Method of addressing messages, method of establishing wireless communications, and communications system

Abstract

A method of 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 representing subgroups of the multiple wireless identification devices, the number of devices in a subgroup in one level being half of the number of devices in the next higher level, the tree search method employing level skipping wherein at least one level of the tree is skipped. 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 levels of the tree are occasionally skipped.

Description

, now U.S. Pat. No. 6,130,602. Other embodiments are possible. A power source 18 is connected to the integrated circuit 16 to supply power to the integrated circuit 16. In one embodiment, the power source 18 comprises a battery. The device 12 further includes at least one antenna 14 connected to the circuitry 16 for wireless or radio frequency transmission and reception by the circuitry 16.

The device 12 transmits and receives radio frequency communications to and from an interrogator 26. An exemplary interrogator is described in commonly assigned U.S. patent application Ser. No. 08/907,689, filed Aug. 8, 1997 and, now U.S. Pat. No. 6,289,209, which is incorporated herein by reference. Preferably, the interrogator 26 includes an antenna 28, as well as dedicated transmitting and receiving circuitry, similar to that implemented on the integrated circuit 16.

Generally, the interrogator 26 transmits an interrogation signal or command 27 via the antenna 28. The device 12 receives the incoming interrogation signal via its antenna 14. Upon receiving the signal 27, the device 12 responds by generating and transmitting a responsive signal or reply 29. The responsive signal 29 typically includes information that uniquely identifies, or labels the particular device 12 that is transmitting, so as to identify any object or person with which the device 12 is associated.

Although only one device 12 is shown in FIG. 1, typically there will be multiple devices 12 that correspond with the interrogator 26, and the particular devices 12 that are in communication with the interrogator 26 will typically change over time. In the illustrated embodiment in FIG. 1, there is no communication between multiple devices 12. Instead, the devices 12 respectively communicate with the interrogator 26. Multiple devices 12 can be used in the same field of an interrogator 26 (i.e., within communications range of an interrogator 26). Similarly, multiple interrogators 26 can be in proximity to one or more of the devices 12.

The radio frequency data communication device 12 can be included in any appropriate housing or packaging. Various methods of manufacturing housings are described in commonly assigned U.S. patent application Ser. No. 08/800,037, filed Feb. 13, 1997, and now U.S. Pat. No. 5,988,510, which is incorporated herein by reference.

FIG. 2 shows but one embodiment in the form of a card or badge 19 including the radio frequency data communication device 12, and a housing 11 including plastic or other suitable material. In one embodiment, the front face of the badge has visual identification features such as graphics, text, information found on identification or credit cards, etc.

FIG. 3 illustrates but one alternative housing supporting the device 12. More particularly, FIG. 3 shows a miniature housing 20 encasing the device 12 to define a tag which can be supported by an object (e.g., hung from an object, affixed to an object, etc.). Although two particular types of housings have been disclosed, the device 12 can be included in any appropriate housing.

If the power source 18 is a battery, the battery can take any suitable form. Preferably, the battery type will be selected depending on weight, size, and life requirements for a particular application. In one embodiment, the battery 18 is a thin profile button-type cell forming a small, thin energy cell more commonly utilized in watches and small electronic devices requiring a thin profile. A conventional button-type cell has a pair of electrodes, an anode formed by one face and a cathode formed by an opposite face. In an alternative embodiment, the power source 18 comprises a series connected pair of button type cells. Instead of using a battery, any suitable power source can be employed.

The circuitry 16 further includes a backscatter transmitter and is configured to provide a responsive signal to the interrogator 26 by radio frequency. More particularly, the circuitry 16 includes a transmitter, a receiver, and memory such as is described in U.S. patent application Ser. No. 08/705,043, now U.S. Pat. No. 6,130,602.

Radio frequency identification has emerged as a viable and affordable alternative to tagging or labeling small to large quantities of items. The interrogator 26 communicates with the devices 12 via an RF link, so all transmissions by the interrogator 26 are heard simultaneously by all devices 12 within range.

If the interrogator 26 sends out a command requesting that all devices 12 within range identify themselves, and gets a large number of simultaneous replies, the interrogator 26 may not be able to interpret any of these replies. Therefore, arbitration schemes are provided.

If the interrogator 26 has prior knowledge of the identification number of a device 12 which the interrogator 26 is looking for, it can specify that a response is requested only from the device 12 with that identification number. To target a command at a specific device 12, (i.e., to initiate point-on-point communication), the interrogator 26 must send a number identifying a specific device 12 along with the command. At start-up, or in a new or changing environment, these identification numbers are not known by the interrogator 26. Therefore, the interrogator 26 must identify all devices 12 in the field (within communication range) such as by determining the identification numbers of the devices 12 in the field. After this is accomplished, point-to-point communication can proceed as desired by the interrogator 26.

Generally speaking, RFID systems are a type of multiaccess communication system. The distance between the interrogator 26 and devices 12 within the field is typically fairly short (e.g., several meters), so packet transmission time is determined primarily by packet size and baud rate. Propagation delays are negligible. In RFID systems, there is a potential for a large number of transmitting devices 12 and there is need for the interrogator 26 to work in a changing environment, where different devices 12 are swapped in and out frequently (e.g., as inventory is added or removed). In such systems, the inventors have determined that the use of random access methods work effectively for contention resolution (i.e., for dealing with collisions between devices 12 attempting to respond to the interrogator 26 at the same time).

