Abstract:
An anti-passback algorithm for an access control system is described. The anti-passback algorithm prevents the use of valid credentials to gain access to an access-controlled area by more than one person within a given period of time. The algorithm is capable of distinguishing between credentials intentionally presented to the access control system and credentials that are unintentionally read by the access control system. Certain variables may be set by the access control system manufacturer or a trusted individual to adapt the algorithm for applications.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/056,361, filed Sep. 26, 2014, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is generally directed to access control systems utilizing credentials encoded in a smart card, UHF tag, or other non-biometric device to control access to a protected resource, and more particularly to preventing circumvention of such access control systems. 
       BACKGROUND 
       [0003]    Access control systems that rely on the presentation, by an authorized user, of credentials encoded in a smart card, UHF tag, or other non-biometric device to control access can in some instances be defeated by passback schemes, wherein an individual uses a given set of credentials to obtain access and then “passes back” the same credentials for use by a second individual. For example, many companies provide a smart card to their employees, which card must be presented to a card reader to gain access to a secured facility. On any given occasion, once an employee uses the card to enter the secured facility, the employee could, while the access control mechanism is still open, pass the card to a non-employee, thus permitting the non-employee to gain unauthorized access to the secured facility. Access control systems used to restrict access to paying customers may be defeated in a similar manner, resulting in lost revenue for the controlled-access area&#39;s owner. 
       SUMMARY 
       [0004]    The present disclosure describes an anti-passback mechanism useful for limiting the use of passback schemes to gain unauthorized access to an access-controlled area, without the need for ground loops, optical sensors, or other equipment commonly used for ensuring the integrity of access control systems. Anti-passback mechanisms according to the present disclosure may be used in card readers or other access control systems to limit the number of times a particular identification device may be used to gain access within a given time period. 
         [0005]    In most Ultra-High Frequency (UHF) access control systems (e.g., systems utilizing UHF signals between approximately 300 MHz and 3 GHZ to exchange information), the UHF reader is an active device that simply reads tag data and reports the data to the access control panel. In the current market, UHF readers poll for an Electronic Product Code (EPC), and, for each poll attempt in which an EPC is read, report that data to the access control panel (either via a wired or wireless connection). If the polling is repetitive, data will be re-read and re-reported to the panel. If the polling frequency is high (e.g., greater than ten polls per second), the repeating data stream may overwhelm the panel and/or require the need for specialized algorithms in the panel to treat (e.g., implement special screening or masking of duplicate events) repeated data entry occurrences. Additionally, UHF readers will read and report UHF tags that are in the vicinity of the reader so long as an adequate return signal is received by the reader. Given that there is no distinction between intentional and stray tag presentations, UHF connected panels must utilize special algorithms to discern which tag is permitted. 
         [0006]    In the current disclosure, duplicate data reports are treated by implementing a firmware-based algorithm to ensure only one event is reported for one intended tag presentation. Using a series of counters and timers and a tag inventory tracking mechanism, an EPC from an intentional tag presentation can be discerned from EPCs from stray tag presentations. The intentionally presented EPC can then be reported to the access control panel once during the intended tag presentation. 
         [0007]    In some applications of UHF readers and tags, such as parking lot access control, it can be expected that the tag will be within the RF field for an extended period of time. The anti-passback mechanism ensures that a tag is only reported to the access control panel once (e.g., even though a tag is read multiple times by the UHF reader, the UHF reader only reports a single instance of a tag read event to the control panel). It is also expected that there may be, for example, a queue of vehicles waiting to enter a parking lot, and tags from vehicles other than the one closest to the reader will occasionally be read—potentially multiple times. The series of counters and timers and the tag inventory tracking mechanism associated with the anti-passback mechanism will prevent a tag that is occasionally read from having its data repeatedly sent to the panel. This is also true for other tags in the vicinity. For example, data from tags of other vehicles in the vicinity, such as vehicles in other traffic lanes, will not be repeatedly sent to the panel. 
         [0008]    In some embodiments, a reader for an access control system comprises a polling module configured to scan for and read an identification code from a data storage device; a counting module configured to calculate a read counter value, a poll counter value, and an anti-passback counter value each time the polling module conducts a scan; a database module configured to store the identification code read by the polling module, and further configured to store the read counter value, the poll counter value, and the anti-passback counter value; and a reporting module configured to transmit identification information obtained from the data storage device when the read counter value reaches a predetermined maximum read counter value. 
         [0009]    The counting module of the access control system reader may be configured to increment the read counter value when the polling module reads the identification code. The counting module may be further configured to decrement the poll counter value from an initial nonzero value each time the polling module scans for an identification code, but only if the anti-passback counter value is equal to zero. The counting module may be still further configured to decrement the anti-passback counter value each time the polling module scans for an identification code, but only if the anti-passback value is nonzero. 
         [0010]    The counting module may also be configured to set the anti-passback counter value to a non-zero predetermined maximum anti-passback counter value when the read counter value reaches the predetermined maximum read counter value. 
         [0011]    In embodiments of the access control system reader described above, at least one of the identification code and the identification information is an EPC. In further embodiments, at least one of the identification code and the identification information is an SIO. 
         [0012]    Also in embodiments of the access control system reader described above, the database module is configured to clear the identification code, the read counter value, the poll counter value, and the anti-passback counter value from storage when both the anti-passback counter value and the poll counter value are equal to zero. 
