Abstract:
A collision resolution system and method for enhanced non-contact automated data collection. The data collection system generally includes a plurality of portable fare tags or token (“smart”) cards and a stationary target terminal. Each tag exchanges a series of messages with the target terminal to establish a communication link for the transfer of financial or other data. Concurrently, each tag and the target terminal process the messages to provide collision resolution, thereby ensuring that only one tag, at any instance, establishes the communication link with the target.

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 08/825,940 filed Apr. 1, 1997, now issued as U.S. Pat. No. 6,010,074, which is based upon and claims priority to United States Provisional Application No. 60/014,444, filed Apr. 1, 1996, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to non-contact automated data collection systems in which a portable token card or fare tag, commonly referred to as a “smart card,” is used with a stationary target terminal to exchange financial and other data in a mass-transit fare transaction system. More particularly, the invention relates to an improved data collection system having collision resolution features to prevent more than one fare tag from establishing a communication link with a single target. 
     2. Description of the Related Art 
     Smart card technology has been effectively used in mass-transit systems. In such an application, the smart card retains a fare value representative of funds available for use by its holder. As the smart card holder uses his card for transportation services, value is subtracted from the smart card in accordance with the applicable fare, or added in exchange for proper consideration. 
     The utilization of smart card technology in mass-transit applications reduces waste and increases efficiency by replacing paper ticket, mechanical coin, and token reading devices. Waste is reduced through the elimination of fare tickets. Efficiency is enhanced by the increased transaction speed and the ease of use of automated non-contact data collection systems for admittance to and departure from the transit system. A typical smart card transaction takes place within a 100 millisecond time period, roughly seven times faster than the time it takes to pass a paper ticket through a standard mechanical transport. For admittance or departure, the smart card need be merely presented in the proximity of the target provided in the stationary target terminal for the fare transaction to take place. Moreover, since data is transmitted via a radio frequency (“RF”) field, no physical contact between a smart card and target is required. The smart card may even be retained in a storage area, such as a purse or wallet, as long as it is presented in the proximity of the target. 
     The capabilities of the smart card system have also been exploited in multi-modal mass-transit systems. In such a system, a smart card is designed to integrate payment schemes for various forms of ground transportation and related services. For instance, smart card technology has been utilized as a common means of payment for local rail, bus, and parking services provided by a particular local transit system. One such system developed by the assignee of this application is disclosed in International Application Number PCT/US92/08892, entitled “Non-Contact Automatic Fare Collection System,” filed Oct. 19, 1992, and published May 13, 1993 as WO 93/09516. 
     A demonstration system generally applying the teachings of this application is currently operating in the Washington Metro Area Transit Authority (WMATA) mass-transit system for rail service, ground transportation (buses), and parking lots. In the WMATA system currently in use, fare data is transmitted between the stationary target terminal and a smart card, referred to herein as a fare tag, via a RF field. A standard target terminal consists of a target and a remotely located controlling computer. The target includes a modulator/demodulator and an antenna designed to transmit and receive, via an RF field with a carrier signal frequency of 2 MHz, a message modulated upon the carrier. During operation, the target emits a continuous RF field designed to evoke a response from a fare tag located in the general proximity of the target. Once a fare tag is brought in range, it is powered by the target&#39;s RF transmission and its responds with a message to the target. The target antenna receives the RF transmission from the responding fare tag, demodulates the message and conveys it to the device to which it is connected. The device determines if the message it has received is in the proper message format and, if so, it responds with a message. The fare tag receives the response and determines if the response is in the proper message format. If the response is in the proper message format, the fare tag responds by communicating with the controlling computer, and the appropriate fare data is read from the tag. The controlling computer then calculates the resulting fare value and that value is transferred and written to the tag&#39;s memory. 