RFID systems have some characteristics that are different from other communications systems. For example, one characteristic of the illustrated RFID systems is that the devices 12 never communicate without being prompted by the interrogator 26. This is in contrast to typical multiaccess systems where the transmitting units operate more independently. In addition, contention for the communication medium is short lived as compared to the ongoing nature of the problem in other multiaccess systems. For example, in a RFID system, after the devices 12 have been identified, the interrogator can communicate with them in a point-to-point fashion. Thus, arbitration in a RFID system is a transient rather than steady-state phenomenon. Further, the capability of a device 12 is limited by practical restrictions on size, power, and cost. The lifetime of a device 12 can often be measured in terms of number of transmissions before battery power is lost. Therefore, one of the most important measures of system performance in RFID arbitration is total time required to arbitrate a set of devices 12. Another measure is power consumed by the devices 12 during the process. This is in contrast to the measures of throughput and packet delay in other types of multiaccess systems.

FIG. 4 illustrates one arbitration scheme that can be employed for communication between the interrogator and devices 12. Generally, the interrogator 26 sends a command causing each device 12 of a potentially large number of responding devices 12 to select a random number from a known range and use it as that device's arbitration number. By transmitting requests for identification to various subsets of the full range of arbitration numbers, and checking for an error-free response, the interrogator 26 determines the arbitration number of every responder station capable of communicating at the same time. Therefore, the interrogator 26 is able to conduct subsequent uninterrupted communication with devices 12, one at a time, by addressing only one device 12.

Three variables are used: an arbitration value (AVALUE), an arbitration mask (AMASK), and a random value ID (RV). The interrogator sends an Identify command (IdentifyCmnd) causing each device of a potentially large number of responding devices to select a random number from a known range and use it as that device's arbitration number. The interrogator sends an arbitration value (AVALUE) and an arbitration mask (AMASK) to a set of devices 12. The receiving devices 12 evaluate the following equation: (AMASK & AVALUE)==(AMASK & RV) wherein “&” is a bitwise AND function, and wherein “==” is an equality function. If the equation evaluates to “1” (TRUE), then the device 12 will reply. If the equation evaluates to “0” (FALSE), then the device 12 will not reply. By performing this in a structured manner, with the number of bits in the arbitration mask being increased by one each time, eventually a device 12 will respond with no collisions. Thus, a binary search tree methodology is employed.

An example using actual numbers will now be provided using only four bits, for simplicity, reference being made to FIG. 4. In one embodiment, sixteen bits are used for AVALUE and AMASK. Other numbers of bits can also be employed depending, for example, on the number of devices 12 expected to be encountered in a particular application, on desired cost points, etc.

Assume, for this example, that there are two devices 12 in the field, one with a random value (RV) of 1100 (binary), and another with a random value (RV) of 1010 (binary). The interrogator is trying to establish communications without collisions being caused by the two devices 12 attempting to communicate at the same time.

The interrogator sets AVALUE to 0000 (or “don't care” for all bits, as indicated by the character “X” in FIG. 4) and AMASK to 0000. The interrogator transmits a command to all devices 12 requesting that they identify themselves. Each of the devices 12 evaluate (AMASK & AVALUE)==(AMASK & RV) using the random value RV that the respective devices 12 selected. If the equation evaluates to “1” (TRUE), then the device 12 will reply. If the equation evaluates to “0” (FALSE), then the device 12 will not reply. In the first level of the illustrated tree, AMASK is 0000 and anything bitwise ANDed with all zeros results in all zeros, so both the devices 12 in the field respond, and there is a collision.

Next, the interrogator sets AMASK to 0001 and AVALUE to 0000 and transmits an identify command. Both devices 12 in the field have a zero for their least significant bit, and (AMASK & AVALUE)==(AMASK & RV) will be true for both devices 12. For the device 12 with a random value of 1100, the left side of the equation is evaluated as follows (0001 & 0000)=0000. The right side is evaluated as (0001 & 1100)=0000. The left side equals the right side, so the equation is true for the device 12 with the random value of 1100. For the device 12 with a random value of 1010, the left side of the equation is evaluated as (0001 & 0000)=0000. The right side is evaluated as (0001 & 1010)=0000. The left side equals the right side, so the equation is true for the device 12 with the random value of 1010. Because the equation is true for both devices 12 in the field, both devices 12 in the field respond, and there is another collision.

Recursively, the interrogator next sets AMASK to 0011 with AVALUE still at 0000 and transmits an Identify command. (AMASK & AVALUE)==(AMASK & RV) is evaluated for both devices 12. For the device 12 with a random value of 1100, the left side of the equation is evaluated as follows (0011 & 0000)=0000. The right side is evaluated as (0011 & 1100)=0000. The left side equals the right side, so the equation is true for the device 12 with the random value of 1100, so this device 12 responds For the device 12 with a random value of 1010, the left side of the equation is evaluated as (0011 & 0000)=0000. The right side is evaluated as (0011 & 1010)=0010. The left side does not equal the right side, so the equation is false for the device 12 with the random value of 1010, and this device 12 does not respond. Therefore, there is no collision, and the interrogator can determine the identity (e.g., an identification number) for the device 12 that does respond.