         [0013]    According to some embodiments of the present disclosure, an access control system, comprises a reader comprising an antenna configured to generate electromagnetic signals and facilitate information exchanges with one or more tags that are presented within a read range of the reader; a driver circuit configured to provide an excitation signal to the antenna at periodic intervals, thereby enabling the antenna to poll for one or more tags within the read range at periodic intervals; a processor configured to receive information regarding whether or not one or more tags responded to the poll initiated by the driver circuit; and a data structure that maintains, for each tag that responds to the poll, an identification code and an associated poll counter, read counter, and anti-passback counter, wherein the poll counter maintains a value corresponding to a number of times that the reader polls for tags after the tag first responds to the poll, wherein the read counter maintains a count for a number of times that the tag is read after the tag first responds to the poll, and wherein the anti-passback counter is assigned a value of zero after the tag first responds to the poll. 
         [0014]    In some embodiments of the access control system, the anti-passback counter is assigned a predetermined nonzero anti-passback counter value when the poll counter reaches a predetermined poll counter value and the read counter reaches a predetermined read counter value. Also in some embodiments, after the anti-passback counter is assigned the predetermined nonzero anti-passback counter value, the anti-passback counter decrements from the predetermined nonzero anti-passback counter value each time the reader polls for tags until the anti-passback counter reaches a value of zero, unless the reader reads the identification code before the anti-passback counter reaches a value of zero. Additionally, in some embodiments the anti-passback counter resets to the predetermined nonzero anti-passback counter value if the reader reads the identification code before the anti-passback counter reaches a value of zero. 
         [0015]    In certain embodiments of the access control system, the data structure clears the identification code and associated poll counter, read counter, and anti-passback counter from storage when the poll counter reaches a predetermined poll counter value and the anti-passback counter has a value of zero. 
         [0016]    In accordance with further embodiments of the present disclosure, a method for preventing circumvention of an access control device having an Ultra-High Frequency reader, comprises polling, at a standard interval and with the Ultra-High Frequency reader, for an identification code from a data storage device; incrementing a read counter every time the identification code is read during the polling step, until the read counter reaches a predetermined maximum value; transmitting, from the Ultra-High Frequency reader, identification information obtained from the data storage device when the read counter reaches the predetermined maximum value; decrementing one of a poll counter and an anti-passback counter after each standard interval; and resetting the read counter to zero or null after either the poll counter or the anti-passback counter is decremented to zero. 
         [0017]    In embodiments, the method further comprises setting the anti-passback counter to a predetermined anti-passback counter value after the read counter reaches the predetermined maximum value and the poll counter reaches a value of zero. 
         [0018]    Also in embodiments, the method further comprises resetting the anti-passback counter to the predetermined anti-passback counter value if the anti-passback counter has a non-zero value and the identification code is read during the polling step. 
         [0019]    In still further embodiments of the method, the identification information is at least one of an EPC and an SIO. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a block diagram of an access control system according to one embodiment of the present disclosure; 
           [0021]      FIG. 2  is a block diagram of a reader according to one embodiment of the present disclosure; 
           [0022]      FIG. 3  is a flowchart depicting the operation of an anti-passback mechanism according to one embodiment of the present disclosure; 
           [0023]      FIG. 4  is a flowchart depicting the operation of an anti-passback mechanism according to another embodiment of the present disclosure; and 
           [0024]      FIG. 5  is a flowchart depicting the operation of an anti-passback mechanism according to yet another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Referring initially to  FIG. 1 , an exemplary access control system  100  includes one or more credentials or tags  104  associated with authorized users of the resource protected by the access control system, and a reader  108  configured to grant access to the protected resource upon recognition of an authorized tag  104 . In this embodiment, reader  108  is configured with an anti-passback mechanism according to the present disclosure. In operation, the reader  108  of access control system  100  polls for data over wireless antenna  120  on a given time interval. The interval is preferably less than or equal to 40,000 microseconds, more preferably less than or equal to 20,000 microseconds, even more preferably less than or equal to 1500 microseconds. The data may include a message having as many as 65,525 bits, but preferably having only 210 bits. The data may also include as many as 255 leading bits, but preferably includes only 25 leading bits. The data may be any data useful for identification purposes, including an EPC or a Secure Identity Object™ (SIO) (HID Global Corp., Austin, Tex.). 
         [0026]    In an embodiment, the access control system reader  108  maintains a table or other data structure  124  in which tag identification data (ID data)  140  is stored along with an associated read counter  128 , poll counter  132 , and anti-passback (APB) counter  136 . In operation, the read counter  128  increments for each read of a given set of tag ID data  140 , from zero up to a predetermined maximum, where it stays until the tag ID data  140  is purged from the table or data structure  124 . Only when the read counter  128  increments to the predetermined maximum is the tag ID data  140  (or credentials associated with the tag ID data  140 ) sent to the access control panel for authentication. 
         [0027]    The poll counter  132  is assigned a predetermined nonzero value (e.g., an integer value between 3 and 100) after the first read of the tag ID data  140 , and thereafter decrements with each poll (whether or not the tag ID data  140  is read again during the poll) as long as the APB counter  136  is set to zero. The APB counter  136  is set to zero after the first read of the tag ID data  140  and remains zero until the read counter  128  hits its predetermined maximum. Once the predetermined maximum value for the read counter  128  is reached, the APB counter  136  is assigned a predetermined nonzero value, and thereafter decrements with each poll until reaching zero. If the reader  108  reads the tag ID data  140  again before the APB counter  136  reaches zero, however, then the APB counter  136  is reset to the predetermined maximum value. 
         [0028]    In an embodiment of the present disclosure, a reader  108  of an access control system  100  that uses EPCs for identification may maintain and update a table or data structure  124  as illustrated in Table 1 below. Each row of the column represents a single poll by the reader  108 . The first column, labeled “Row,” is added for purposes of convenience for this disclosure only. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Row 
                 EPC 
                 Read Counter 
                 Poll Counter 
                 APB Counter 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 null 
                 null 
                 null 
                 null 
               