     This demonstration system did not include any means or method for resolving message collisions because, as a result of its low signal power transmission characteristics, the target could only power one fare tag at a time. If two tags were presented in the proximity of the target, neither tag&#39;s internal circuitry would reach the required threshold voltage to generate and transmit a message. However, at higher power levels, message collisions can occur if more than one fare tag is presented in the proximity of a target and those fare tags are activated. Once activated, the tags simultaneously transmit information to the target. The simultaneously transmitted information collides and creates a signal which the target cannot recognize. Accordingly, there is a need for an effective collision resolution system that will prevent message collisions from multiple tags in higher power systems. 
     SUMMARY OF THE INVENTION 
     Applicant has met this need by providing a system and method of resolving message collisions in a non-contact automated data collection system having a target configured to receive message transmissions from an electronic fare tag and convey the message transmissions to a microcontroller. According to the method, messages originating from more than one fare card, if simultaneously conveyed to the microcontroller, are processed to determine that the simultaneously received messages do not conform to a valid message form. The microcontroller then generates and transmits an invalidity indication message indicating that the messages simultaneously conveyed to the microcontroller do not conform to a valid message form. The invalidity indication message effects each of the plurality of tags to transmit a new message similar to the first message after a preselected period of time, where the preselected period of time is determined for each of the tags individually. The first new message, which is transmitted by the tag with the shortest preselected time period, is conveyed to the microcontroller and processed. In response, the microcontroller generates and transmits a validity indication message indicating that the first new message to the microcontroller conforms to a valid message form. The tags receive the validity indication message and all but the tag which transmitted the first new message discontinue transmissions until they receive an appropriate message. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram representing an improved target constructed according to the principles of the invention in communication with a schematically shown fare tag; 
     FIG. 2 is a block diagram illustrating the microprocessor in the target of the invention; 
     FIG. 3 is a block diagram of a fare tag that may be used in the improved target of the invention; 
     FIG. 4 illustrates a normal communication situation in which a single fare tag is located in the proximity of a target; 
     FIG. 5 illustrates a situation in which two fare tags are in the proximity of, and attempting to communicate with, a single target; and 
     FIG. 6 is a flowchart illustrating the collision resolution communication protocol of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is described herein in an electronic fare collection system for rapid transit or toll applications. However, it should be apparent that the principles of invention may have broader applicability to other systems in which non-contact data exchange is utilized. 
     The improved target of the invention may be used advantageously in a fare collection system similar to that described in WO 93/09516 discussed above, the disclosure of which is incorporated by reference herein. Thus, only the features of the invention that differ from the system disclosed in WO 93/09516 or that are required to understand the collision resolution protocol of the invention are described in detail herein. 
     In the illustrated embodiment, the target  10  of the invention is shown schematically in FIG.  1 . The target  10  consists of an coil antenna  12 , a modulator/demodulator  14 , a microcontroller  16 , a ceramic resonator or quartz crystal  18 , and a serial interface port  20 . Microcontroller  16  has an internal clock (not shown) whose frequency is determined by the quartz crystal  18 . Additionally, a controlling computer  21  is coupled to the serial interface port  20 . Controlling computer  21  is preferably remotely positioned, although it may be located directly on the target itself. Any known, commercially available parts may be employed for these components, but the following parts are preferred: a P5876820 microcontroller manufactured by Dallas Semiconductor; a RS-232 interface manufactured by Linear Tech, a 14.7436 MHz quartz crystal manufactured by ECS Inc., and a coil antenna consisting of a 3 μHy, 1.0 A inductor. In a transit application, controlling computer  21  will likely be in the entrance and/or exit gates, and any vending machines. Both microcontroller  16  and controlling computer  21  have software residing therein to assist in the control of various operations of target  10 , including the administration of a communications protocol between the target  10  and a fare tag  50 . During operation, messages and data are transmitted to the target  10  through RF field  22 , and from the target  10  via RF field  24 . Preferably, the RF field is modulated at 115.2 KBaud. However, the RF field is not limited to such a modulation frequency and other frequencies may be used as will be appreciated by those of ordinary skill in the art. 