De-recursion takes place, and the devices 12 to the right for the same AMASK level are accessed when AVALUE is set at 0010, and AMASK is set to 0011.

The device 12 with the random value of 1010 receives a command and evaluates the equation (AMASK & AVALUE)==(AMASK & RV). The left side of the equation is evaluated as (0011 & 0010)=0010. The right side of the equation is evaluated as (0011 & 1010)=0010. The right side equals the left side, so the equation is true for the device 12 with the random value of 1010. Because there are no other devices 12 in the subtree, a good reply is returned by the device 12 with the random value of 1010. There is no collision, and the interrogator can determine the identity (e.g., an identification number) for the device 12 that does respond.

By recursion, what is meant is that a function makes a call to itself. In other words, the function calls itself within the body of the function. After the called function returns, de-recursion takes place and execution continues at the place just after the function call; i.e. at the beginning of the statement after the function call.

For instance, consider a function that has four statements (numbered 1,2,3,4) in it, and the second statement is a recursive call. Assume that the fourth statement is a return statement. The first time through the loop (iteration 1) the function executes the statement 2 and (because it is a recursive call) calls itself causing iteration 2 to occur. When iteration 2 gets to statement 2, it calls itself making iteration 3. During execution in iteration 3 of statement 1, assume that the function does a return. The information that was saved on the stack from iteration 2 is loaded and the function resumes execution at statement 3 (in iteration 2), followed by the execution of statement 4 which is also a return statement. Since there are no more statements in the function, the function de-recurses to iteration 1. Iteration 1, had previously recursively called itself in statement 2. Therefore, it now executes statement 3 (in iteration 1). Following that it executes a return at statement 4. Recursion is known in the art.

Consider the following code which can be used to implement operation of the method shown in FIG. 4 and described above.

Arbitrate(AMASK, AVALUE)
{
collision=IdentifyCmnd(AMASK, AVALUE)
if (collision) then
{
/* recursive call for left side */
Arbitrate((AMASK<<1)+1, AVALUE)
/* recursive call for right side */
Arbitrate((AMASK<<1)+1, AVALUE+(AMASK+1))
}   /* endif */
} /* return */

The symbol “<<” represents a bitwise left shift. “<<1” means shift left by one place. Thus, 0001<<1 would be 0010. Note, however, that AMASK is originally called with a value of zero, and 0000<<1 is still 0000. Therefore, for the first recursive fall, AMASK=(AMASK<<1)+1. So for the first recursive call, the value of AMASK is 0000+0001=0001. For the second call, AMASK=(0001<<1)+1=0010+1=0011. For the third recursive call, AMASK=(0011<<1)+1=0110+1=0111.

The routine generates values for AMASK and AVALUE to be used by the interrogator in an identify command “IdentifyCmnd.” Note that the routine calls itself if there is a collision. De-recursion occurs when there is no collision. AVALUE and AMASK would have values such as the following assuming collisions take place all the way down to the bottom of the tree.

AVALUE AMASK

0000   0000
0000   0001
0000   0011
0000   0111
0000   1111*
1000   1111*
0100   0111
0100   1111*
1100   1111*

This sequence of AMASK, AVALUE binary numbers assumes that there are collisions all the way down to the bottom of the tree, at which point the Identify command sent by the interrogator is finally successful so that no collision occurs. Rows in the table for which the interrogator is successful in receiving a reply without collision are marked with the symbol “*”. Note that if the Identify command was successful at, for example, the third line in the table then the interrogator would stop going down that branch of the tree and start down another, so the sequence would be as shown in the following table.

AVALUE AMASK

0000   0000
0000   0001
0000   0011*
0010   0011
. . .  . . .

This method is referred to as a splitting method. It works by splitting groups of colliding devices 12 into subsets that are resolved in turn. The splitting method can also be viewed as a type of tree search. Each split moves the method one level deeper in the tree.

Either depth-first or breadth-first traversals of the tree can be employed. Depth first traversals are performed by using recursion, as is employed in the code listed above. Breadth-first traversals are accomplished by using a queue instead of recursion. The following is an example of code for performing a breadth-first traversal.

Arbitrate(AMASK, AVALUE)
{
enqueue(0,0)
while (queue != empty)
(AMASK,AVALUE) = dequeue( )
collision=IdentifyCmnd(AMASK, AVALUE)
if (collision) then
{
TEMP = AMASK+1
NEW_AMASK = (AMASK<<1)+1
enqueue(NEW_AMASK, AVALUE)
enqueue(NEW_AMASK, AVALUE+TEMP)
}  /* endif */
endwhile
}/* return */

The symbol “!=” means not equal to. AVALUE and AMASK would have values such as those indicated in the following table for such code.

AVALUE AMASK

0000   0000
0000   0001
0001   0001
0000   0011
0010   0011
0001   0011
0011   0011
0000   0111
0100   0111
. . .  . . .

Rows in the table for which the interrogator is successful in receiving a reply without collision are marked with the symbol “*”.

FIG. 5 illustrates an embodiment wherein levels in the tree are skipped. The inventors have determined that skipping levels in the tree, after a collision caused by multiple devices 12 responding, reduces the number of subsequent collisions without adding significantly to the number of no replies. In real-time systems, it is desirable to have quick arbitration sessions on a set of devices 12 whose unique identification numbers are unknown. Level skipping reduces the number of collisions, both reducing arbitration time and conserving battery life on a set of devices 12.