               
                 2 
                 1234 
                 1 
                 4 
                 0 
               
               
                 3 
                 1234 
                 2 
                 3 
                 0 
               
               
                 4 
                 1234 
                 3 
                 2 
                 6 
               
               
                 5 
                 1234 
                 3 
                 2 
                 6 
               
               
                 6 
                 1234 
                 3 
                 2 
                 6 
               
               
                 7 
                 1234 
                 3 
                 2 
                 5 
               
               
                 8 
                 1234 
                 3 
                 2 
                 4 
               
               
                 9 
                 1234 
                 3 
                 2 
                 3 
               
               
                 10 
                 1234 
                 3 
                 2 
                 2 
               
               
                 11 
                 1234 
                 3 
                 2 
                 1 
               
               
                 12 
                 1234 
                 3 
                 2 
                 0 
               
               
                 13 
                 null 
                 null 
                 null 
                 null 
               
               
                 14 
                 4321 
                 1 
                 4 
                 0 
               
               
                 15 
                 4321 
                 2 
                 3 
                 0 
               
               
                 16 
                 4321 
                 2 
                 2 
                 0 
               
               
                 17 
                 4321 
                 2 
                 1 
                 0 
               
               
                 18 
                 4321 
                 2 
                 0 
                 0 
               
               
                 19 
                 null 
                 null 
                 null 
                 null 
               
               
                   
               
             
          
         
       
     