     Incoming RF fare tag messages and data  22  transmitted to the target  10  from the fare tag  50  are received by the coil antenna  12 . Once received, an incoming fare tag messages or data  22  is conveyed to modulator/demodulator  14  for demodulation, and the incoming fare tag message is conveyed to the microcontroller  16 , whereupon, depending on the message type, it is either processed or relayed through the serial port interface  20  to the controlling computer  22 . The microcontroller  16  processes data and messages in accordance with the application it has been configured to achieve. Likewise, controlling computer  21  is configured to process data and messages in accordance with its software. The operating speed of clock  18  is sufficient to drive microcontroller  16  to enable a data transmission rate of approximately 115.2 KBaud between target  10  and fare tag  50 . However, the invention is not limited to such a transmission rate, and other data transmission rates may also be utilized. 
     Outgoing RF tag messages and data  24  are transmitted from the target&#39;s coil antenna  12  to the fare tag  50 . These messages may be conveyed by the microcontroller  16  through the modulator/demodulator  14 . Or, messages and data may be conveyed by the controlling computer  21  through the serial port interface  20  to the microcontroller  16 , and from microcontroller  16  to the modulator/demodulator  14 . The message or data is modulated onto an RF carrier signal by modulator/demodulator  14  to form an outgoing target message. After modulation, the outgoing target message is conveyed to the coil antenna  12 , whereupon it is transmitted to fare tag  50  as an outgoing RF target message  24 . 
     The microcontroller  16  of target  10  is depicted in FIG.  2 . The microcontroller  16  consists of an execution and control unit  32  and a memory bank  34  containing software, the operation of which will be explained herein. Memory bank  34  may contain any memory know in the art such as RAM, ROM, etc., but EPROM is preferred. In operation, incoming messages  36  are conveyed to the execution and control unit  32 , whereupon the incoming messages  38  are processed. The execution and control unit  32  then proceeds under control of instructions read from memory bank  37 . 
     The fare tag  50  that may be used with the invention is shown in greater detail in FIG.  3 . Any fare tag and combination may be used. The fare tag  50  includes a coil antenna  52 , a modulator/demodulator  54 , and a controller  56 . Any known, commercially available devices may be employed, including a custom application specific integrated circuit. Similar to the operation of target  10 , messages are transmitted to the fare tag  50  via RF field  24  and from the fare tag  50  via RF field  22 . Preferably, the RF field is modulated at 115.2 KBaud, but other frequencies may be used as well. 
     Incoming RF fare tag messages  22  are transmitted to the fare tag  50  from the target  10  and are received by the coil antenna  52 . Once received, the incoming target messages  22  are conveyed to modulator/demodulator  54  for demodulation. The demodulated incoming target message is conveyed to the controller  56  and processed in accordance with the configuration of controller  56 . 
     Outgoing target messages  24  are transmitted from the fare tag&#39;s coil antenna  52  to the target  10 . These messages originate as data generated by the controller  56 , and are conveyed to the modulator/demodulator  54 . The message is modulated onto a RF carrier signal by modulator/demodulator  54  to form an outgoing target message. After modulation, the outgoing target message is conveyed to the coil antenna  52 , whereupon it is transmitted to target  10  as an outgoing RF target message  24 . 
     A message, either an incoming fare tag message  22  or an outgoing target message  24  transmitted between target  10  and fare tag  50 , is generated by either microcontroller  16 , controlling computer  21 , or controller  56  in accordance with the software or logic residing therein. A message is typically, but not necessarily, approximately 1 byte or greater in length, and may represent control information for controlling the operation of the target  10  or fare tag  50 , authentication information to ensure the authenticity of any incoming fare tag message  22  or an outgoing target message  24 , message identification information, or other information desired for the particular application in which the invention is employed. 
     Under normal conditions, illustrated by FIG. 4, a single fare tag  50  establishes communication with a single target  10  before fare data is transferred between target  10  and fare tag  50 . Data communications between the target  10  and the fare tag  50  will take place at 115.2 kilobits per second, half duplex. However, the invention is not limited to such a communications scheme, other data transmission rates and a full duplex mode of communication may be utilized. Before communication is established with a fare tag  50 , target  10  lies in a pulsing mode in which it periodically transmits, under the control of microcontroller  16 , a “wakeup” message along transmission path  64  generated by the controlling computer  21 . The “wakeup” message contains a sync or start of message, character, a message identification character, and a message digest containing a random number. When a fare tag  50  is presented in the proximity of target  10 , it receives the “wakeup” message transmitted by target  10 . The fare tag  50  responds, after a random wait period, with a “ping” message. The random wait period of the fare tag  50  is a random multiple, preferably, but not limited to, an integer from 