Thus, FIG. 5 illustrates a binary search tree 32 being defined for a tree search method similar to the tree search method described in connection with FIG. 4. The tree 32 has multiple levels 34, 36, 38, 40, and 42 representing subgroups of the multiple devices 12. The number of devices in a subgroup in one level 34, 36, 38, 40, and 42 is half of the number of devices in the next higher level 34, 36, 38, 40, and 42. Although only five levels are shown, if more bits are employed, (e.g., sixteen bits or an integer multiple of eight or sixteen bits for each of AMASK and AVALUE), there will of course be more levels. The tree search method illustrated in FIG. 5 employs level skipping wherein at least one level of the tree is skipped.

A first predetermined number of bits, e.g. sixteen or an integer multiple of eight or sixteen bits, are established to be used as unique identification numbers. Respective devices 12 are provided with unique identification numbers respectively having the first predetermined numbers of bits, in addition to their random values RV. For example, such unique identification numbers are stored in memory in the respective devices 12.

A second predetermined number of bits are established to be used for the random values RV. The devices 12 are caused to select random values, RV. This is done, for example, by the interrogator 26 sending an appropriate command. Respective devices choose random values independently of random values selected by the other devices 12. Random number generators are known in the art.

The interrogator transmits a command requesting devices 12 having random values RV within a specified group of random values to respond, using a methodology similar to that described in connection with FIG. 4, except that levels are skipped. Four subsets of random values, instead of two, are probed when moving down the tree and skipping a level. This means that instead of eliminating half of the remaining devices 12 and re-trying, after a collision, the interrogator eliminates three quarters of the remaining devices 12 and re-tries (by sending a command). In other words, a new specified group is created that is one quarter of the set of random values of the previous group.

Each devices 12 that receives the command determines if its chosen random value falls within the specified group by evaluating the equation (AMASK & AVALUE)==(AMASK & RV) and, if so, sends a reply, to the interrogator. The reply includes the random value of the replying device 12 and the unique identification number of the device 12. The interrogator determines if a collision occurred between devices that sent a reply and, if so, creates a new, smaller, specified group, by moving down the tree, skipping a level.

In the illustrated embodiment, every other level is skipped. In alternative embodiments, more than one level is skipped each time.

The trade off that must be considered in determining how many (if any) levels to skip with each decent down the tree is as following. Skipping levels reduces the number of collisions, thus saving battery power in the devices 12. Skipping deeper (skipping more than one level) further reduces the number of collisions. The more levels that are skipped, the greater the reduction in collisions. However, skipping levels results in longer search times because the number of queries (Identify commands) increases. The more levels that are skipped, the longer the search times. The inventors have determined that skipping just one level has an almost negligible effect on search time, but drastically reduces the number of collisions. If more than one level is skipped, search time increases substantially.

The inventors have determined that skipping every other level drastically reduces the number of collisions and saves battery power with out significantly increasing the number of queries.

After receiving a reply without collision from a device 12, the interrogator 26 can send a command individually addressed to that device by using its now known random value or its now known unique identification number.

The above described code for depth-first traversal is modified to provide for level skipping by increasing the number of recursive calls as shown below. For example, the above described code for depth-first traversal is replaced with code such as the following to provide for depth-first traversal employing level skipping.

Arbitrate(AMASK, AVALUE)
{
collision=IdentifyCmnd(AMASK, AVALUE)
if (collision) then
{
TEMP = AMASK+1
NEW_AMASK = (AMASK<<2)+3
Arbitrate(NEW_AMASK, AVALUE)
Arbitrate(NEW_AMASK, AVALUE+TEMP)
Arbitrate(NEW_AMASK, AVALUE+2*TEMP)
Arbitrate(NEW_AMASK, AVALUE+3*TEMP)
}/* endif */
}/* return */

AVALUE and AMASK would have values such as those indicated in the following table for such code.

AVALUE AMASK

0000   0000
0000   0011
0000   1111*
0100   1111*
1000   1111*
1100   1111*
0001   0011
0001   1111*
0101   1111*
1001   1111*
1101   1111*
0010   0011
0010   1111*
0110   1111*
1010   1111*
1110   1111*
. . .  . . .

Rows in the table for which the interrogator is successful in receiving a reply without collision are marked with the symbol “*”.

Similarly, the code provided above for breadth-first traversal can be readily modified to employ level skipping. Instead of inserting two items into the queue each time through the loop, four items are inserted into the queue each time through the loop. For either breadth-first traversal or depth-first traversal, AMASK will be shifted by two bits instead of one, and AVALUE will take on twice as many different values as in the case where level skipping is not employed.

Another arbitration method that can be employed is referred to as the “Aloha” method. In the Aloha method, every time a device 12 is involved in a collision, it waits a random period of time before retransmitting. This method can be improved by dividing time into equally sized slots and forcing transmissions to be aligned with one of these slots. This is referred to as “slotted Aloha.” In operation, the interrogator asks all devices 12 in the field to transmit their identification numbers in the next time slot. If the response is garbled, the interrogator informs the devices 12 that a collision has occurred, and the slotted Aloha scheme is put into action. This means that each device 12 in the field responds within an arbitrary slot determined by a randomly selected value. In other words, in each successive time slot, the devices 12 decide to transmit their identification number with a certain probability.