         [0029]    Rows 1 through 13 of Table 1 illustrate the anti-passback steps taken by reader  108  when an EPC tag is brought within read range of the reader  108 , remains in range for several polling cycles, and then leaves read range. For purposes of illustration, assume the EPC tag is being used by a driver in a car who is seeking access to a parking structure. 
         [0030]    Before the driver approaches the parking structure entrance, the entrance is clear (e.g., no other vehicles are in a read range of the reader  108  protecting the parking structure entrance). As a result, when the tag reader  108  at the parking structure entrance polls for data, it receives no response (row 1). The driver then pulls up next to the tag reader  108  and presents her EPC tag  104  within a read range of the reader  108 . When the reader  108  again polls for data, it reads tag  104 &#39;s identification data  140 , which in this case is EPC 1234. Upon reading EPC 1234 for the first time, the reader  108  stores the read EPC in its table or data structure  124 , sets the read counter  128  to 1, sets the poll counter  132  to a predetermined nonzero value (in this example, 4), and sets the APB counter  136  to 0 (row 2). As the driver continues to present her EPC tag  104 , the EPC tag  104  remains in the reader  108 &#39;s field, such that each time the reader  108  polls for data, its reads EPC 1234. As a result, the reader  108  increments the read counter  128  and decrements the poll counter  132  (rows 3 and 4). When the read counter  128  reaches the predetermined maximum (in this example, 3), the reader  108  sends EPC 1234 (or credentials associated therewith) to the access control panel (not shown) of access control system  100  for authentication, and sets the APB counter  136  to a predetermined value (in this example, 6) (row 4). 
         [0031]    As long as the driver presents the EPC tag  104  to the reader  108 , the reader  108 , upon polling, continues to read EPC 1234 Once the read counter  128  has reached the predetermined maximum, however, the read counter  128  stays at that maximum (rows 5 and 6). And, as long as the APB counter  136  has a non-zero value, the reader  108  does not decrement the poll counter  132  (rows 5 and 6). Also as long as the APB counter  136  has a non-zero value, the reader  108  resets the APB counter  136  to its maximum value (in this example, 6) every time its reads EPC 1234 (rows 5 and 6). 
         [0032]    Because polling happens on an interval periodic basis, typically measured in microseconds, the above events occur quickly with respect to user perception. When the driver is granted access to the parking structure, she enters, removing the EPC tag  104  from the reader  108 &#39;s scanning field (which is usually on the order of 5 to 20 meters from the reader  108 ). Thus, as the reader  108  continues polling, it no longer reads EPC 1234. As a result, the reader  108  decrements the APB counter  136  after each poll (rows 7 through 12). When the APB counter  136  reaches 0 (row 12), the reader  108  purges EPC 1234 from the table or data structure  124 , and sets each field to null (row 13). 
         [0033]    Another car with a second EPC tag  104  for the parking structure&#39;s access control system  100  may then pass close by the access control system reader  108 . If the reader  108 , upon polling, reads the second EPC tag  104 &#39;s identification data  140  (i.e. EPC 4321), it will create a table entry for EPC 4321 in table or data structure  124  and, as with EPC 1234, set the read counter  128  to 1, the poll counter  132  to 4, and the APB counter  136  to 0 (row 14). The second EPC tag  104  may stay within range for a second polling cycle, such that the read counter  128  increments and the poll counter  132  decrements (row 15). However, if the second EPC tag  104  passes out of range before the read counter  128  hits the predetermined maximum (in this example, 3), then the APB counter  136  will remain zero. As a result, the poll counter  132  will decrement after each polling cycle (rows 15-18). Once the poll counter  132  reaches zero, the reader  108  purges EPC 4321 from the table  124 , and sets each field to null (row 19). 
         [0034]    The predetermined maximum value of the read counter  128  may be selected by the access control system manufacturer or by a trusted individual, such as the access control system administrator. The value selected determines a minimum number of times the reader  108  must read the identification device  104  before sending the credentials associated with the device  104  to the access control panel for authentication. The lower the value, the greater the chance that credentials from a stray identification device  104  (i.e. one that does not belong to an individual seeking access) will be sent to the access control panel for authentication. The greater the value, the longer an individual seeking access must wait before his credentials are sent to the access control panel for authentication. These factors can be weighed depending upon the environment in which the reader  108  is situated. 
         [0035]    The predetermined value of the poll counter  132  may be selected by the access control system manufacturer or by a trusted individual, such as the access control system administrator. The value selected determines how long (i.e. how many polling cycles) the reader  108  will attempt to read identification data  140  from a given identification device  104  before purging the table of the identification data  140  in question. If the value is lower, then an identification device  104  must be read by the reader  108  with a proportionately higher consistency to have the credentials encoded therein sent to the access control panel. If the value is higher, then an identification device  104  must be read by the reader  108  with a proportionately lower consistency to have the credentials encoded therein sent to the access control panel. 
         [0036]    The predetermined value of the APB counter  136  may be selected by the access control system manufacturer or by a trusted individual, such as the access control system administrator. The value selected determines the length of time (i.e. how many polling cycles) that must pass before the identification device  104  can again be used to gain access through the access control system  100 . A higher value will prevent the identification device  104  from being used again to gain access for a longer period of time than will a lower value. 
