Abstract:
Messages may be passed between Radio Frequency Identification (RFID) tags using RFID readers. A first tag with a message intended for a second tag sends the message to an RFID reader. The reader then determines that the destination of the message is the second tag and sends the message to the second tag. The second tag may confirm receipt of the message by sending a receipt confirmation message to the reader for forwarding to the first tag, and/or the reader may itself confirm that the message was sent to the second tag by sending a transmit confirmation message to the first tag.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/059,070 filed on Oct. 2, 2014. The disclosures of the above applications are hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND 
     Radio-Frequency Identification (RFID) systems typically include RFID readers, also known as RFID reader/writers or RFID interrogators, and RFID tags. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package. 
     In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field. The RF wave may encode one or more commands that instruct the tags to perform one or more actions. 
     A tag that senses the interrogating RF wave may respond by transmitting back another RF wave. The tag either generates the transmitted back RF wave originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways. 
     The reflected-back RF wave may encode data stored in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on. Accordingly, when a reader receives tag data it can learn about the item that hosts the tag and/or about the tag itself. 
     An RFID tag typically includes an antenna section, a radio section, a power-management section, and frequently a logical section, a memory, or both. In some RFID tags the power-management section included an energy storage device such as a battery. RFID tags with an energy storage device are known as battery-assisted, semi-active, or active tags. Other RFID tags can be powered solely by the RF signal they receive. Such RFID tags do not include an energy storage device and are called passive tags. Of course, even passive tags typically include temporary energy- and data/flag-storage elements such as capacitors or inductors. 
     BRIEF SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended as an aid in determining the scope of the claimed subject matter. 
     Embodiments are directed to RFID tag-to-tag communication using RFID readers. A first tag with a message intended for a second tag sends the message to an RFID reader. The reader then determines that the destination of the message is the second tag and sends the message to the second tag. The second tag may confirm receipt of the message by sending a receipt confirmation message to the reader for forwarding to the first tag, and/or the reader may itself confirm that the message was sent to the second tag by sending a transmit confirmation message to the first tag. 
     According to some embodiments, a method for an RFID reader to pass messages between RFID tags is provided. The method may include receiving a first message including data from a first tag, determining the data are intended for a second tag, and attempting to establish communications with the second tag. The method may further include transmitting a second message including at least a portion of the data to the second tag in response to establishing communications with the second tag, and in response to not establishing communications with the second tag, at least one of storing the data for subsequent transmission to the second tag, storing an error indication, and transmitting the error indication to the first tag. 
     According to other embodiments, an RFID reader configured to pass messages between RFID tags is provided. The reader may include a transceiver configured to transmit and receive signals from RFID tags and a processor block coupled to the transceiver. The processor block may be configured to receive a first message including data from a first tag, determine the data are intended for a second tag, attempt to establish communications with the second tag, and transmit a second message including at least a portion of the data to the second tag in response to establishing communications with the second tag. 
     According to further embodiments, a method for a first RFID tag to transmit data to a second tag is provided. The method may include determining at the first tag that data is to be transmitted to the second tag, determining at the first tag at least a portion of an identifier for the second tag, and transmitting the data and the portion of the identifier to a reader for subsequent transmission to the second tag. 
     These and other features and advantages will be apparent from a reading of the following detailed description and a review of the associated drawings. It is to be understood that both the foregoing general description and the following detailed description are explanatory only and are not restrictive of aspects as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following Detailed Description proceeds with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of components of an RFID system. 
         FIG. 2  is a diagram showing components of a passive RFID tag, such as a tag that can be used in the system of  FIG. 1 . 
         FIG. 3  is a conceptual diagram for explaining a half-duplex mode of communication between the components of the RFID system of  FIG. 1 . 
         FIG. 4  is a block diagram showing a detail of an RFID tag, such as the one shown in  FIG. 2 . 
         FIGS. 5A and 5B  illustrate signal paths during tag-to-reader and reader-to-tag communications in the block diagram of  FIG. 4 . 
         FIG. 6  is a block diagram showing a detail of an RFID reader system, such as the one shown in  FIG. 1 . 
         FIG. 7  is a block diagram illustrating an overall architecture of an RFID system according to embodiments. 
         FIG. 8  is a conceptual diagram of how RFID readers can be used to pass messages between RFID tags, according to embodiments. 
         FIG. 9  depicts messages that a reader may send to tags, according to embodiments. 
         FIG. 10  is a conceptual diagram of how RFID readers can be used to pass notifications between RFID tags, according to embodiments. 
         FIG. 11  is a flowchart depicting a process for an RFID tag to send a message to another tag via an RFID reader, according to embodiments. 
         FIG. 12  is a flowchart depicting a process for an RFID reader to relay a message between RFID tags, according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, references are made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments or examples. These embodiments or examples may be combined, other aspects may be utilized, and structural changes may be made without departing from the spirit or scope of the present disclosure. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents. 
     As used herein, “memory” is one of ROM, RAM, SRAM, DRAM, NVM, EEPROM, FLASH, Fuse, MRAM, FRAM, and other similar information-storage technologies as will be known to those skilled in the art. Some portions of memory may be writeable and some not. “Command” refers to a reader request for one or more tags to perform one or more actions, and includes one or more tag instructions preceded by a command identifier or command code that identifies the command and/or the tag instructions. “Instruction” refers to a request to a tag to perform a single explicit action (e.g., write data into memory). “Program” refers to a request to a tag to perform a set or sequence of instructions (e.g., read a value from memory and, if the read value is less than a threshold then lock a memory word). “Protocol” refers to an industry standard for communications between a reader and a tag (and vice versa), such as the Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz-960 MHz by GS1 EPCglobal, Inc. (“Gen2 Specification”), versions 1.2.0 and 2.0 of which are hereby incorporated by reference. 
       FIG. 1  is a diagram of the components of a typical RFID system  100 , incorporating embodiments. An RFID reader  110  transmits an interrogating RF signal  112 . RFID tag  120  in the vicinity of RFID reader  110  senses interrogating RF signal  112  and generate signal  126  in response. RFID reader  110  senses and interprets signal  126 . The signals  112  and  126  may include RF waves and/or non-propagating RF signals (e.g., reactive near-field signals). 
     Reader  110  and tag  120  communicate via signals  112  and  126 . When communicating, each encodes, modulates, and transmits data to the other, and each receives, demodulates, and decodes data from the other. The data can be modulated onto, and demodulated from, RF waveforms. The RF waveforms are typically in a suitable range of frequencies, such as those near 900 MHz, 13.56 MHz, and so on. 
     The communication between reader and tag uses symbols, also called RFID symbols. A symbol can be a delimiter, a calibration value, and so on. Symbols can be implemented for exchanging binary data, such as “0” and “1”, if that is desired. When symbols are processed by reader  110  and tag  120  they can be treated as values, numbers, and so on. 
     Tag  120  can be a passive tag, or an active or battery-assisted tag (i.e., a tag having its own power source). When tag  120  is a passive tag, it is powered from signal  112 . 