The Aloha method is based on a system operated by the University of Hawaii. In 1971, the University of Hawaii began operation of a system named Aloha. A communication satellite was used to interconnect several university computers by use of a random access protocol. The system operates as follows. Users or devices transmit at any time they desire. After transmitting, a user listens for an acknowledgment from the receiver or interrogator. Transmissions from different users will sometimes overlap in time (collide), causing reception errors in the data in each of the contending messages. The errors are detected by the receiver, and the receiver sends a negative acknowledgment to the users. When a negative acknowledgment is received, the messages are retransmitted by the colliding users after a random delay. If the colliding users attempted to retransmit without the random delay, they would collide again. If the user does not receive either an acknowledgment or a negative acknowledgment within a certain amount of time, the user “times out” and retransmits the message.

There is a scheme known as slotted Aloha which improves the Aloha scheme by requiring a small amount of coordination among stations. In the slotted Aloha scheme, a sequence of coordination pulses is broadcast to all stations (devices). As is the case with the pure Aloha scheme, packet lengths are constant. Messages are required to be sent in a slot time between synchronization pulses, and can be started only at the beginning of a time slot. This reduces the rate of collisions because only messages transmitted in the same slot can interfere with one another. The retransmission mode of the pure Aloha scheme is modified for slotted Aloha such that if a negative acknowledgment occurs, the device retransmits after a random delay of an integer number of slot times.

Aloha methods are described in a commonly assigned patent application (attorney docket number MI40-089) naming Clifton W. Wood, Jr. as an inventor, titled “Method of Addressing Messages and Communications System,” filed concurrently herewith Ser. No. 09/026,248, filed Feb. 19, 1998, and now U.S. Pat. No. 6,275,476, which is incorporated herein by reference.

In one alternative embodiment, an Aloha method is combined with level skipping, such as the level skipping shown and described in connection with FIG. 5. For example, in one embodiment, devices 12 sending a reply to the interrogator 26 do so within a randomly selected time slot of a number of slots.

In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.

Claims

1. A method of establishing wireless communications between an interrogator and wireless identification devices, the method comprising utilizing a tree search technique to establish communications without collision between the interrogator and individual ones of the multiple wireless identification devices, the method including using a binary search tree having multiple levels representing subgroups of the multiple wireless identification devices, the number of devices in a subgroup in one level being less than the number of devices in the next level, the tree search technique employing level skipping wherein every second level of the tree is skipped.

2. 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.

3. A method in accordance with claim 1 wherein when a subgroup contains both a device that is within communications range of the interrogator, and a device that is not within communications range of the interrogator, the device that is not within communications range of the interrogator does not respond to the command.

4. A method in accordance with claim 1 wherein when a subgroup contains both a device that is within communications range of the interrogator, and a device that is not within communications range of the interrogator, the device that is within communications range of the interrogator responds to the command.

5. A method in accordance with claim 1 wherein a device in a subgroup changes between being within communications range of the interrogator and not being within communications range, over time.

6. 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;

causing the devices to select random values, wherein respective devices choose random values independently of random values selected by the other devices;

transmitting a communication, from the interrogator, requesting devices having random values within a specified group of random values to respond;

receiving the communication at multiple devices, devices receiving the communication 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, using a search tree, that is one quarter of the first mentioned specified group, wherein at least one level of a search tree is skipped.

7. A method of addressing messages from an interrogator to a selected one or more of a number of communications devices in accordance with claim 6 wherein sending a reply to the interrogator comprises transmitting the unique identification number of the device sending the reply.

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 6 wherein sending a reply to the interrogator comprises transmitting the random value 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 6 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.

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 6 wherein, after receiving a reply without collision from a device, the interrogator sends a command individually addressed to that device.

11. A method of addressing messages from a transponder to a selected one or more of a number of communications devices, the method comprising:

causing the devices to select random values, wherein respective devices choose random values independently of random values selected by the other devices;

transmitting a communication, from the transponder, 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 or equal to the entire set of random values, the plurality of possible groups being organized in a binary tree having a plurality of levels, wherein groups of random values decrease in size with each level descended;

devices receiving the communication respectively determining if the random value chosen by the device falls within the specified group and, if so, sending a reply to the transponder; and, if not, not sending a reply; and

determining using the transponder if a collision occurred between devices that sent a reply and, if so, creating a new, smaller, specified group by descending at least two levels in the tree.

12. A method of addressing messages from a transponder to a selected one or more of a number of communications devices in accordance with claim 11 and further comprising establishing unique identification numbers for respective devices.

13. A method of addressing messages from a transponder 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 a transponder 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 sixteen bits.

15. A method of addressing messages from a transponder to a selected one or more of a number of communications devices in accordance with claim 13 wherein devices sending a reply to the transponder 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 unique identification numbers;

causing the devices to select random values, wherein respective devices choose random values independently of random values selected by the other devices;

transmitting from the interrogator 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 or equal to the entire set of random values, the plurality of possible groups being organized in a binary tree having a plurality of levels, wherein groups of random values decrease in size with each level;

receiving the command at multiple of the devices, the 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;

determining using the interrogator 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, wherein at least one level of the tree is skipped, 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 wherein every second level is skipped.