         [0037]    Referring now to  FIG. 2 , an access control system reader  200  configured with an anti-passback mechanism according to embodiments of the present disclosure may comprise a polling module  204 , a counting module  208 , a database module  212 , and a reporting module  216 . Polling module  204  is configured to scan for and read an identification code from a data storage device (e.g. a credential or tag). Counting module  208  is configured to calculate a read counter value, a poll counter value, and an anti-passback counter value each time the polling module  204  conducts a scan. Database module  212  is configured to store the identification code read by the polling module  204 , and is further configured to store the read counter value, the poll counter value, and the anti-passback counter value calculated by counting module  208  each time the polling module  204  conducts a scan. Reporting module  216  is configured to transmit identification information obtained from the data storage device to an access control panel of the access control system when the read counter value reaches a predetermined maximum read counter value. 
         [0038]    In embodiments of the present disclosure, the counting module  208  of the reader  200  may be further configured to increment a read counter value when the polling module  204  reads an identification code from a credential or tag. In embodiments, the counting module  208  is further configured to decrement a poll counter value from an initial nonzero value each time the polling module  204  scans for an identification code, but only if the anti-passback counter value is equal to zero. Additionally, in embodiments, the counting module  208  is configured to decrement an anti-passback counter value each time the polling module  204  scans for an identification code, but only if the anti-passback value is nonzero. 
         [0039]    An embodiment of an anti-passback mechanism as implemented in an access control system having a card reader will be further described by reference to  FIG. 3 . Beginning at item  300 , the card reader scans, or polls, for identification data from an identification card. If it recognizes ID data from a card (node  304 ), then it queries the data table to determine whether the ID data is already stored in the table (node  336 ). If the ID data is not already stored in the table, then the reader adds the ID data to the table, sets the read counter to 1, sets the poll counter to the predetermined value, and sets the APB counter to zero (node  340 ). If the ID data is in the table, then the reader queries the table to determine whether the APB counter associated with the ID data is zero (node  332 ). If the APB counter is not zero, then the reader sets the APB counter to the maximum value and restarts the process (node  328 ). If the APB counter is zero, then the reader increments the read counter and decrements the poll counter (node  344 ). If, after decrementing the poll counter, the poll counter value is zero, then the reader purges the ID data from the table and restarts the process (nodes  348 ,  360 ). If the poll counter value is not zero, then the reader evaluates whether the read counter has reached the predetermined maximum or threshold value (nodes  348 ,  352 ). If the read counter has reached the predetermined maximum value, then the reader sends the ID data (or credentials associated therewith) to the access control panel for authorization (which, if successful, will cause the access control system to grant access to the holder of the authorized tag), and sets the APB counter to the predetermined maximum value before restarting the process (node  356 ). If the read counter has not reached the maximum value, then the reader restarts the process (node  352 ). 
         [0040]    If, upon scanning or polling, the card reader does not read ID data from an identification card, then the reader determines whether the APB counter for the most recently read ID data, if any, is zero (node  308 ). If the APB counter is not zero, then the reader decrements the APB counter and restarts the process (node  312 ). If the APB counter is zero, then the reader determines whether the poll counter is zero (node  316 ). If the poll counter is not zero, then the reader decrements the poll counter and restarts the process (node  320 ). If the poll counter is zero, then the reader purges the ID data from the table before restarting the process (node  324 ). 
         [0041]    Referring now to  FIG. 4 , in some embodiments, once the ID data or associated credentials are forwarded to the access control panel for authorization, the reader will read and authenticate an SIO (node  472 ). If authentication is successful, the reader will output the SIO and access will be granted (node  452 ). If authentication is unsuccessful, then access will not be granted (node  476 ). In some embodiments, the access control system will retry the read and authentication process one or more times, granting access once authentication is successful but denying access if authentication remains unsuccessful once a predetermined maximum number of tries has been reached. The predetermined maximum number of tries may be set by the access control system manufacturer in some embodiments, or by a trusted individual in other embodiments. If the tag receives a “partial” access grant at node  468  but the reader never successfully reads and authenticates an SIO at node  472 , then if the tag is found during the next inventory cycle  400  at node  404 , the reader will determine at node  432  whether the anti-passback counter for that tag is 0. If not, the reader will reset the anti-passback counter to the predetermined maximum value (node  428 ) and will determine that the tag is in a “partial” grant access state (node  444 ). At that point, the reader will again attempt to read and authenticate an SIO for the tag (node  456 ) and, if authentication is successful, the reader will output the SIO and access will be granted (node  488 ). If authentication is unsuccessful, then access will not be granted (node  480 ). 
         [0042]    As evident from comparing  FIGS. 4 and 5 , in some embodiments the reader will output an SIO in conjunction with granting access (node  452 ) and an EPC if access is not granted (node  492 ), while in other embodiments the reader will output both an EPC and an SIO in the process of granting access (nodes  572 ,  596 ), and will not output either an EPC or an SIO if access is not granted. In other embodiments, the reader outputs an EPC (but not an SIO) in conjunction with granting access (as shown in  FIG. 3 , node  364 ). 
         [0043]    In some embodiments, an access control system having anti-passback functionality as described herein includes a graphical user interface (whether on a reader associated with the access control system or on an administrative device of the access control system) containing various fields and/or controls such as those described in Table 2 below. 
         [0000]    
       
