periods, for receiving an acknowledge signal from said interrogator during the acknowledge period.

48. In a communication system for communicating in a communication region over a common communication channel between an interrogator and a plurality of tags, where the interrogator includes,

interrogator memory for storing interrogator code including a broadcast command and a directed command, interrogator processor means connected for accessing said interrogator memory, interrogator transceiver means connected to said interrogator processor means,

and where each of said tags includes,

a tag transceiver for sending and receiving communications to and from said interrogator transceiver means over said common communication channel, a tag memory for storing information including tag code, a tag processor, connected to said tag transceiver,

the method comprising,

processing, in said interrogator processor, said interrogator code to sequence said broadcast command and said directed command a plurality of times,

sending and receiving communications in said communication region over said common communication channel by said interrogator transceiver, including sending said broadcast command and said directed command said plurality of times, said communications having a sequence including a plurality of collection periods, said collection periods including interrogator communication periods for said interrogator to send commands over said communication channel to said tags and including tag communication periods for said tags to send responses over said communication channel to said interrogator, where said responses from different tags collide if tag communication periods for said different tags coincide and where said responses from said different tags do not collide if the tag communication periods for said different tags do not coincide,

in each of said tags,

receiving said broadcast command and said directed command said plurality of times by said tag transceiver,

processing said tag code to send a response to said interrogator through said tag transceiver for one or more of said plurality of times,

for each of two or more of said plurality of tags, sending a response over said common communication channel such that at some times, a collision of responses occurs during one of said plurality of times and such that at another one of said plurality of times, no collision of responses occurs.

49. The communication method of claim 48 wherein,

said broadcast command requests tag ID'"'"'s from all tags,

for each tag, said tag processor sends a tag ID with said response upon receipt of said broadcast command,

for each tag for which said interrogator receives a tag ID, said interrogator processor sends said directed command to acknowledge that said tag ID has been received.

50. The communication method of claim 49 has a unique tag ID and said interrogator collects the tag ID'"'"'s from tags in the communication region using communications between said interrogators and said tags during a plurality of collection periods, said interrogator,

executing synchronizing code in said interrogator code for sending synchronizing signals to said tags to initiate each of said collection periods,

executing bandwidth code in said interrogator code for controlling the bandwidth of said collection periods,

each said tag having means for inhibiting sending a tag ID after receipt of a command.

51. The communication method of claim 50 wherein said interrogator establishes said collection periods as collection periods CP(1), CP(2), . . . , CP(p), . . . , CP(P) having respectively listen periods LP(1), LP(2), . . . , LP(p), . . . , LP(P) and having respectively acknowledge periods AP(1), AP(2), . . . , AP(p), . . . , AP(P).

52. The communication method of claim 51 wherein said bandwidth code sets the bandwidth of each collection period by setting the duration of the respective listen period.

53. The communication method of claim 52 wherein for one or more subsequent listen periods after the first listen period LP(1), the listen periods are shorter in duration than the listen period LP(1).

54. The communication method of claim 53 wherein said first listen period LP(1) and said one or more subsequent listen periods each have durations that permit receipt of separate transmissions of tag IDs from a plurality of tags.

55. The communication system of claim 54 wherein for a plurality of said one or more listen periods, each successive listen period is approximately one-half the duration of the preceding listen period.

56. The communication method of claim 52 wherein for said listen periods, each of said tags has response means for distributing the tag ID to a particular response time during the respective listen period.

57. The communication method of claim 56 wherein said particular response time is determined for each said tag by a hash table algorithm using the tag ID for said tag.

58. The communication method of claim 56 wherein said response means includes means to randomly determine said particular response time for each said tag.

59. The communication method of claim 49 wherein each of said tags has a unique tag ID and said interrogator collects the tag ID'"'"'s from tags in the communication region using communications between said interrogators and said tags during a plurality of collection periods, said interrogator executing said interrogator code including,

executing synchronizing code for sending synchronizing signals to said tags to initiate each of said collection periods,

executing bandwidth code for controlling the bandwidth of said collection periods,

each said tag inhibiting sending a tag ID after receipt of said directed command and for sending a tag ID only once for each collection period.

60. The communication method of claim 48 includes a controllable power means for providing power to said tag transceiver in a low-power state for a sleep mode and in a normal-power state for an awake mode where said tag processor controls said power means to select said low-power state or said normal-power state.