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 the unique identification numbers are respectively defined by a predetermined number of bits.

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 the unique identification numbers are respectively defined by a predetermined number of bits and wherein the random values are respectively defined by a predetermined number of bits.

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 and further comprising, after the interrogator transmits a command requesting devices having random values within the new specified group of random values to respond:

devices receiving the command respectively determining if their chosen random values fall within the new smaller specified group and, if so, sending a reply to the interrogator.

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 20 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.

22. A system comprising:

an interrogator;

a number of communications devices capable of wireless communications with the interrogator;

means for establishing for respective devices unique identification numbers respectively having the first predetermined number of bits;

means for causing the devices to select random values, wherein respective devices choose random values independently of random values selected by the other devices;

means for causing the interrogator to transmit a command requesting devices having random values within a specified group of random values to respond;

means for causing devices receiving the command to determine if their chosen random values fall within the specified group and, if so, to 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, to create a new, smaller, specified group that is one quarter of the first mentioned specified group, wherein at least one level of the tree is skipped.

23. A system in accordance with claim 22 wherein sending a reply to the interrogator comprises transmitting the unique identification number of the device sending the reply.

24. A system in accordance with claim 22 wherein sending a reply to the interrogator comprises transmitting the random value of the device sending the reply.

25. A system in accordance with claim 22 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.

26. A system in accordance with claim 22 wherein the interrogator further includes means for, after receiving a reply without collision from a device, sending a command individually addressed to that device.

27. A method for performing radio frequency communications, the method comprising:

determining by an interrogator a group of a plurality of groups from a range of possible random numbers;

transmitting by the interrogator a first request for a response from each of one or more radio frequency identification (rfid) devices that has generated a random number included in the group;

receiving by the one or more rfid devices the first request;

communicating by each of one or more responding rfid devices a response, wherein each of the one or more responding rfid devices has generated a random number within the group;

receiving by the interrogator one or more responses from the one or more responding rfid devices; and

determining by the interrogator if there is a collision between the one or more responses, and if such a collision is determined to occur, then determining by the interrogator a subgroup of a plurality of subgroups of possible random numbers within the group and transmitting by the interrogator a second request for response from each of one or more rfid devices that has generated a random number included in the subgroup, wherein, in a hierarchy of the group and the plurality of subgroups having multiple levels, the subgroup is more than one level lower than the group.

28. The method of claim 27, wherein the first request includes a selection indicator, the selection indicator selecting each of one or more rfid devices that has generated a random number included in the group for communication.

29. The method of claim 27, further comprising transmitting by the interrogator a wake-up signal, the wake-up signal causing the one or more rfid devices to transition from a non-responsive state to a responsive state.

30. The method of claim 27, wherein the hierarchy forms a binary tree and each of the multiple levels corresponds to a number of valid bits common to corresponding subgroups.

31. The method of claim 27, wherein the communicating by the one or more responding rfid devices comprises the one or more responding rfid devices communicating a value randomly generated by each of the one or more responding rfid devices.

32. The method of claim 27, further comprising transmitting by the interrogator a second request indicating a number of slots from which each of the one or more responding rfid devices is to randomly select a slot in which to respond with a randomly generated multi-bit identifier.

33. The method of claim 32, wherein the second request comprises a multi-bit value to select the at least one rfid device for response to the second request.

34. The method of claim 33, further comprising determining by the interrogator at least one identification number stored in at least one of the one or more responding rfid devices, the at least one identification number to identify a person.

35. The method of claim 32, further comprising determining by the interrogator at least one identification number stored in at least one of the one or more responding rfid devices, the at least one identification number to identify a person.

36. The method of claim 27, further comprising determining by the interrogator at least one identification number stored in at least one of the one or more responding rfid devices, the at least one identification number to identify a person associated with the at least one of the one or more rfid responding devices.

37. A system for performing radio frequency communications, the system comprising:

a radio frequency identification (rfid) device comprising a random number generator to generate a random value;

one or more antennas positioned in a first region; and

an interrogator communicatively coupled to the one or more antennas, the interrogator to repeatedly select a subgroup of numbers from a plurality of possible subgroups of numbers from a group of numbers and to transmit a first request for responses from one or more rfid devices;

storing a number in the selected subgroup until the interrogator receives one or more responses from one or more rfid devices without a collision, wherein the group and the plurality of subgroups correspond to a plurality of levels, with each subgroup containing a fewer number of numbers than the group, and at least one level is skipped at least during one of repeated subgroup selections.

38. The system of claim 37, wherein the first request includes a selection indicator, the selection indicator identifying one or more rfid devices storing a number in the selected subgroup for communication.

39. The system of claim 37, wherein the interrogator is further configured to transmit a wake-up signal, and one or more rfid devices are further configured to transition out of a sleep state upon receipt of the wake-up signal, wherein the sleep state is a non-responsive state.

40. The system of claim 37, wherein the plurality of levels correspond to a binary tree with each of the plurality of levels corresponding to a number of valid bits common to associated subgroups.

41. The system of claim 37, wherein the rfid device is one of the one or more rfid devices from which the interrogator receives the one or more responses.

42. The system of claim 37, wherein the interrogator is to transmit a second request indicating a number of slots from which at least one rfid device is to randomly select a slot in which to respond with an identifier.