       FIG. 2  is a diagram of an RFID tag  220 , which may function as tag  120  of  FIG. 1 . Tag  220  is drawn as a passive tag, meaning it does not have its own power source. Much of what is described in this document, however, applies also to active and battery-assisted tags. 
     Tag  220  is typically (although not necessarily) formed on a substantially planar inlay  222 , which can be made in many ways known in the art. Tag  220  includes a circuit which may be implemented as an IC  224 . In some embodiments IC  224  is implemented in complementary metal-oxide semiconductor (CMOS) technology. In other embodiments IC  224  may be implemented in other technologies such as bipolar junction transistor (BJT) technology, metal-semiconductor field-effect transistor (MESFET) technology, and others as will be well known to those skilled in the art. IC  224  is arranged on inlay  222 . 
     Tag  220  also includes an antenna for exchanging wireless signals with its environment. The antenna is often flat and attached to inlay  222 . IC  224  is electrically coupled to the antenna via suitable antenna contacts (not shown in  FIG. 2 ). The term “electrically coupled” as used herein may mean a direct electrical connection, or it may mean a connection that includes one or more intervening circuit blocks, elements, or devices. The “electrical” part of the term “electrically coupled” as used in this document shall mean a coupling that is one or more of ohmic/galvanic, capacitive, and/or inductive. 
     IC  224  is shown with a single antenna port, comprising two antenna contacts electrically coupled to two antenna segments  226  and  228  which are shown here forming a dipole. Many other embodiments are possible using any number of ports, contacts, antennas, and/or antenna segments. 
     Diagram  250  depicts top and side views of tag  252 . Tag  252  differs from tag  220  in that it includes a substantially planar strap substrate  254  having strap contacts  256  and  258 . The IC  224  is mounted on the strap substrate  254  such that antenna contacts/pads on the IC  224  electrically connect with the strap contacts  256  and  258  via suitable contact connections (not shown). The strap substrate  254  is then placed on the inlay  222  such that the strap contacts  256  and  258  electrically connect to the antenna segments  226  and  228 . The strap substrate  254  may be affixed to the inlay  222  via pressing, an interface layer, one or more adhesives, or any other suitable means for securing the strap substrate  254  to the inlay  222 . 
     Diagram  260  depicts a side view of an alternative way to place strap substrate  254  onto the inlay  222 . Instead of having the surface of strap substrate  254  with strap contacts  256 / 258  face the surface of inlay  222  with antenna segments  226 / 228 , strap substrate  254  is placed such that the surface opposite the strap contacts  256 / 258  faces the surface of inlay  222  with antenna segments  226 / 228 . Strap contacts  256 / 258  can then be electrically connected to antenna segments  226 / 228  capacitively through strap substrate  254 . In some embodiments, the relative positions of strap substrate  254  and inlay  222  may be reversed, with strap contacts  256 / 258  capacitively coupled to antenna segments  226 / 228  through inlay  222  instead of through strap substrate  254 . Of course, in other embodiments strap contacts  256 / 258  may capacitively couple to antenna segments  226 / 228  through both inlay  222  and strap substrate  254 . 
     In operation, the antenna receives a signal and communicates it to IC  224 , which both harvests power and responds if appropriate, based on the incoming signal and the IC&#39;s internal state. If IC  224  uses backscatter modulation then it responds by modulating the antenna&#39;s reflectance, which generates response signal  126  from signal  112  transmitted by the reader. Electrically coupling and uncoupling the antenna contacts of IC  224  can modulate the antenna&#39;s reflectance, as can varying the admittance of a shunt-connected circuit element which is coupled to the antenna contacts. Varying the impedance of a series-connected circuit element is another means of modulating the antenna&#39;s reflectance. 
     In the embodiment of  FIG. 2 , antenna segments  226  and  228  are separate from IC  224 . In other embodiments the antenna segments may alternatively be formed on IC  224 . Tag antennas according to embodiments may be designed in any form and are not limited to dipoles. For example, the tag antenna may be a patch, a slot, a loop, a coil, a horn, a spiral, or any other suitable antenna. 
     The components of the RFID system of  FIG. 1  may communicate with each other in any number of modes. One such mode is called full duplex. Another such mode is called half-duplex, and is described below. 
       FIG. 3  is a conceptual diagram  300  for explaining half-duplex communications between the components of the RFID system of  FIG. 1 , in this case with tag  120  implemented as passive tag  220  of  FIG. 2 . The explanation is made with reference to a TIME axis, and also to a human metaphor of “talking” and “listening”. The actual technical implementations for “talking” and “listening” are now described. 
     RFID reader  110  and RFID tag  120  talk and listen to each other by taking turns. As seen on axis TIME, when reader  110  talks to tag  120  the communication session is designated as “R→T”, and when tag  120  talks to reader  110  the communication session is designated as “T→R”. Along the TIME axis, a sample R→T communication session occurs during a time interval  312 , and a following sample T→R communication session occurs during a time interval  326 . Of course interval  312  is typically of a different duration than interval  326 —here the durations are shown approximately equal only for purposes of illustration. 
     According to blocks  332  and  336 , RFID reader  110  talks during interval  312 , and listens during interval  326 . According to blocks  342  and  346 , RFID tag  120  listens while reader  110  talks (during interval  312 ), and talks while reader  110  listens (during interval  326 ). 
     In terms of actual behavior, during interval  312  reader  110  talks to tag  120  as follows. According to block  352 , reader  110  transmits signal  112 , which was first described in  FIG. 1 . At the same time, according to block  362 , tag  120  receives signal  112  and processes it to extract data and so on. Meanwhile, according to block  372 , tag  120  does not backscatter with its antenna, and according to block  382 , reader  110  has no signal to receive from tag  120 . 
     During interval  326 , tag  120  talks to reader  110  as follows. According to block  356 , reader  110  transmits a continuous wave (CW) signal, which can be thought of as a carrier that typically encodes no information. This CW signal serves both to transfer energy to tag  120  for its own internal power needs, and also as a carrier that tag  120  can modulate with its backscatter. Indeed, during interval  326 , according to block  366 , tag  120  does not receive a signal for processing. Instead, according to block  376 , tag  120  modulates the CW emitted according to block  356  so as to generate backscatter signal  126 . Concurrently, according to block  386 , reader  110  receives backscatter signal  126  and processes it. 
       FIG. 4  is a block diagram showing a detail of an RFID IC, such as IC  224  in  FIG. 2 . Electrical circuit  424  in  FIG. 4  may be formed in an IC of an RFID tag, such as tag  220  of  FIG. 2 . Circuit  424  has a number of main components that are described in this document. Circuit  424  may have a number of additional components from what is shown and described, or different components, depending on the exact implementation. 
     Circuit  424  shows two IC contacts  432 ,  433 , suitable for coupling to antenna segments such as antenna segments  226 / 228  of RFID tag  220  of  FIG. 2 . When two IC contacts form the signal input from and signal return to an antenna they are often referred-to as an antenna port. IC contacts  432 ,  433  may be made in any suitable way, such as from metallic pads and so on. In some embodiments circuit  424  uses more than two IC contacts, especially when tag  220  has more than one antenna port and/or more than one antenna. 
     Circuit  424  includes signal-routing section  435  which may include signal wiring, signal-routing busses, receive/transmit switches, and so on that can route a signal to the components of circuit  424 . In some embodiments IC contacts  432 / 433  couple galvanically and/or inductively to signal-routing section  435 . In other embodiments (such as is shown in  FIG. 4 ) circuit  424  includes optional capacitors  436  and/or  438  which, if present, capacitively couple IC contacts  432 / 433  to signal-routing section  435 . This capacitive coupling causes IC contacts  432 / 433  to be galvanically decoupled from signal-routing section  435  and other circuit components. 
     Capacitive coupling (and resultant galvanic decoupling) between IC contacts  432  and/or  433  and components of circuit  424  is desirable in certain situations. For example, in some RFID tag embodiments IC contacts  432  and  433  may galvanically connect to terminals of a tuning loop on the tag. In this situation, capacitors  436  and/or  438  galvanically decouple IC contact  432  from IC contact  433 , thereby preventing the formation of a short circuit between the IC contacts through the tuning loop. 
     Capacitors  436 / 438  may be implemented within circuit  424  and/or partly or completely external to circuit  424 . For example, a dielectric or insulating layer on the surface of the IC containing circuit  424  may serve as the dielectric in capacitor  436  and/or capacitor  438 . As another example, a dielectric or insulating layer on the surface of a tag substrate (e.g., inlay  222  or strap substrate  254 ) may serve as the dielectric in capacitors  436 / 438 . Metallic or conductive layers positioned on both sides of the dielectric layer (i.e., between the dielectric layer and the IC and between the dielectric layer and the tag substrate) may then serve as terminals of the capacitors  436 / 438 . The conductive layers may include IC contacts (e.g., IC contacts  432 / 433 ), antenna segments (e.g., antenna segments  226 / 228 ), or any other suitable conductive layers. 
     Circuit  424  also includes a rectifier and PMU (Power Management Unit)  441  that harvests energy from the RF signal received by antenna segments  226 / 228  to power the circuits of IC  424  during either or both reader-to-tag (R→T) and tag-to-reader (T→R) sessions. Rectifier and PMU  441  may be implemented in any way known in the art. 
     Circuit  424  additionally includes a demodulator  442  that demodulates the RF signal received via IC contacts  432 ,  433 . Demodulator  442  may be implemented in any way known in the art, for example including a slicer, an amplifier, and so on. 
     Circuit  424  further includes a processing block  444  that receives the output from demodulator  442  and performs operations such as command decoding, memory interfacing, and so on. In addition, processing block  444  may generate an output signal for transmission. Processing block  444  may be implemented in any way known in the art, for example by combinations of one or more of a processor, memory, decoder, encoder, and so on. 
     Circuit  424  additionally includes a modulator  446  that modulates an output signal generated by processing block  444 . The modulated signal is transmitted by driving IC contacts  432 ,  433 , and therefore driving the load presented by the coupled antenna segment or segments. Modulator  446  may be implemented in any way known in the art, for example including a switch, driver, amplifier, and so on. 
     In one embodiment, demodulator  442  and modulator  446  may be combined in a single transceiver circuit. In another embodiment modulator  446  may modulate a signal using backscatter. In another embodiment modulator  446  may include an active transmitter. In yet other embodiments demodulator  442  and modulator  446  may be part of processing block  444 . 
     Circuit  424  additionally includes a memory  450  to store data  452 . At least a portion of memory  450  is preferably implemented as a nonvolatile memory (NVM), which means that data  452  is retained even when circuit  424  does not have power, as is frequently the case for a passive RFID tag. 
     In some embodiments, particularly in those with more than one antenna port, circuit  424  may contain multiple demodulators, rectifiers, PMUs, modulators, processing blocks, and/or memories. 
     In terms of processing a signal, circuit  424  operates differently during a R→T session and a T→R session. The different operations are described below, in this case with circuit  424  representing an IC of an RFID tag. 
       FIG. 5A  shows version  524 -A of components of circuit  424  of  FIG. 4 , further modified to emphasize a signal operation during a R→T session during time interval  312  of  FIG. 3 . Demodulator  442  demodulates an RF signal received from IC contacts  432 ,  433 . The demodulated signal is provided to processing block  444  as C_IN. In one embodiment, C_IN may include a received stream of symbols. 
     Version  524 -A shows as relatively obscured those components that do not play a part in processing a signal during a R→T session. Rectifier and PMU  441  may be active, such as for converting RF power. Modulator  446  generally does not transmit during a R→T session, and typically does not interact with the received RF signal significantly, either because switching action in section  435  of  FIG. 4  decouples modulator  446  from the RF signal, or by designing modulator  446  to have a suitable impedance, and so on. 
     Although modulator  446  is typically inactive during a R→T session, it need not be so. For example, during a R→T session modulator  446  could be adjusting its own parameters for operation in a future session, and so on. 
       FIG. 5B  shows version  524 -B of components of circuit  424  of  FIG. 4 , further modified to emphasize a signal operation during a T→R session during time interval  326  of  FIG. 3 . Processing block  444  outputs a signal C_OUT. In one embodiment, C_OUT may include a stream of symbols for transmission. Modulator  446  then modulates C_OUT and provides it to antenna segments such as segments  226 / 228  of RFID tag  220  via IC contacts  432 ,  433 . 
     Version  524 -B shows as relatively obscured those components that do not play a part in processing a signal during a T→R session. Rectifier and PMU  441  may be active, such as for converting RF power. Demodulator  442  generally does not receive during a T→R session, and typically does not interact with the transmitted RF signal significantly, either because switching action in section  435  of  FIG. 4  decouples demodulator  442  from the RF signal, or by designing demodulator  442  to have a suitable impedance, and so on. 
     Although demodulator  442  is typically inactive during a T→R session, it need not be so. For example, during a T→R session demodulator  442  could be adjusting its own parameters for operation in a future session, and so on. 
     In typical embodiments, demodulator  442  and modulator  446  are operable to demodulate and modulate signals according to a protocol, such as the Gen2 Specification mentioned above. In embodiments where circuit  424  includes multiple demodulators and/or modulators, each may be configured to support different protocols or different sets of protocols. A protocol specifies, in part, symbol encodings, and may include a set of modulations, rates, timings, or any other parameter associated with data communications. In addition, a protocol can be a variant of a stated specification such as the Gen2 Specification, for example including fewer or additional commands than the stated specification calls for, and so on. In such instances, additional commands are sometimes called custom commands. 