61. The communication method of claim 60 wherein,

said broadcast command requests all tags but one specific tag to go to said sleep mode,

for each tag except said one specific tag, said tag processor upon receipt of said broadcast command causes said tag to go to said sleep mode,

for said specific tag, said interrogator processor sends said directed command to command an action by said specific tag,

said tag processor for said specific tag upon receipt of said directed command sends said response including the tag ID to said interrogator.

62. The communication method of claim 60 wherein said interrogator includes wake-up means for sending a wake-up signal to said tags and wherein each of said tags includes wake-up detection means connected to said tag transceiver and responsive to said wake-up signal from said interrogator for providing a signal to said tag processor to cause switching to said normal-power state.

63. Apparatus for identifying items through transmission and reception of signals comprising:

a plurality of tags, one for each of said items;

an interrogator for communicating with said tags,

said tags and said interrogator communicating over a common communication channel with a communication protocol, said communication protocol including a sequence common to all tags, said sequence including a plurality of collection periods, each collection period having,

a synchronization period during which said interrogator can communicate with said plurality of tags,

a listen period having time slots during which one or more of said plurality of tags can communicate a tag identifying signal to said interrogator over said communication channel, where tag identifying signals from different tags collide if the time slots for said tag identifying signals for said different tags coincide and where tag identifying signals from different tags do not collide if the time slots for said tag identifying signals for said different tags do not coincide, and

an acknowledge period during which said interrogator can communicate acknowledge signals to said tags over said communication channel,

said interrogator including,

interrogator memory for storing interrogator code including one or more broadcast commands and one or more directed commands and including bandwidth code for controlling the bandwidth of said tag communication periods,

interrogator processor means connected for accessing said interrogator memory and for processing said interrogator code to sequence said broadcast commands and said directed commands,

signal sending means, operable for each of said collection periods, for sending during said synchronization period one of said broadcast commands over the common communication channel to the tags to signal the commencement of the listen period,

receiver means, operable for each of said collection periods, for receiving tag identifying signals over the common communication channel from said tags during said listen period,

acknowledge sending means, operable for each of said collection periods, for sending acknowledge signals as directed commands over the common communication channel to said tags during said acknowledge period,

each of said tags including,

signal receiving means, operable for one or more of said collection periods, for receiving said one of said broadcast commands from said signal sending means during said synchronization period so as to identify the start of the listen period,

identification sending means, operable for one or more of said collection periods, for sending a tag identifying signal during one of a plurality of time slots during the listen period,

acknowledge receiving means, operable for one of said collection periods, for receiving an acknowledge signal from said interrogator during the acknowledge period,

control means, operable for said one or more of said collection periods, for controlling said identification sending means,

said control means operative, before said acknowledge receiving means receives said acknowledge signal and during each of said one or more of said collection periods, for selecting one of said time slots for sending a tag identifying signal whereby two or more of said plurality of tags may each send a respective tag identifying signal over said common communication channel such that during one time slot of a listen period a collision of tag identifying signals may occur and such that during another time slot of a listen period no collision of tag identifying signals occurs,

said control means operative, after said acknowledge receiving means receives said acknowledge signal, for inhibiting said identification sending means from sending said tag identifying signal.

64. The communication system of claim 63 wherein said interrogator code includes code for controlling the duration of said collection periods.

65. The communication system of claim 64 wherein said interrogator code establishes said collection periods as the P collection periods CP(1), CP(2), . . . , CP(p), . . . , CP(P) having respectively P synchronization periods SP(1), SP(2), . . . , SP(p), . . . , SP(P), having respectively the P listen periods LP(1), LP(2), . . . , LP(p), . . . , LP(P) and having respectively the P acknowledge periods AP(1), AP(2), . . . , AP(p), . . . , AP(P) and wherein each of said listen periods has a plurality of time slots.

66. The communication system of claim 65 period LP(1) is followed by one or more subsequent listen periods LP(2), . . . , LP(p), . . . , LP(P) and where one or more of the subsequent listen periods LP(2), . . . , LP(p), . . . , LP(P) are shorter in duration than the first listen period LP(1) whereby one or more of said subsequent listen periods have fewer time slots than said first listen period.