43. The system of claim 42, wherein the second request comprises a plurality of bits to select the at least one rfid device corresponding to the bits.

44. The system of claim 43, wherein the rfid device stores an identification number to identify a person with whom the rfid device is associated.

45. The system of claim 42, wherein the number stored in the one or more rfid devices is a random number generated by the one or more rfid devices, and the identifier is randomly generated by the at least one rfid device.

46. The system of claim 42, wherein the rfid device stores an identification number to identify a person with whom the rfid device is associated.

47. The system of claim 37, wherein the rfid device stores an identification number to identify a person with whom the rfid device is associated.

48. An interrogator comprising:

one or more antennas;

a receiver communicatively coupled to at least one of the one or more antennas to receive one or more messages from one or more radio frequency identification (rfid) devices;

a transmitter communicatively coupled to at least one of the one or more antennas to transmit one or more messages; and

a control unit communicatively coupled to the transmitter and the receiver, the control unit to implement a sequence comprising a loop to repeatedly determine a subgroup for a first request such that the subgroup includes a subset of possible numbers of a previous subgroup and to repeatedly cause the transmitter to transmit the first request until the control unit receives a response without a collision occurring between a plurality of responding rfid devices, the first request including an indication of the subgroup, wherein in accordance with the sequence, in at least one of repeated subgroup determinations, the determined subgroup includes less than half of possible numbers of the successively previous subgroup.

49. The interrogator of claim 48, wherein the control unit is further configured to utilize a binary search tree to determine the subgroup.

50. The interrogator of claim 49, wherein the control unit is further configured to skip one or more intermediate levels in the binary search tree.

51. The interrogator of claim 48, wherein the response includes a random number generated by a responding rfid tag.

52. The interrogator of claim 48, wherein the transmitter is to transmit a second request to indicate a number of slots from which at least one rfid device is to randomly select a slot in which to respond with an identifier.

53. The interrogator of claim 52, wherein the second request is to comprise a plurality of bits to select the at least one rfid device, corresponding to the bits, to respond.

54. The interrogator of claim 53, wherein the receiver is to receive an identification number from the at least one rfid device to identify a person.

55. The interrogator of claim 53, wherein the subgroup includes a subset of possible random numbers generated by the one or more rfid devices, the response includes a number randomly generated by a responding rfid device of the one or more rfid devices, and the identifier is randomly generated by the at least one rfid device.

56. The interrogator of claim 52, wherein the subgroup includes a subset of possible random numbers generated by the one or more rfid devices, the response includes a number randomly generated by a responding rfid device of the one or more rfid devices, and the identifier is randomly generated by the at least one rfid device.

57. The interrogator of claim 56, wherein the receiver is to receive an identification number from the one or more rfid devices to identify a person.

58. The interrogator of claim 48, wherein the receiver is to receive an identification number from the one or more rfid devices to identify a person.

59. A method of operating a target radio frequency identification (rfid) device to operate in accordance with a process that enables the target device to be individually selected for further communication, the method comprising:

causing one or more rfid devices including the target rfid device to communicate a respective identifier, and if a collision occurs, performing operations comprising:

defining a first subgroup of a plurality of subgroups of possible identifiers;

transmitting a first request to rfid devices for each of rfid devices having a respective identifier within the first subgroup to respond;

receiving one or more responses, and if a collision occurs, repeatedly redefining the first subgroup to include fewer possible identifiers, and retransmitting the first request for each of rfid devices having a respective identifier within the redefined first subgroup to respond until a response is received without a collision, wherein one or more levels in a hierarchical structure of subgroups is skipped between at least one of the first subgroups and the redefined first subgroup that follows the at least one first subgroup in succession.

60. The method of claim 59, wherein the request includes a selection indicator, the selection indicator identifying one or more rfid devices from which a response is being requested.

61. The method of claim 59, wherein the target device is to transition from a non-responsive state to a responsive state in response to a wake-up signal.

62. The method of claim 59, wherein the defining and the redefining are performed at least in part in accordance with a binary search tree.

63. The method of claim 59, wherein the one or more responses include one or more respective random numbers generated by one or more respective rfid devices.

64. The method of claim 59, wherein the operations further comprise transmitting a second request indicating a number of slots, and at least one rfid device responding in a randomly selected slot of the number of slots indicated by the second request.

65. The method of claim 64, wherein in accordance with the process the second request comprises a multi-bit value that selects the at least one rfid device for response to the second request.

66. The method of claim 65, further comprising an identification number stored in a memory region of the target rfid device, the identification number corresponding to a person associated with the target rfid device.

67. The method of claim 64, wherein the identifiers are randomly generated by the one or more rfid devices, and the receiving the one or more responses comprises receiving one or more randomly generated numbers.

68. The method of claim 64, further comprising an identification number stored in a memory region of the target rfid device, the identification number corresponding to a person associated with the target rfid device.

69. The method of claim 59, further comprising an identification number stored in a memory region of the target rfid device, the identification number corresponding to a person associated with the target rfid device.