       FIG. 6  is a block diagram of an RFID reader system  600  according to embodiments. RFID reader system  600  includes a local block  610 , and optionally remote components  670 . Local block  610  and remote components  670  can be implemented in any number of ways. It will be recognized that RFID reader  110  of  FIG. 1  is the same as local block  610 , if remote components  670  are not provided. Alternately, RFID reader  110  can be implemented instead by RFID reader system  600 , of which only the local block  610  is shown in  FIG. 1 . 
     In some embodiments, one or more of the blocks or components of reader system  600  may be implemented as integrated circuits. For example, local block  610 , one or more of the components of local block  610 , and/or one or more of the remote component  670  may be implemented as integrated circuits using CMOS technology, BJT technology, MESFET technology, and/or any other suitable implementation technology. 
     Local block  610  is responsible for communicating with the tags. Local block  610  includes a block  651  of an antenna and a driver of the antenna for communicating with the tags. Some readers, like that shown in local block  610 , contain a single antenna and driver. Some readers contain multiple antennas and drivers and a method to switch signals among them, including sometimes using different antennas for transmitting and for receiving. Some readers contain multiple antennas and drivers that can operate simultaneously. A demodulator/decoder block  653  demodulates and decodes backscattered waves received from the tags via antenna/driver block  651 . Modulator/encoder block  654  encodes and modulates an RF wave that is to be transmitted to the tags via antenna/driver block  651 . 
     Local block  610  additionally includes an optional local processor  656 . Local processor  656  may be implemented in any number of ways known in the art. Such ways include, by way of examples and not of limitation, digital and/or analog processors such as microprocessors and digital-signal processors (DSPs); controllers such as microcontrollers; software running in a machine such as a general purpose computer: programmable circuits such as Field Programmable Gate Arrays (FPGAs), Field-Programmable Analog Arrays (FPAAs), Programmable Logic Devices (PLDs), Application Specific Integrated Circuits (ASIC), any combination of one or more of these; and so on. In some cases, some or all of the decoding function in block  653 , the encoding function in block  654 , or both, may be performed instead by local processor  656 . In some cases local processor  656  may implement an encryption or authentication function; in some cases one or more of these functions can be distributed among other blocks such as encoding block  654 , or may be entirely incorporated in another block. 
     Local block  610  additionally includes an optional local memory  657 . Local memory  657  may be implemented in any number of ways known in the art, including, by way of example and not of limitation, any of the memory types described above as well as any combination thereof. Local memory  657  can be implemented separately from local processor  656 , or in an IC with local processor  656 , with or without other components. Local memory  657 , if provided, can store programs for local processor  656  to run, if needed. 
     In some embodiments, local memory  657  stores data read from tags, or data to be written to tags, such as Electronic Product Codes (EPCs), Tag Identifiers (TIDs) and other data. Local memory  657  can also include reference data that is to be compared to EPCs, instructions and/or rules for how to encode commands for the tags, modes for controlling antenna  651 , secret keys, key pairs, and so on. In some of these embodiments, local memory  657  is provided as a database. 
     Some components of local block  610  typically treat the data as analog, such as the antenna/driver block  651 . Other components such as local memory  657  typically treat the data as digital. At some point there is a conversion between analog and digital. Based on where this conversion occurs, a reader may be characterized as “analog” or “digital”, but most readers contain a mix of analog and digital functionality. 
     If remote components  670  are provided, they are coupled to local block  610  via an electronic communications network  680 . Network  680  can be a Local Area Network (LAN), a Metropolitan Area Network (MAN), a Wide Area Network (WAN), a network of networks such as the internet, or a local communication link, such as a USB, PCI, and so on. Local block  610  may include a local network connection  659  for communicating with communications network  680 . Communications on the network can be secure, such as if they are encrypted or physically protected, or insecure if they are not encrypted or otherwise protected. 
     There can be one or more remote component(s)  670 . If more than one, they can be located at the same location, or in different locations. They can access each other and local block  610  via communications network  680 , or via other similar networks, and so on. Accordingly, remote component(s)  670  can use respective remote network connections. Only one such remote network connection  679  is shown, which is similar to local network connection  659 , etc. 
     Remote component(s)  670  can also include a remote processor  676 . Remote processor  676  can be made in any way known in the art, such as was described with reference to local processor  656 . Remote processor  676  may also implement an authentication function, similar to local processor  656 . 
     Remote component(s)  670  can also include a remote memory  677 . Remote memory  677  can be made in any way known in the art, such as was described with reference to local memory  657 . Remote memory  677  may include a local database, and a different database of a standards organization, such as one that can reference EPCs. Remote memory  677  may also contain information associated with commands, tag profiles, keys, or the like, similar to local memory  657 . 
     Of the above-described elements, it may be useful to consider a combination of these components, designated as operational processing block  690 . Operational processing block  690  includes those components that are provided of the following: local processor  656 , remote processor  676 , local network connection  659 , remote network connection  679 , and by extension an applicable portion of communications network  680  that links remote network connection  679  with local network connection  659 . The portion can be dynamically changeable, etc. In addition, operational processing block  690  can receive and decode RF waves received via antenna/driver  651 , and cause antenna/driver  651  to transmit RF waves according to what it has processed. 
     Operational processing block  690  includes either local processor  656 , or remote processor  676 , or both. If both are provided, remote processor  676  can be made such that it operates in a way complementary with that of local processor  656 . In fact, the two can cooperate. It will be appreciated that operational processing block  690 , as defined this way, is in communication with both local memory  657  and remote memory  677 , if both are present. 
     Accordingly, operational processing block  690  is location independent, in that its functions can be implemented either by local processor  656 , or by remote processor  676 , or by a combination of both. Some of these functions are preferably implemented by local processor  656 , and some by remote processor  676 . Operational processing block  690  accesses local memory  657 , or remote memory  677 , or both for storing and/or retrieving data. 
     RFID reader system  600  operates by operational processing block  690  generating communications for RFID tags. These communications are ultimately transmitted by antenna/driver block  651 , with modulator/encoder block  654  encoding and modulating the information on an RF wave. Then data is received from the tags via antenna/driver block  651 , demodulated and decoded by demodulator/decoder block  653 , and processed by operational processing block  690 . 
     Embodiments of an RFID reader system can be implemented as hardware, software, firmware, or any combination. Such a system may be subdivided into components or modules. A person skilled in the art will recognize that some of these components or modules can be implemented as hardware, some as software, some as firmware, and some as a combination. An example of such a subdivision is now described, together with the RFID tag as an additional module. 
       FIG. 7  is a block diagram illustrating an overall architecture of an RFID system  700  according to embodiments. RFID system  700  may be subdivided into modules or components, each of which may be implemented by itself or in combination with others. In addition, some of them may be present more than once. Other embodiments may be equivalently subdivided into different modules. Some aspects of  FIG. 7  are parallel with systems, modules, and components described previously. 
     An RFID tag  703  is considered here as a module by itself. RFID tag  703  conducts a wireless communication  706  with the remainder, via the air interface  705 . Air interface  705  is really a boundary, in that signals or data that pass through it are not intended to be transformed from one thing to another. Specifications as to how readers and tags are to communicate with each other, for example the Gen2 Specification, also properly characterize that boundary as an interface. 
     RFID system  700  includes one or more reader antennas  710 , and an RF front-end module  720  for interfacing with reader antenna(s)  710 . These can be made as described above. 
     RFID system  700  also includes a signal-processing module  730 . In one embodiment, signal-processing module  730  exchanges waveforms with RF front-end module  720 , such as I and Q waveform pairs. 
     RFID system  700  also includes a physical-driver module  740 , which is also known as data-link module. In some embodiments physical-driver module  740  exchanges bits with signal-processing module  730 . Physical-driver module  740  can be the stage associated with the framing of data. 
     RFID system  700  additionally includes a media access control module  750 . In one embodiment, media access control layer module  750  exchanges packets of bits with physical driver module  740 . Media access control layer module  750  can make decisions for sharing the medium of wireless communication, which in this case is the air interface. 
     RFID system  700  moreover includes an application-programming library-module  760 . This module  760  can include application programming interfaces (APIs), other objects, etc. 
     All of these RFID system functionalities can be supported by one or more processors. One of these processors can be considered a host processor. Such a host processor might include a host operating system (OS) and/or central processing unit (CPU), as in module  770 . In some embodiments, the processor is not considered as a separate module, but one that includes some of the above-mentioned modules of RFID system  700 . In some embodiments the one or more processors may perform operations associated with retrieving data that may include a tag public key, an electronic signature, a tag identifier, an item identifier, and/or a signing-authority public key. In some embodiments the one or more processors may verify an electronic signature, create a tag challenge, and/or verify a tag response. 
     User interface module  780  may be coupled to application-programming-library module  760 , for accessing the APIs. User interface module  780  can be manual, automatic, or both. It can be supported by the host OS/CPU module  770  mentioned above, or by a separate processor, etc. 
     It will be observed that the modules of RFID system  700  form a chain. Adjacent modules in the chain can be coupled by appropriate instrumentalities for exchanging signals. These instrumentalities include conductors, buses, interfaces, and so on. These instrumentalities can be local, e.g. to connect modules that are physically close to each other, or over a network, for remote communication. 
     The chain is used in one direction for receiving RFID waveforms and in the other direction for transmitting RFID waveforms. In receiving mode, reader antenna(s)  710  receives wireless waves, which are in turn processed successively by the various modules in the chain. Processing can terminate in any one of the modules. In transmitting mode, waveform initiation can be in any one of the modules. Ultimately, signals are routed to reader antenna(s)  710  to be transmitted as wireless waves. 
     The architecture of RFID system  700  is presented for purposes of explanation, and not of limitation. Its particular, subdivision into modules need not be followed for creating embodiments. Furthermore, the features of the present disclosure can be performed either within a single one of the modules, or by a combination of them. 
     As mentioned previously, embodiments are directed to RFID tag-to-tag communication using RFID readers. Embodiments additionally include programs, and methods of operation of the programs. A program is generally defined as a group of steps or operations leading to a desired result, due to the nature of the elements in the steps and their sequence. A program may be implemented as a sequence of steps or operations for a processor, but may be implemented in other processing elements such as FPGAs, DSPs, or other devices as described above. 
     Performing the steps, instructions, or operations of a program requires manipulating physical quantities. Usually, though not necessarily, these quantities may be transferred, combined, compared, and otherwise manipulated or processed according to the steps or instructions, and they may also be stored in a computer-readable medium. These quantities include, for example, electrical, magnetic, and electromagnetic charges or particles, states of matter, and in the more general case can include the states of any physical devices or elements. It is convenient at times, principally for reasons of common usage, to refer to information represented by the states of these quantities as bits, data bits, samples, values, symbols, characters, terms, numbers, or the like. It should be borne in mind, however, that all of these and similar terms are associated with the appropriate physical quantities, and that these terms are merely convenient labels applied to these physical quantities, individually or in groups. 
     Embodiments furthermore include storage media. Such media, individually or in combination with others, have stored thereon instructions, data, keys, signatures, and other data of a program made according to the embodiments. A storage medium according to the embodiments is a computer-readable medium, such as a memory, and is read by a processor of the type mentioned above. If a memory, it can be implemented in any of the ways and using any of the technologies described above. 
     Even though it is said that the program may be stored in a computer-readable medium, it should be clear to a person skilled in the art that it need not be a single memory, or even a single machine. Various portions, modules or features of it may reside in separate memories, or even separate machines. The separate machines may be connected directly, or through a network such as a local access network (LAN) or a global network such as the Internet. 
     In some situations, a tag population may include specialized tags having particular features or capabilities and other tags that do not have those particular features. For example, specialized tags may be configured to sense the environment (e.g., to sense vibration, temperature, humidity, light levels, chemical presence, radiation, etc.). As another example, specialized tags may be configured to perform cryptographic or authentication operations. 
     In some embodiments, a tag population that includes specialized tags with certain features and other tags without those features may be configured to allow the other tags to take advantage of those certain features, by using a reader as a message relay. For example, suppose that a refrigerated container stores a number of RFID-tagged items that are temperature sensitive, and the items are to be monitored to ensure that their temperatures do not exceed a particular threshold. Instead of having each RFID tag configured to independently determine temperature, some subset of the tags, or one or more other tags associated with the refrigerated container, may be configured with temperature-sensing capability. These temperature-sensing tags may then be configured to measure the temperature within the container (based on some predetermined or dynamic schedule) and update the other, non-temperature-sensing tags by using one or more RFID readers as relays. For example, a temperature-sensing tag may occasionally transmit temperature data to other tags for storage, as a kind of temperature log, or may cause other tags to store a temperature warning or flag if the sensed temperature exceeds the threshold. Subsequently, a user or customer may inventory one or more of the other tags and be able to determine that the temperature of the tagged items have exceeded the temperature threshold. Accordingly, even though only a subset of tags are configured with temperature-sensing capabilities, all of the tags in the population may be able to share the temperature-sensing capabilities of the subset. 
     In some embodiments, the specialized tags described herein may be further configured to at least partially control reader-tag interactions. Many RFID systems with readers and tags are configured such that a reader (or a controller coupled to a reader) initiates reader-tag interactions while the tags respond. For example, a reader determines whether, when, and how reader commands are to be transmitted to tags, and the tags respond accordingly. In some embodiments, an RFID system may be configured such that tags take some role in controlling reader-tag interactions. For example, a tag may determine that some data is to be written to or read from another tag. In response, the tag may use a reader as a message relay, by indicating to the reader how and/or when the data is to be written to or read from the other tag. In some embodiments, the tag itself may act as a controller for the reader. 
       FIG. 8  is a conceptual diagram of how RFID readers can be used to pass messages between RFID tags, according to embodiments. Diagram  800  depicts RFID reader  810  and RFID tags  802 ,  820 , and  830 . Reader  810  may be able to communicate with tags  802 ,  820 , and  830  at some point in time. For example, tags  802 ,  820 , and  830  may be within the field-of-view of reader  810  at substantially the same time. As another example, only one or two of tags  802 ,  820 , and  830  may be in the field-of-view of reader  810  at a particular time, but all three tags  802 ,  820 , and  830  may be within the field-of-view of reader  810  over some period of time. 
     In some embodiments, tag  802  may wish to transmit a message  806  containing commands, data, or other suitable data to tag  820  and/or tag  830 . For example, suppose that tag  802  is configured to sense or receive sensor data about its environment, such as vibration, temperature, humidity, light levels, and/or presence of chemicals. In contrast, tags  820  and  830 , present in the same environment, may not be capable of sensing the environment, and furthermore may be associated with items that are sensitive to some aspect of the environment, such as being light-, temperature-, or chemically-sensitive. In this situation, tag  802  may transmit the sensed environmental data to tags  820  and  830 . Tags  820  and  830  may then store the sensed environmental data to provide a record of potentially harmful environmental exposures for their associated items. 
     Message  806  may represent a fully-formed tag response (for example, a tag response to a previous reader command or request), or may represent the payload of such a tag response. In some embodiments, message  806  may be a portion of a longer message. In some embodiments, some or all of message  806  or a longer message of which message  806  is a component may be cryptographically protected using encryption and/or authentication techniques. These techniques might be based on either symmetric or asymmetric cryptography. The secret or private keys for any cryptographic operation might be known to a specific set of tags, to a single tag, as well as potentially to a set of readers or otherwise authorized entities. In some embodiments, tag  802  may store message  806  in a tag memory  804 , or may generate message  806  as it is sent. 
     Tag  802  may then indicate to reader  810  that it has a message intended for one or more other tags. For example, tag  802  may assert a message flag for reader  810  to read, or may include a message indicator or the message  806  in a response to reader  810 . Reader  810  may then read message  806  from tag  802 , if tag  802  has not already provided message  806  to reader  810  in a previous tag response. 
     In some embodiments, message  806  may be intended for one or more particular tags. In these situations, tag  802  may provide a destination tag identifier to reader  810 . Tag  802  may include the destination tag identifier in message  806  (e.g., inserted or concatenated) or may send the destination tag identifier separately to reader  810  (e.g., in a separate message). In some embodiments, tag  802  may store the destination tag identifier near message  806  in tag memory  804  or may store the destination tag identifier at an entirely different location in tag memory  804 . Reader  810  may then use the destination tag identifier associated with message  806  to identify one or more destination tags for message  810 . The destination tag identifier may include identifier(s) for one or more specific tags, such as a tag identifier (TID), an electronic product code (EPC), and/or a serialized global trade item number (SGTIN). The destination tag identifier may also (or instead) include identifier(s) for a group or class of tags, such as a global trade item number (GTIN), a non-serialized or non-unique portion of a TID or EPC, a tag or tag IC manufacturer identifier, a customer identifier, or any other suitable identifier associated with one or more tags. In some embodiments, the destination tag identifier may identify tags with particular features. For example, the destination tag identifier may identify tags that lack certain capabilities, such as environmental sensing, cryptographic operations, or other capabilities. In other embodiments, the destination tag identifier may identify tags in a particular location. For example, the destination tag identifier may identify tags in a particular container but not in an adjacent container. 
     Reader  810  may also provide the destination tag identifiers instead of or in addition to tag  802 . In some embodiments, tag  802  may know the identity of tags  820 / 830 , but may not know the particular identifiers used to identify tags  820 / 830  during reader-tag communication. For example, reader  810  may identify tags  820 / 830  using tag handles or random/pseudorandom numbers (for example, as described in the Gen2 Specification) that are not necessarily known to tag  802 . In this situation, reader  810  may convert destination tag identifiers provided by tag  802  into the appropriate tag identifiers. In some embodiments, tag  802  may not know the identity of tags  820 / 830  at all. In this case, reader  810  may identify the appropriate tags to receive message  806 , for example based on one or more defining characteristics provided by tag  802  or known by reader  810 . In one embodiment, tag  802  may indicate that message  806  is to be provided to any tag that reader  810  can communicate with, or only to tags associated with items in a particular location or a particular container. In response, reader  810  may identify the corresponding tags (for example, via querying or inventorying) and provide message  806  to the identified tags. 
     In embodiments where message  806  is a portion of a longer message, message  806  may include or be associated with a portion identifier to enable a message recipient to reassemble the longer message from the message portions. The portion identifier associated with a particular message portion may identify the particular message portion, the total number of portions in the message, the originator of the message, or any other information related to the message or the message reassembly process. 
     Reader  810 , upon determining that it has received a message intended for one or more identified destination tags, may attempt to establish communications with the identified destination tag(s), for example by determining whether the destination tag(s) are visible (i.e., in its field-of-view) and successfully receive and respond to reader commands. For example, if message  806  is associated with a destination tag identifier identifying tag  820  and/or tag  830 , reader  810  may determine whether tag  820  and/or tag  830  are visible and can successfully receive and respond to commands. Reader  810  may determine whether communications can be established with a particular tag by performing a querying or inventorying process (for example, as described in the Gen2 Specification) to retrieve identifiers associated with visible tags. 
     If reader  810  determines that communications can be established to at least one destination tag, it may send message  812  to the visible destination tag(s). For example, as depicted in  FIG. 8 , reader  810  may send message  812  to tag  820  and tag  830 . In some embodiments, message  812  may be identical to message  806 . In other embodiments, message  812  may include additional information over message  806  and/or exclude information present in message  806 . Such information may include routing information, destination tag identifiers (as described above), originating tag identifiers (e.g., an identifier for tag  802 ), relaying reader identifiers (e.g., an identifier for reader  810 ), timestamps for receipt and/or transmission of messages  806  and/or  812 , or any other suitable information. 
     On the other hand, if reader  810  determines that communications cannot be established with any destination tags, reader  810  may enter a failure state. In the failure state, reader  810  may discard the message  806  and/or transmit an error indication to tag  802  indicating that message  806  could not be successfully transmitted to the destination tag(s). In some embodiments, reader  810  may also (or instead) store the error indication, along with identifiers for the originating tag (tag  802 ) and the destination tags and optionally timestamps for the failed transmission(s). 
     In some embodiments, reader  810  may not discard the message  806  and/or transmit/store the error indication immediately, and instead may attempt to re-deliver message  806  at a later time. In this situation, reader  810  may store the message  806  until communications can be established with a destination tag or until a particular time period has elapsed. For example, reader  810  may be configured to store message  806  for a dynamically-determined or predefined time period. In some embodiments, reader  810  may be configured to store message  806  until the memory space occupied by message  806  is to be used for another purpose. In other embodiments, message  806  may include or be associated with an expiration time, and reader  810  may be configured to discard message  806  when the expiration time is reached. Similarly, in embodiments where message  806  is associated with multiple destination tags (e.g., tag  820  and tag  830 ) and reader  810  is only able to send message  806  to one of the tags, reader  810  may retain message  806  until one or more of the above criteria have been met. 
     Tags  820  and/or  830  may then process the received message  812 . For example, if message  812  includes one or more commands, tags  820 / 830  may comply with the included commands. If message  812  includes data, tags  820 / 830  may store the received data, use the data to modify existing stored data, and/or modify a tag operating condition or state. 
     In some embodiments, tags  820 / 830  may validate the message  812  by determining whether the message  812  is complete and/or authentic before processing any data included in the message  812 . For example, message  812  may include or be associated with a portion identifier (described above) if it is a portion of a longer message. Tags  820 / 830  may read the portion identifiers associated with received messages and use the portion identifiers to determine if a complete message has been received or if some message portions have not been received. 
     In some embodiments, messages  806  and/or  812  may include check code(s) that can be used to verify the correctness of an associated message or message portion. For example, a message or message portion may include a check code such as a parity bit or bits, a checksum, a cyclic redundancy check, a hash function output, an error-correcting code, or any other suitable error-checking or correcting code. The reader  810  or the tags  820 / 830  may then use the check code to determine if the message has been corrupted or otherwise damaged during transit. 
     Messages  806 / 812  may also (or instead) be electronically secured by tag  802 , reader  810 , or some other authority. For example, the message(s) may be encrypted and/or include a cryptographically-derived authentication code such as a digital signature, message authentication code, or any other suitable cryptographic code. A receiving entity, such as reader  810  or tags  820 / 830 , may then be able to verify that the message is correct, was sent from a particular source, and was not modified during transit. 
     In some embodiments, tags  802 ,  820 ,  830 , and reader  810  may also (or instead) be cryptographically authenticated to each other via a separate process. For example, tags  802 ,  820 , and  830  may be authenticated to the reader via a challenge-response protocol, or the reader may be authenticated to the tag, or both parties might be mutually authenticated. Alternatively tags and readers might have previously set up cryptographically-secure communication channel(s) with reader  810  using, for example, a handshake process involving a secret key, a public key, a private key, and/or a shared key. The tags  802 ,  820 , and  830  may then exchange messages  806 / 812  with reader  810  over the cryptographically-secure communication channels. 
     In certain embodiments, tag  802 , reader  810 , and/or tags  820 / 830  may authenticate each other by determining whether they appear on an approval list. The approval list may include the tags and/or readers that are authorized (for example, by an authority) to send messages to other tags, receive messages from other tags, or perform message relaying. In the example described above, tag  802  may first determine whether reader  810  is on an approval list before sending message  806  to reader  810 . Similarly, reader  810  may determine whether tag  802  is on an approval list before accepting message  806 , and may also determine whether tags  820 / 830  are on an approval list before sending message  812 . Tags  820 / 830  in turn may determine whether tag  802  and/or reader  810  are on an approval list before executing and/or storing the contents of message  812 . 
     As described above, a message from a reader (e.g. message  812 ) that includes a message from a tag and intended for one or more destination tags may be identical to the message from the tag, may include additional information over the message from the tag, or may exclude information present in the message from the tag.  FIG. 9  depicts messages that a reader may send to tags, according to embodiments. A reader such as reader  810  may receive a tag message  908  (e.g., message  806 ) from a tag and intended for one or more destination tags. In one embodiment, the reader may construct a reader message to send to the destination tags by concatenating or inserting additional data to the received tag message  908 . The reader may do this because the tag message  908  is not a fully-formed command or is otherwise unsuitable for transmission to tags according to a protocol. In some embodiments, tag message  908  may specify the form of the reader message to be transmitted to the destination tags, and may include instructions and parameters necessary for the reader to assemble the reader message. If tag message  908  is a fully-formed command, in some situations the reader may encapsulate the tag message  908  in the payload of another reader command. The reader may also add additional data to identify the reader, to provide a timestamp that indicates when tag message  908  was received or when the reader message is sent to the destination tag(s), to cryptographically secure the message, or to provide any other suitable logging information. 
     In one embodiment, the reader may construct message  900  from tag message  908 , by concatenating or inserting an ID  902 , a command code  904 , optional other parameter(s)  906 , and optional check code  910 . ID  902  identifies one or more destination tags, and may include one or more of the destination tag identifiers described in  FIG. 8s . Command code  904  may be a command identifier or command code that identifies a particular command according to a protocol or a specification, as described above and in the Gen2 Specification. 
     In some embodiments, message  900  may include optional other parameters  906 . Parameters  906  may include data necessary to complete a reader command as identified by command code  904 , may include additional data as described above (e.g., reader identifier, timestamp, electronic signature, logging information, etc.), or may include any other suitable data. Message  900  also includes tag message  908 , which may be identical to the message received from the tag, stripped of redundant data (e.g., destination tag identifiers), and/or encrypted. Message  900  may further include optional check code  910 , which may be an error-checking or error-correction code (e.g., a cyclic redundancy check) computed over the entire message  900  or a portion of message  900 . 
     In other embodiments, the reader may send the tag message  908  substantially as-is, or with the inclusion of minimal necessary data. For example, tag message  908  may be a bitstream corresponding to a fully-formed command according to a protocol that is known or unknown to the reader. In this case the reader may forward tag message  908  to the destination tag(s) as message  950 , with little or no added information. In a situation where tag message  908  corresponds to a fully-formed command according to a protocol that is unknown to the reader, tag  802  may also provide transmission timing parameters, form tag message  908 , or otherwise cause the reader to be adjusted such that the reader can transmit tag message  908  according to the protocol. 
     Message  950  includes tag message  908 , and may also include optional ID  902 , other parameter(s)  906 , and/or check code  910 , as described above. As previously mentioned, in some embodiments the originating tag that sent tag message  908  may not know the particular identifier used to identify the destination tag(s) during reader-tag communication. For example, a reader may identify a target tag using a tag handle or random/pseudorandom number that is not necessarily known to the originating tag. In this situation, the reader may add the tag handle to the tag message, and may also calculate check code  910  based on the added tag handle. 
     Upon the successful transmission of a tag message to one or more destination tags, the destination tags and/or the reader may notify the originating tag accordingly.  FIG. 10  is a conceptual diagram of how RFID readers can be used to pass notifications between RFID tags, according to embodiments. Diagram  1000 , similar to diagram  800 , illustrates an example situation after reader  810  has sent message  812  to tag  820  and optionally tag  830 . After tags  820  and/or  830  receive message  812  and optionally confirms the completeness and/or authenticity of message  812 , tags  820 / 830  may send notifications  1002 / 1004  to reader  810  confirming receipt of message  812 . Notifications  852 / 854  may include information about the tags  820 / 830 , the reader  810 , the messages  806  or  812 , and/or other receipt information (e.g., timestamps), and may be electronically secured (e.g., electronically signed or encrypted). 
     Reader  810  may then send notification  1006  to tag  802  to notify tag  802  that tags  820 / 830  have received message  806 . If tags  820 / 830  determines that message  812  is incomplete or fails to authenticate message  812 , tags  820 / 830  may instead send notifications  1002 / 1004  to reader  810  indicating a message reception failure. In some embodiments, reader  810  may send notification  1006  to notify tag  802  that it has either successfully sent message  806  to tags  820 / 830  or has failed to send message  806  to tags  820 / 830 . In these embodiments, tags  820 / 830  may or may not also provide notifications  1002 / 1004  indicating successful or failed receipt of message  812 . 
     In some embodiments, notifications  1002 ,  1004 , and/or  1006  may be encrypted or include cryptographically-generated authentication codes to indicate that they originated from tags  820 ,  830 , and reader  810 , respectively. Tag  802  may then be able to verify that the notifications  1002 ,  1004 , and/or  1006  did in fact originate from tags  820 ,  830 , and/or reader  810 , respectively, and were not tampered with in transit. In some embodiments, tags  802 ,  820 ,  830  and reader  810  may instead (or also) communicate over cryptographically-secured channels, as described above in  FIG. 8 . 
     As described above, if reader  810  determines that communications cannot be established with any destination tags, it may enter a failure state. In some embodiments, reader  810  may also attempt to identify one or more relay tags or readers to forward the message  806  onward to its eventual destination. In these embodiments, reader  810  may select relay tags or readers based on one or more criteria. For example, reader  810  may select relay tags or readers based on their current or future proximity to the destination tag(s). 
     In some embodiments, tags suitable for relaying purposes may store information about the different readers that can read them. For example, if a tag is visible to two or more different readers, the tag may store identifiers for the two or more different readers. The tag may also store one or more time indicators associated with the reader identifiers to determine when it is visible to a particular reader. Similarly, a reader suitable for relaying purposes may store information about different tags (suitable for relaying or not) in its field-of-view. For example, if a reader has two or more different tags in its field-of-view, the reader may store identifiers for the tag(s), and may also store time indicator(s) associated with the tag identifiers to determine when the different tags are visible to the reader. In some embodiments, a reader may store a list of possible destination readers and tags and the potential relay pathways (via both tags and readers) to those destination readers or tags. By consulting the list, the reader can determine which particular relay pathways (including tags and/or readers) it can use to pass messages to particular destinations. 
     While in the embodiments above notifications are sent to the originating tag (i.e., tag  802 ) by the same reader (i.e., reader  810 ) used to send the message to the destination tags, this need not be the case. For example, tags  820  and  830  may send notifications  852  and  854  to different readers for eventual transmission to tag  802 . 
       FIG. 11  is a flowchart depicting a process  1100  for an RFID tag to send a message to another tag via an RFID reader. Process  1100  begins at step  1102 , where a tag (e.g., tag  802 ) determines that a message is to be sent to one or more other destination tags (e.g., tags  820 / 830 ). The message may be data that is stored at the tag, may be data generated by the tag as a result of the determination, or may be a fully-formed command that complies with one or more protocols, such as the Gen2 Specification. In some embodiments, the message may include or be associated with one or more destination tag identifiers that identify the destination tags, as described above. At step  1104 , the tag sends the message to a reader (e.g., reader  810 ), along with one or more destination tag identifiers. In some embodiments, the tag may provide an indication to the reader that the tag has a message to send to the destination tags. The tag may store the message in tag memory and backscatter the message to the reader in response to a reader command (e.g., a read command). 
     In some embodiments, at optional step  1106  the tag may receive one or more notifications originating from the destination tags and/or the reader, as described above. In some embodiments, the tag may take action based on the received notifications. For example, if the received notifications indicate that a destination tag was unable to confirm the completeness or authenticity of the message the tag may re-send the message at optional step  1108 . 
       FIG. 12  is a flowchart depicting a process  1200  for an RFID reader to relay a message between RFID tags. Process  1200  begins at step  1202 , where a reader (e.g., reader  810 ) may receive a message from an originating tag. At step  1204 , the reader may determine that the received message is for one or more other destination tags. For example, the received message may include a destination tag identifier as described above, or the reader may read a separate destination tag identifier from the tag. The destination tag identifier may identify a set or class of tags, or may only identify a single tag. 
     At step  1206 , the reader determines whether it can establish communications with the other tags. For example, the reader may determine whether the other tag(s) are visible and can successfully receive and respond to reader commands. In some embodiments, the reader may also determine whether the other tag(s) can be reached via one or more relay tags or readers, as described above. 
     If at step  1206  the reader determines that it cannot establish communications with the other tags, then at step  1208  the reader may enter a failure state. In the failure state, the reader may discard the message, transmit an error indication to the originating tag, store the error indication, and/or store identifiers for the originating tag and/or the other tags. The reader may immediately perform one or more of these actions, or wait for a particular, predetermined or dynamic time before performing these action(s). In the latter situation, the reader may periodically attempt to establish communications with the other tags. If successful, the reader may proceed to send the message as described below. 
     If at step  1206  the reader determines that it can establish communications with the other tags, then at optional step  1210  the reader may process the message for transmission. For example, the reader may include or exclude routing information, destination tag identifiers, originating tag identifiers, relaying reader identifiers, timestamps, or any other suitable information. In some embodiments, the reader may add or remove information from the message as described above in  FIG. 9 . 
     At step  1212  the reader may then send the message, processed or not, to the other tag(s). If only a subset of destination tags are visible to the reader, the reader may only send the message to the visible destination tags and store the message as described above until the other destination tags are visible. In some embodiments, the reader may identify one or more other tags or readers to use as relays for transmitting the message to the destination tag(s). For example, if the reader determines that none of the destination tags are visible and none will likely be visible within a particular time duration, then the reader may select one or more relay tags or readers to forward the message onward to the destination tags, as described above. 
     In some embodiments, at optional step  1214  the reader may generate a notification as described above in  FIG. 10 , and may send the notification to the originating tag and/or store the notification at the reader. The notification may originate from the reader and/or from the destination tag(s), and may indicate whether the message was successfully sent to the destination tag(s), whether the destination tag(s) successfully processed the message, whether the message is complete, and/or whether the destination tag(s) successfully authenticated the message. 
     The steps described in processes  1100  and  1200  are for illustration purposes only. Tag-to-tag message passing may be performed employing additional or fewer steps and in different orders using the principles described herein. Of course the order of the steps may be modified, some steps eliminated, or other steps added according to other embodiments. 
     The foregoing detailed description has set forth various embodiments of the devices and/or processes via the use of block diagrams and/or examples. Insofar as such block diagrams and/or examples contain one or more functions and/or aspects, it will be understood by those within the art that each function and/or aspect within such block diagrams or examples may be implemented, according to embodiments formed, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof. 
     The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is to be understood that this disclosure is not limited to particular methods, configurations, antennas, transmission lines, and the like, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. 
     With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. 
     It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should be interpreted to mean “at least one” or “one or more”) the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations.” without other modifiers, means at least two recitations, or two or more recitations). 
     Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.” 
     As will be understood by one skilled in the art, for any and all purposes, such as in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.