0-7, of a “slot time.” The slot time is typically chosen to be greater than the round-trip communication time, from fare tag  50  and back to fare tag  50 , of the “ping” and “pong-valid” messages discussed below. A “ping” message may be two characters in length and contains a randomly generated number followed by its duplicate inverted. Although this specification is not limited to such a method, Applicant&#39;s preferred method of inverting a number is to change each bit of the number&#39;s binary representation from high to low or low to high, depending upon the corresponding original bit&#39;s representation. The microcontroller  16  verifies that the “ping” message contains a random number followed by its inverse, and generates a “pong-valid” message. The “pong-valid” message may be one character in length. The fare tag receives the “pong-valid” message, responds with an “imawake” message, and communication between the controlling computer  21  and fare tag  50  is established. Thereafter, fare data residing in the memory of tag  50  is read and transmitted to application  21 , which manipulates the fare data in accordance with its software and generates new fare data to be written onto the memory of tag  50 . The “imawake” message includes a synchronizing, or start of message, character, a message identification character, and a message digest consisting of random number followed by its duplicate inverted. 
     FIG. 5 illustrates a tag collision resolution condition. Collisions occur when two fare tags, such as fare tag  50 ( a ) and fare tag  50 ( b ), are presented in the proximity of a target  10  during the establishment of message communication, as discussed above, between the target  10  and either fare tag  50 ( a ) or fare tag  50 ( b ). To avoid such a situation, the microcontroller  16  is programmed to administer the collision resolution protocol of the invention. 
     The collision resolution protocol of the invention is described in conjunction with the flowchart shown in FIG. 6, starting at step  70 . At step  71 , before communications are established between a target  10  and a fare tag  50 , the microcontroller  16  controls target  10  to periodically generate and transmit a data “wakeup” message originated from controlling computer  21 , along transmission path  66  (shown in FIG.  5 ). Progressing to step  72 , if multiple fare tags  50 ( a ) and  50 ( b ) are in the proximity of target  10 , each fare tag  50 ( a ) and  30 ( b ) responds, as indicated at step  74 , after a random wait period, to the “wakeup” message  80  with a “ping” message transmitted along paths  68  and  69 , respectively, as shown in FIG.  5 . The random wait period of each fare tag,  50 ( a ) and  50 ( b ), is a random multiple, preferably, but not limited to, an integer from 0-7, of a “slot time.” The slot time is typically chosen to be greater than the round-trip communication time, from fare tag  50  and back to fare tag  50 , of the “ping” and “pong-valid” messages discussed above. The preferred slot time is 0.3 milliseconds. If, at step  76 , fare tags  50 ( a ) and  50 ( b ) generate equivalent random wait periods and collide by responding simultaneously and transmit a response in the form of a “ping” message along transmission paths  68  and  69 , the target  10  does not receive a coherent “ping” message, which as discussed above should consist of a random number followed by its inverse. The incoherent “ping” message resulting from the simultaneous reception of two “ping” messages, transmitted along paths  62  and  64 , is not recognized as valid by the microcontroller  16  of target  10 . In the case of non-recognition, microcontroller  16  controls the target  10  such that it transmits, along transmission path  66  a “pong-invalid” message in accordance with step  78 . A “pong invalid” message may be one character in length. The colliding fare tags  50 ( a ) and  50 ( b ) receive the “pong-invalid” message which causes, at step  80 , each fare tag to once again prepare to transmit a “ping” message along transmission paths  68  and  69 , respectively, after its randomly generated random wait period. If, at step  82 , the microcontroller  16  of target  10  receives a recognizable “ping” message it immediately replies with a “pong-valid” message, in accordance with step  84 , via transmission path  66 . Both fare tags  50 ( a ) and  50 ( b ) receive the “pong-valid” message. The fare tag  50 ( a ) or  50 ( b ) which has yet to transmit a “ping” message as a result of its randomly generated wait period, remains silent, as reflected by step  86 , after it receives the “pong-valid” message transmitted by target  10 . The fare tag  50 ( a ) or  50 ( b ) which did transmit the “ping” message receives the “pong-valid” message generated by microcontroller  16  and engages in communication with the controlling computer  21  by responding with an “imawake” message, illustrated by step  88 . Finally, at step  90 , if the controlling computer  21  does not recognize the “imawake” message transmitted by the chosen fare tag, collision is again assumed and the controlling computer  21  transmits a “wakeup” message to be transmitted by the target  10  periodically, under control of microcontroller  16 . 
     Table I is a table describing, in further detail, one preferred embodiment of the operating program of microcontroller  16  of target  10 . 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 A. 
                 Start Program 
               
             
          
           
               
                   
                 B. 
                 Initialize computer components and program variables 
               
               
                   
                   
                 (Establish communication mode) 
               
               
                   
                 C. 
                 Output “wakeup” command periodically 
               
               
                   
                 D. 
                 Input a responsive message (“ping”) 
               
             
          
           
               
                   
                 1. 
                 If responsive message (“ping”) is valid, 
               
               
                   
                   
                 output a validity indication (“pong-valid”); 
               
               
                   
                   
                 Else, 
               
             
          
           
               
                   
                 (i) 
                 output a non-validity indication (“pong-valid”) 
               
               
                   
                 (ii) 
                 return to input a responsive message 
               
             
          
           
               
                   
                 E. 
                 End 
               
               
                   
                   
               
             
          
         
       
     
     Table II is a table describing, in further detail, one preferred embodiment of the functional operation or operating program of the controller  56  of tag  50 . 
     
       
         
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
             
           
               
                 TABLE II 
               
               
                   
               
             
             
               
                 A. 
                 Start Function or Program 
               
               
                 B. 
                 Initialize computer components and program variables 
               
               
                   
                 (Establish communication mode) 
               
               
                 C. 
                 Input “wakeup” command 
               
               
                 D. 
                 Generate a random integer 
               
               
                 E. 
                 Wait for a time period equivalent to a preselected time period 
               
               
                   
                 multiplied by the random integer 
               
             
          
           
               
                   
                 1. 
                 if validity indication (“pong”) is input before wait time period 
               
               
                   
                   
                 has expired, remain silent; 
               
               
                   
                   
                 Else, 
               
             
          
           
               
                   
                 (i) 
                 output a responsive message (“ping”) 
               
               
                   
                 (ii) 
                 input an indication message 
               
             
          
           
               
                   
                 (a) 
                 if indication message a validity indication (“pong”) 
               
               
                   
                   
                 output “imawake”; 
               
               
                   
                 (b) 
                 if indication message a non-validity indication 
               
               
                   
                   
                 (“pong-invalid”) return fo generate a random integer 
               
               
                   
                   
                 Else, 
               
             
          
           
               
                   
                 (i) 
                 return to input “wakeup” command 
               
             
          
           
               
                 F. 
                 End 
               
               
                   
               
             
          
         
       
     
     Suitable code in any known programming language, or its functional equivalent, can be written by any artisan having ordinary skill in the art. The written code can be transferred to a read-only memory microchip to implement the logic carried out in the collision resolution communication protocol illustrated in Tables I and II and the flowchart of FIG. 6, as is also known by any artisan having ordinary skill in the art.