         
               
               
             
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Field 
                 Description 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Clock &amp; Data 
               
             
          
           
               
                 Bit Time (us) 
                 Range: 0-40000. Default: 1500 Microseconds 
               
               
                 Message Length 
                 Range: 0-65525. Default: 210 
               
               
                 Leading Bits 
                 Range: 0-255. Default: 25 
               
               
                 Grant Access Reads 
                 Defines the number of times a tag must be read by the reader before the 
               
               
                   
                 data is reported to the access control system panel. Range: 0-255. 
               
               
                   
                 Default: 3. 
               
               
                   
                 Note: This field works together with the Grant Access Polls setting. 
               
               
                 Grant Access Polls 
                 Defines the number of reader polls after the first successful read of a tag, 
               
               
                   
                 to achieve the required Grant Access Reads value defined above. 
               
               
                   
                 Range: 0-255. Default: 25. 
               
               
                   
                 Note: The data will not be reported to the access control system panel if 
               
               
                   
                 the reader cannot achieve the required number of reads within the 
               
               
                   
                 defined number of polls. The combination of Grant Access Reads and 
               
               
                   
                 Grant Access Polls ensures that a tag is in the reader&#39;s area of interest 
               
               
                   
                 and not a passing vehicle or an environmentally influenced reflection. 
               
               
                 Grant Access APB 
                 Set the Anti-Passback for n polling. Range: 0-255. Default: 50. 
               
               
                   
                 Note: The reader will only send data to the access control system panel 
               
               
                   
                 once for the period of time that the tag is read plus the time defined in 
               
               
                   
                 this field. A tag that leaves and re-enters the field while APB is still 
               
               
                   
                 active will not be reported and will reset the timer to the defined value. 
               
               
                   
                 All tags that have been read and are active in the APB timer will be 
               
               
                   
                 displayed in the Access Table on the Reader Information tab. 
               
               
                 Data Output 
                 SIO and EPC (Default) 
               
               
                   
                 SIO only 
               
               
                   
                 EPC only 
               
               
                 Apply Changes 
                 The Apply Changes button will apply any of the changes that have been 
               
               
                   
                 made to the configuration (fields will be green) and save them to the 
               
               
                   
                 configuration. Once saved the green fields should turn back to black. 
               
               
                   
               
             
          
         
       
     
         [0044]    Using the graphical user interface, the access control system manufacturer or a trusted individual (e.g. an administrator of the access control system) can set the maximum values for the read counter, the poll counter, and the APB counter, and can select whether to output an SIO only, and EPC only, or both an SIO and an EPC in conjunction with granting access to an authorized tag. The administrator can also configure the bit time (i.e. polling interval), the message length to be used for communications among components of the access control system (e.g. between the reader and the tag), and the number of leading bits that will be included in each message. These configuration settings allow the anti-passback mechanism of a given access control system to be optimized for various applications with which the access control system might be used. 
         [0045]    As can be seen from the above examples, the anti-passback mechanism disclosed herein is useful for preventing valid credentials from being used more than once in a given time period to gain access to an access controlled area. If an attempt is made to reuse the credentials after they have already been presented to the access control panel and before the APB counter has reached zero, then access is denied. The APB counter is then reset to its predetermined maximum value, thus extending the time during which the same credentials cannot be reused. 
         [0046]    Specific details were given in the description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments.