Patent Publication Number: US-2007096874-A1

Title: Distributed RFID interrogation system and method of operating the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      The present application claims the benefit of and priority to U.S. Provisional Application Ser. No. 60/622,562 entitled “Distributed RFID Interrogation System and Method,” filed on Oct. 26, 2004, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention is directed to system and methods of interrogating ‘passive’ radio-frequency identification (RFID) transponders, and more particularly to a distributed RFID interrogation system for interrogating RFID transponders located in spatially-partitioned areas.  
     BACKGROUND OF THE INVENTION  
      RFID technologies are widely used for automatic identification. A basic RFID system includes an RFID tag or transponder carrying identification data and an RFID interrogation device or reader that reads and/or writes the identification data. An RFID tag typically includes a microchip for data storage and processing, and a coupling element, such as an antenna coil, for communication. Tags may be classified as active or passive. Active tags have built-in power sources while passive tags are powered by radio waves received from the reader and thus cannot initiate any communications. Instead, passive RFID transponders sense the presence of an interrogating signal, and respond to it by varying their reflection coefficient as a function of time. An RFID reader operates by writing data into the tags or interrogating the tags for their data through a radio-frequency (RF) interface. An RFID reader for interrogating passive tags is typically designed to receive from the tags a backscattered portion of a signal transmitted from the reader and to extract tag data from received signal.  
      RFID technology has found wide applications including those in retailing, where it is often of interest to use RFID tags and interrogation devices to monitor products offered to the consumer. In such an application, the RFID system is intended to replace the laborious and expensive manual count of the number of units for each product type remaining on shelves or in a display area.  
      RFID transponders may operate in various frequency bands, such as 13.56 MHz (‘HF’), approximately 860-960 MHz (UHF), or 2.4-2.5 GHz (microwave). Low-frequency transponders typically employ relatively expensive multi-turn inductive coils to extract power from the radio-frequency signals from the interrogating device. Because such coils are expensive and relatively difficult to construct, high-frequency transponders are relatively larger and more expensive compared to UHF or microwave transponders. Thus, from the viewpoint of reduced size and cost, transponders operating at higher frequencies are preferred for identification of consumer products.  
      On the other hand, conventional UHF and microwave interrogation devices are standalone devices, typically costing up to several thousand dollars each, so that it is impractical to use more than a few readers to monitor a retailing area. In consequence, high-power readers with a large coverage area must be employed for higher frequency transponders. The size of the coverage area for a UHF or microwave reader can be as large as several meters in diameter, depending on the radiated power, type of transponders, and environment. The exact location of the responding transponders is then difficult to ascertain, since they may be anywhere within the coverage area of the reader. Furthermore, particularly when metallic shelving is employed to contain items for display, the metallic constituents of the shelving may scatter the impinging radiation from the readers, causing shadowed regions where transponders do not respond to the reader. Absorption and scattering from the packaging and contents of the stocked items themselves may also interfere with propagation of the signals from the reader and the responding signals from the transponders. To ensure good coverage with only a few readers, the readers must be physically moved through the region to be monitored by stocking clerks. As a result, the cost of monitoring is increased, and the consistency of coverage is limited by the skill and attention of the monitoring personnel.  
      What is needed is a system for interrogating UHF and/or microwave RFID tags in retail display areas and other similar environments that provides consistent coverage without manual intervention.  
     BRIEF SUMMARY OF THE INVENTION  
      The embodiments of the present invention provide a system and method for interrogating passive radio frequency identification (RFID) transponders located in compartmentalized areas such as on shelves or in other spatially-partitioned storage areas. The system includes a controller and a plurality of minimal function readers coupled to the controller via a network-interface. The controller is configured to address a subset of at least one reader at a time to interrogate the RFID transponders located in at least one of the compartmentalized areas. The readers are each given a physical location and a unique address, through which the controller would know the locations of the RFID transponders being interrogated.  
      In one embodiment, the plurality of minimal-function readers are a number of simplified, highly-miniaturized, low-power readers dispersed along the length of a substantially-conventional networking cable. Each reader receives its power over the cable, and simultaneously employs the cable to communicate with the controller, which may be a relatively-sophisticated controlling device, and optionally with the other readers on the cable. Low reader power ensures that the range of coverage is small, so that a transponder responding to a particular reader must be located very close to the reader. The total coverage area is large because many readers can be conveniently and precisely installed in a single cable placement. Conventional networking technology can be used to keep total system cost low. The system may be used to interrogate UHF and/or microwave RFID tags in retail display areas and similar environments, in which the system can provide approximate location of the counted items and consistent coverage even in the presence of metallic shelving without manual intervention and with low installation and operating costs.  