70. A method of operating a target radio frequency identification (rfid) device to operate in accordance with a process that enables the target device to be individually selected for further communication, the method comprising:

causing one or more rfid devices including the target rfid device to communicate a respective identifier, and if a collision occurs, performing operations comprising:

defining a first subgroup of a plurality of subgroups of possible identifiers;

transmitting a first request to rfid devices for each of rfid devices having a respective identifier within the first subgroup to respond; and

receiving one or more responses, and if a collision occurs, repeatedly redefining the first subgroup to include fewer possible identifiers, and retransmitting the first request for each of rfid devices having a respective identifier within the redefined first subgroup to respond until a response is received without a collision, wherein one or more levels in a hierarchical structure of subgroups is skipped between at least one of the first subgroups and the redefined first subgroup that follows the at least one first subgroup in succession;

determining an owner of the target rfid device based at least in part on said respective identifier of said target rfid device; and

debiting an account held by said owner.

71. The method of claim 70, wherein the debiting of the account held by the owner is associated with the payment of a toll.

72. The method of claim 71, wherein a master wireless device is disposed within a toll booth, and said method further comprises operating said master wireless device disposed within said toll both at least when said target rfid device communicating the respective identifier is in proximity thereto.

73. The method of claim 71, wherein the debiting of the account comprises receiving a credit card number against which the toll can be charged.

74. The method of claim 70, wherein the debiting of the account comprises receiving a credit card number which can be charged.

75. The method of claim 70, wherein the debiting of the account held by the owner is for payment for goods or services.

76. A method for performing radio frequency communications in a system comprised of one or more radio frequency identification (rfid) devices, one or more antennas positioned in a first region and an interrogator communicatively coupled to the one or more antennas, the method comprising:

repeatedly selecting a subgroup of numbers from a plurality of possible subgroups of numbers from a group of numbers;

transmitting a first request from the interrogator for responses from the one or more rfid devices;

storing a number in the selected subgroup until the interrogator receives one or more responses from a target rfid device without a collision, wherein the group and the plurality of subgroups correspond to a plurality of levels, with each subgroup containing a fewer number of numbers than the group, and at least one level is skipped at least during one of repeated subgroup selections;

determining an owner of the target rfid device based at least in part on the one or more responses from the target rfid device; and

debiting an account held by the owner.

77. The method of claim 76, wherein the debiting of the account held by the owner is associated with the payment of a toll.

78. The method of claim 77, wherein the interrogator is disposed within a toll both, and said method further comprises operating the interrogator disposed within said toll both at least when said target rfid device communicating is in proximity thereto.

79. The method of claim 77, wherein the debiting of the account comprises receiving a credit card number against which the toll can be charged.

80. The method of claim 76, wherein the debiting of the account comprises receiving a credit card number which can be charged.

81. The method of claim 76, wherein the debiting of the account held by the owner is for payment for goods or services.

82. A method of conducting a financial transaction, the method enabling a target radio frequency device to be individually selected for further communication, the method comprising:

causing one or more radio frequency devices including the target radio frequency device to communicate a respective identifier, and performing operations comprising:

defining a first subgroup of a plurality of subgroups of possible identifiers;

transmitting a first request to radio frequency devices for each of radio frequency devices having a respective identifier within the first subgroup to respond; and

receiving one or more responses, and if a collision occurs, repeatedly redefining the first subgroup to include fewer possible identifiers, and

retransmitting the first request for each of radio frequency devices having a respective identifier within the redefined first subgroup to respond until a response is received without a collision, wherein one or more levels in a hierarchical structure of subgroups is skipped between at least one of the first subgroups and the redefined first subgroup that follows the at least one first subgroup in succession; and

debiting a financial account associated with the target radio frequency device based at least in part on said respective identifier of said target radio frequency device.

83. The method of claim 82, wherein the debiting of the account is associated with the payment of a toll.

84. The method of claim 83, wherein a master wireless device is disposed within a toll booth, and said method further comprises operating said master wireless device disposed within said toll booth at least when said target radio frequency device communicating the respective identifier is in proximity thereto.

85. The method of claim 83, wherein the debiting of the account comprises receiving a credit card number against which the toll can be charged.

86. The method of claim 82, wherein the debiting of the account comprises receiving a credit card number which can be charged.

87. The method of claim 82, wherein the debiting of the account is for payment for goods or services.

88. A method for performing radio frequency communications in a system comprised of one or more radio frequency devices, one or more antennas positioned in a first region, and an interrogator communicatively coupled to the one or more antennas, the method comprising:

repeatedly selecting a subgroup of numbers from a plurality of possible subgroups of numbers from a group of numbers;

transmitting a first request from the interrogator for responses from the one or more radio frequency devices;

storing a number in the selected subgroup until the interrogator receives one or more responses from a target radio frequency device without a collision, wherein the group and the plurality of subgroups correspond to a plurality of levels, with each subgroup containing a fewer number of numbers than the group, and at least one level is skipped at least during one of repeated subgroup selections; and

debiting a financial account associated with the target radio frequency device based at least in part on the one or more responses from the target radio frequency device.

89. The method of claim 88, wherein the debiting of the account is associated with the payment of a toll.

90. The method of claim 89, wherein the interrogator is disposed within a toll both, and said method further comprises operating the interrogator disposed within said toll both at least when said target radio frequency device communicating is in proximity thereto.

91. The method of claim 89, wherein the debiting of the account comprises receiving a credit card number against which the toll can be charged.

92. The method of claim 88, wherein the debiting of the account comprises receiving a credit card number which can be charged.

93. The method of claim 88, wherein the debiting of the account is for payment for goods or services.