      The embodiments of the present invention also provide a method for interrogating RFID tags in compartmentalized areas. The method comprises the steps of placing a plurality of RFID readers coupled to a controller through a network interface in the compartmentalized areas, addressing a first subset of at least one of the plurality of RFID readers from the controller to interrogate RFID tags located in a first subset of at least one of the compartmentalized areas during a first time period, and addressing a second subset of at least one of the plurality of RFID readers from the controller to interrogate RFID tags located in a second subset of at least one of the compartmentalized areas during a second time period after the first time period.  
      In one embodiment, the RFID readers are placed in the compartmentalized areas such that at least one of the plurality of RFID readers is situated in each area, the plurality of RFID readers are coupled to the controller via a network-compatible cable, and the controller addresses each subset of the readers by sending a reader interface packet down the network-compatible cable, the reader interface packet including at least one unique address corresponding to a subset of RFID readers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a distributed RFID interrogation system according to one embodiment of the inventive system.  
       FIG. 2  is a block diagram of an exemplary application scenario for the distributed RFID interrogation system according to one embodiment of the inventive system.  
       FIG. 3  is a block diagram illustrating a network interface in the distributed RFID interrogation system according to one embodiment of the present invention.  
       FIG. 4  is a block diagram of an exemplary encapsulated reader interface packet used by the distributed RFID interrogation system according to one embodiment of the present invention.  
       FIG. 5  is a block diagram of a minimal-function reader in the distributed RFID interrogation system according to one embodiment of the present invention.  
       FIG. 6  is a block diagram of an RFID radio in the minimal-function reader according to one embodiment of the present invention.  
       FIG. 7  is a block diagram of an exemplary interface for the RFID radio according to one embodiment of the present invention.  
       FIG. 8  is a block diagram of a portion of a network-compatible cable in the distributed RFID interrogation system according to one embodiment of the present invention.  
       FIG. 9  is a block diagram of a portion of a network-compatible cable in the distributed RFID interrogation system according to an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The embodiments of the present invention provide a distributed RFID interrogation system. As shown in  FIG. 1 , an overall block diagram of a distributed RFID system  100  according to one embodiment of the present invention comprises a plurality of networked minimal-function readers (MFR)  110  and a controller  120  in communication with the MFRs via a network interface. In one embodiment, the MFRs  110  are distributed along at least one network-compatible cable (cable)  130 . Each MFR  110  is configured to transmit a continuous or amplitude-modulated signal and to detect backscattered signals from nearby tags. The controller  120  is a relatively sophisticated controller and is configured to manage higher-level RFID communication protocols and functions, such as inventory functions, access control, collision resolution, etc., and optionally some physical layer functions such as instantaneous power of transmitted signals and interpretation of the received amplitude. The controller  120  may use a conventional network interface, such as the Institute of Electrical and Electronics Engineers (IEEE) 802.3 standard “Ethernet” network interface, to communicate with the minimal-function readers  110  along the cable  130 . The network-compatible cable  130  is equipped with an interface connector at one or both ends that is compatible with the appropriate standard network interface, such as the well-known ‘Category 5 Ethernet’ RJ45 interface. The cable  130  can be formed using one or more of conventional network cables that are slightly modified, as described in more detail below, to incorporate and support operation of the minimal-function readers  110 . In one embodiment, each MFR  110  is associated with a unique address in the network, such as a unique IEEE-802.3-compliant medium access control (MAC) address.  
      The distributed RFID system (system)  100  is useful for interrogating tags located in compartmentalized areas. One application scenario for system  100  is in inventory control, as depicted in  FIG. 2 , where the network-compatible cable  130  is installed on a retail display shelf  210  having a plurality of partitioned areas  220  separated by shelf partitions  230  made of, for example, a metallic material. At least some of the partitioned areas  220  are occupied by stocked merchandise or items  222  each having a RFID tag  224  attached thereto. In one embodiment, the cable placement and MFR locations on the cable are adjusted to ensure that each partitioned area  220  is completely visible from at least one MFR  110 . The effect of the shelf partitions  230  on the effective coverage area can be found by experiment, simulation, or past experience. If the shelf partitions  230  do not act to limit the effective coverage area of the minimal-function readers  110 , two or more neighboring partitioned areas  220  can share one MFR  110 . But the condition that each partitioned area  220  be covered may simply be satisfied by placing at least one minimal-function reader  110  in each partitioned area  220 , especially when shelf partitions  230  are made of metal. Optionally, more than one readers  110  may be placed in each partitioned region  220 , in order to provide redundant reading capability, ensure thorough coverage of the addressed area, and/or ensure coverage in the event of failure of a particular reader. The cable  130  and the MFRs  110  should also be placed on the shelf  210  in a convenient fashion so as to minimize interference with normal shelf functions, such as display, consumer access, and restocking.  
      In one embodiment of the present invention, the controller  120  addresses one or more MFRs  110 , causing them to transmit appropriate interrogation signals that energize and activate the tags  224  on the items within the coverage areas of the at least one MFR. The interrogation signals may be combinations of continuous-wave (CW) and varying-amplitude signals, such as Amplitude Shift Keying (ASK) or non-constant-envelope Binary Phase Shift Keying (BPSK) signals. In inventory applications of this type, updates may only need to be accomplished on an occasional or periodic basis, such as once per hour or once per day. Thus there is no need for all MFRs  110  on a single cable  130  to operate simultaneously. In general, only a subset of as few as a single MFR  110  should be in operation at any given time. The number of MFRs  110  used simultaneously may be chosen through a tradeoff of the required DC power, data capacity on the network cable  130 , and desired frequency for updating the information. Since each MFR  110  has a unique physical address within the network, the controller  120  can always establish the identity of the MFR  110  providing information on any given tag, and thus through knowledge of the physical location of that reader  110  can infer to good precision the location of the tagged item. The physical location of each MFR  110  can be manually programmed into the controller  120 . Alternatively, in some circumstances, one or more tags  224  with known ID&#39;s in known locations can be temporarily or permanently placed in close proximity to each MFR  110  to help the controller locate the MFR. By examining which tags  224  can be seen by each reader, this approach can also be used to establish the approximate readable volume associated with each MFR  110 .  
      An exemplary communication architecture for the distributed reader system  100  using, for example, the Open Systems Interconnect (OSI) protocol is shown in  FIG. 3 . OSI is a well-known industry-wide protocol standard consisting of seven well-defined layers. In the simplest implementation where the MFRs  110  are distributed along the cable  130  so that there is no need for a NETWORK layer or full network capability for system  100 , only the first two layers (the PHYSICAL and LINK layers) of the OSI hierarchy are supported in system  100 . As shown in  FIG. 3 , an OSI hierarchy  310  within the controller  120  includes a physical layer  311 , a link layer  312  having a logic link control part  315  and a medium access control part  313  for communicating with the physical layer  311 , a custom reader interface driver  317  for communicating with the logic link control part  315 , and user applications  319  for communicating with the reader interface driver  317  via an Application Program Interface. In each MFR  110 , an OSI hierarchy  320  includes an RFID radio  321 , a custom RFID radio driver  323  for communicating with the RFID radio, a link layer  326  having a medium access control part  327  and a logic link control part  325  for communicating with the custom RFID radio driver  323 , and a physical layer  329  for communicating with the medium access control part  327 . The physical layer  311  in the controller  120  and the physical layer  329  in the MFR  110  communicate with each other via the cable  130 .  
      In one exemplary embodiment, the IEEE 802.3 standard is used as a communications standard. In this example, the custom reader interface driver  317  in the controller  120  generates specialized Reader Interface (RIN) packets, each being associated a unique IEEE-802.3-compliant MAC address that identifies a particular minimal-function reader  110  to receive the packet. Other layer-2 technologies could be used, with appropriate addressing substituted as needed. The packets may be encapsulated in standard headers, such as IEEE 802.3-compliant preamble and header, and are transmitted over the network-compatible cable  130 . The relevant minimal-function reader  110  to which a packet is addressed recognizes the packet&#39;s unique MAC address and extracts the RIN packet from the 802.3 header. The custom driver  323  in the MFR  110  interprets the RIN packet and controls the RFID radio  321 . Data from the minimal function reader  110  undergoes the reverse process of encapsulation and decapsulation in the controller  120  to provide data to the custom reader interface driver  317  and hence to the user applications  319 .  
      Alternative implementations can employ conventional IEEE 802.3 layer-2 switching to allow a single controller to communicate with a number of physical collision domains, but such implementations must contend with switch buffer latency. Potential delays in packet delivery in larger systems would necessitate longer buffering times and thus more complex function support in the minimal-function reader units  110 , with consequent increase in cost. A single reader controller  130  could in principle control a number of physically-separated minimal-function-readers  110  by employing a conventional networked architecture such as TCP/IP (Transmission Control Protocol/Internet Protocol), but the potentially long and possibly variable latencies in a networked environment would tend to force the individual reader units  110  to become more intelligent and thus more expensive.  
      An exemplary architecture for a RIN packet  400  is shown in  FIG. 4 . The packet  400  is encapsulated by a header such as an IEEE 802.3 header  410 , and includes some or all of a RIN header  420 , a command name  430 , one or more appropriate data fields  440 , and possibly some additional post-fixed error check data or flags  450 . Within the RIN packet  400 , the RIN header  420  describes a protocol version, type of packet, and other control data, the command name  430  describes the requisite action, and the data fields  440  are appended as needed. The error check  450  may be appended on the RIN packet, or the encapsulating layer  410  may be relied upon for reliable packet delivery. Using a register-based control system as described below, a small number of commands suffices to provide considerable flexibility in the operation of the minimal-function reader  110 .  
      A simplified block diagram of an exemplary minimal-function reader  110  is shown in  FIG. 5 . The MFR  110  as shown in  FIG. 5  allows the system  100  to be implemented without incurring excessive costs, but more elaborate RFID readers could also be used as the MFRs  110  when appropriate. As shown in  FIG. 5 , the reader  110  includes an RFID radio  510  connected to one or more antennas  512 , an Ethernet interface  520  coupled to the cable  130 , an interface module  530  coupled between the RFID radio  510  and the Ethernet interface  520 , a crystal oscillator  540  coupled to the RFID radio  510 , the Ethernet interface  520 , and the interface module  530 , and a DC power module  550  coupled to each of the above components in the reader  110  and to the cable  130 .  
      The RFID radio  510  is configured to generate continuous wave or amplitude-modulated output signals and to extract information from signals backscattered from the tags  224  by detecting variations therein. The RFID radio  110  includes a frequency synthesizer (not shown) locked to a low-frequency reference signal provided by the crystal oscillator  540 , and modules for facilitating instantaneous output RF power control, transmit amplification, and I and Q down-conversion of received signals. The interface module  530  is configured to instruct the RFID radio  510  as to the frequency and instantaneous output power desired, and to receive the I and Q baseband outputs. The interface module  530  may be configured with only the capability of sending sampled values of the I and Q outputs to the controller  120 , but this is very inefficient, and in general the interface module  530  would incorporate added capability to save a sequence of sampled values and send them in a single longer packet to the controller. The Ethernet interface  520  is a standard commercial single-chip Ethernet controller, capable of sending and receiving packets using the network-compatible cable  130 . The Ethernet interface  520  is also configured to de-encapsulate packets from the controller  120  and provide them over a bus to the interface chip  530 , and to encapsulate responses therefrom.  
      In one embodiment, cable  130  includes a plurality of twisted pairs including, for example, a transmit (TX) twisted pair  132 , a receive (RX) twisted pair  134 , a DC power twisted pair  136 , etc. The DC power twisted pair  136  of the cable  120  is used to provide DC power to the minimal-function readers  110  through the DC power module  550 . While standard approaches such as those according to the IEEE 802.3af standard might be used, a custom powering arrangement may also be used to avoid complex power regulation circuitry in the minimal-function readers  110 . It is also possible to include comparators and decoding logic to return digital symbols from the interface module  530  to the Ethernet module  520  rather than digitized analog data, or to incorporate a simple Peripheral Interface Controller (PIC) microprocessor and to implement full RFID communications protocols within the reader  110 , so that it becomes a conventional reader with a simplified interface.  
      In a conventional switched Ethernet network, the transmit (TX) pair for one transceiver is the receive (RX) pair for the transceiver on the other end of the cable. Such an arrangement can be employed for the distributed reader system  100 , but a simpler alternative arrangement is to revert to the true common-medium approach of the original Ethernet networks and use the same TX twisted pair  132  to transmit to all distributed readers, and the corresponding RX pair  134  to receive from all distributed readers. Since the state of shelves changes slowly, in practice only one or a few readers will be operational at any given time, so the probability of packet collisions is low and can be dealt with at the application level rather than at the physical layer.  
      The antennas may be any conventional small antennas. For example, a tip-loaded dipole or folded dipole may be incorporated into the reader package, and a patch antenna or patch array may alternatively or additional used. Various surface-mount antennas are also available, and provide some size reduction at the cost of decreased efficiency. A single transmit/receive antenna may be employed, using a directional coupler or circulator to extract the reflected signal from the tags. Alternatively, a separate decoupled receive antenna can be employed for the reverse link from tag to reader. In another alternative implementation two separate TX/RX antennas can be provided to ensure diversity—that is, if a tag is in a local null of the field excited by one antenna, it is unlikely to be in the null of the second antenna given that the two antennas are spaced at least a quarter of a wavelength apart. The reader can be provided with a switch arrangement to alternately employ the main or diversity antenna, in order to ensure reliable read of all tags present on the shelf.  
       FIG. 6  is a simplified block diagram of the RFID reader  510  according to one embodiment of the present invention. As shown in  FIG. 6 , reader  510  includes a frequency synthesizer  604  configured to generate a continuous wave (CW) signal with reference to the clock signal from the crystal oscillator  540 , and a local oscillator (LO) buffer amplifier  606  coupled to synthesizer  604  and configured to amplify the CW signal. LO buffer amplifier  606  also protects the synthesizer from disturbances created from other parts of reader  510 . LO buffer amplifier  606  may be implemented using conventional means.  
      Reader  510  further includes a transmit (TX) chain  610  configured to form and transmit TX signals for interrogating the tags  224 , and a receive (RX) chain  630  configured to receive backscattered RF signals from tags  224 , and to generate I and Q output signals. TX chain  610  includes an output power control module  612 , a modulator  614 , and an amplifier  616 . RX chain  630  includes a splitter  632 , a 90° hybrid  634 , an I-branch  640 , and a Q-branch  650 .  
      Reader  510  further includes a splitter  608  coupled between LO buffer amplifier  606  and TX/RX chains  610  and  630  and configured to split the CW signal from LO buffer amplifier  606  into a TX CW signal for the TX chain and a RX LO signal for the RX chain. When more than one antenna can be used by reader  510 , reader  510  may also include an antenna select module  622  configured to select one of a plurality of antenna  624  for broadcasting the TX signal or receiving the RF signal. Reader  510  further includes a directional coupler  620  coupled between antenna select module  622  and TX/RX chains  610  and  630 . Directional coupler  620  is configured to pass the TX signal from the TX chain  610  to at least one antenna through antenna select module  622  and to couple the RF signals by the antenna to the RX chain  630 .  
      A simple and flexible means of controlling of the RFID radio  510  is through the use of a number of data registers, which can be included in a register block  660  in the RFID radio  510 . Bits in the registers may be assigned to control and/or reflect the state of various functions in the radio  510 , such as power management, digital-analog converters (DACs), attenuators, clock buffers, comparators, local oscillator (LO) status, modulation, anti-alias and baseband filtering, pulse shaping, frequency and amplitude locking, and phase-locked loop (PLL) status. In addition, values of ranges of bits within the registers may be used to adjust parameters relevant to the operation of the reader  510 , such as the modulus of dividers in the synthesizer  604 , values of taps used in finite-impulse-response filters also in the synthesizer  604 , or other variable parameters. Registers in the register block  660  may be written to and/or read from by the controller  120  via a serial port interface (SPI)  662 , as discussed below.  
      Referring still to  FIG. 6 , in one embodiment of the present invention, in TX chain  610 , output power control module  612  is configured to adjust the power level of the TX CW signal according to corresponding bits stored in the register block  660 , and modulator  614  is configured to form the TX signal by modulating and optionally amplifying the modulated TX CW signal.  
      In one embodiment of the present invention, RX chain  630  includes I-branch  640  configured to generate at least one in-phase signal I-SIG and/or digitized in-phase signal I-DIG based on a RF signal received from a tag  224 , and Q-branch  650  configured to generate at least one quadrature signal Q-SIG and/or digitized quadrature signal Q_DIG based on the RF signal received from the tag. RX chain  630  further includes splitter  632  configured to receive the RF signal from the directional coupler  630  and to split the received RF signal into two RF_receive signals going separately into the I-branch  640  and the Q-branch  650 . RX chain  630  further includes a 90° (quarter wavelength) hybrid  634  configured to receive the RX LO signal from the splitter  608  and to split the RX LO signal into a first LO signal in-phase with the RX LO signal and going into the I-branch  640 , and a second LO signal with a 90° phase shift from the RX LO signal and going into the Q-branch  650 .  
      I-branch  640  and Q-branch  650  function to demodulate ASK or EPCglobal class-1 signals from the tags and may include conventional heterodyne or super-heterodyne topology for I/Q demodulators. As shown in  FIG. 6 , I-branch  640  includes a mixer  641  excited by the first LO signal and configured to convert the RF_receive signal into a first baseband signal. The RF_receive signal may be filtered by a preselection filter (not shown), amplified by a low-noise amplifier (not shown) and then further filtered by a second preselection filter (not shown) before being applied to mixer  641 . I_branch  640  further includes a first low-pass filter  642  coupled to mixer  641  and configured to filter out the LO signal component in the downconverted signal, at least one baseband gain amplifier  644  coupled to low-pass filter  642 , and a second low-pass filter  646  coupled to baseband gain amplifier(s)  646  and configured to filter out noises caused by the baseband gain amplifier(s)  644 . The output of filter  646  is the in-phase signal I_SIG. I-branch  640  may further include a comparator functioning as an analog to digital (A/D) converter  648  configured to generate a digital in-phase signal I_DIG from the I_SIG signal.  
      Likewise, Q-branch  650  includes a mixer  651  excited by the second LO signal and configured to convert the RF_receive signal into a second baseband signal. As in the I-branch, the RF_receive signal may be filtered by a preselection filter, amplified by a low-noise amplifier and then further filtered by a second preselectionfilter before being applied to mixer  651 . Q_branch  650  further includes a first low-pass filter  652  coupled to the mixer and configured to filter out the LO signal component in the second baseband signal, at least one baseband gain amplifier  654  coupled to low-pass filter  652 , and a second low-pass filter  656  coupled to baseband gain amplifier(s)  652  and configured to filter out noises caused by the baseband gain amplifier(s). The output of filter  656  is the quadrature signal Q_SIG. Q-branch may further include a comparator functioning as an A/D converter  658  configured to convert the Q_SIG signal into a digital quadrature signal Q_DIG.  
      A conceptual input/output interface with the radio  510  is shown in  FIG. 7 . A convenient way to pass values into and out of the registers in reader  520  is to use a three-line emulation serial port interface (SPI)  662 , though other means, such as a parallel bus interface, could be employed. Some key inputs, such as power-on and transmit-data-enable functions, and an input for transmitting digital data itself, must be attended to without delays contingent on writing to and reading from the register and are thus provided with dedicated digital inputs. The output of a homodyne receiver in the reader  510 , which output is representative of the scattered signal from the tag, is divided into in-phase (I) and quadrature (Q) signal values, with their phases being defined with respect to the local oscillator. The output of the reader  510  can be the actual instantaneous analog I and Q voltages, which would then be sampled and digitized locally in the interface module  530 . Digital sampling could alternatively take place within the RFID radio  510 , as discussed above. Or, the I and Q outputs could drive comparators to provide a simple single-valued digital output, as in the case of a digitized voltage output with 1-bit resolution.  
      From the point of view of the custom reader interface driver  317  in the controller  120 , all bits within the register block  660  in the RFID radio  510  in any minimal-function reader  110  can be adjusted by a WRITE command generally with the following syntax:  
      WRITE(location, length, values) associated with the MAC address of the relevant minimal-function reader  110 . Similarly, all register values can be queried using generally the following READ command:  
      READ(location, length, values)  
      which is again associated with the relevant MAC address.  
      Transmit digital data could be sent one bit per packet, but it is much more efficient to send transmit data in RIN packets of a form such as:  
      TXDATA(length, values, timing information)  
      Data values from TXDATA packets are buffered by the interface module  530  and delivered at a requested rate to the RFID radio  510 . Similarly, the interface module  530  optimally should buffer I and Q data from the radio  510 , and then send packets of a form such as:  
      RXDATA(format, length, values, timing information)  
      to communicate the received signals back to the controller  120 .  
      In one embodiment, each of the minimal-function readers  110  is incorporated into the network-compatible cable  130  during manufacture. Alternatively, the network-compatible cable  130  may be constructed of connectable sections  810  each having connectors  812  at one or both ends so that the cable sections  810  can be connected by adaptors  820  each bearing receptacles  822  for connecting with the cable sections  810 , as shown in  FIG. 8 . An adaptor may either incorporate a minimal-function reader  110 , as shown in  FIG. 8 , or provide an additional receptacle  824  allowing a relatively remotely-situated reader  510  to be connected thereto through an additional network-compatible cable  830  having a variable length, as shown in  FIG. 9 .  
      This invention has been described in terms of a number of embodiments, but this description is not meant to limit the scope of the invention. Numerous variations will be apparent to those skilled in the art, without departing from the spirit and scope of the invention disclosed herein.