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Passive proximity cards contain:
Parity bits
Facility or Site Codes
Identification code
This data is fed to the reader as a bit stream of information.
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Parity Bits -Most of the proximity cards/tokens contain at least 1 error detection or parity bit. This bit is used to count the number of zeroes or ones contained on the card. If this bit is zero, it typically is referring to the fact there are an even number of zeroes or ones on the card.
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Identification Code
This is the employee ID number. Often times this is printed on the card.
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More format details?
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Site codesor families were originally created to provide for the production of a larger number of "26 bits" cards by duplicating cards rather than increasing the available number of user codes.
The cards were ordered for a specific site and the manufacturers, in the beginning, were keeping track of what had been shipped where. When a new site was installed, the site was provided with a site code that had never shipped to that immediate geographic area, thus, maintaining the level of security of the card. Also, the fact that all the cards on a specific site had the same site code, the control equipment did not have to store the site code in each of the user's code, thus reducing memory requirements in the hardware.
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As the market for access control exploded, it became almost impossible to maintain such a strategy.
Although there are still some manufacturers working this way (the change represents considerable modifications to both hardware and software), the trend is to provide cards with a greater number of bits and making obsolete the notion of site codes. This new method also means faster delivery of cards, even for add-ons to existing sites, right out of the manufacturer's or distributor's regular stock.
As long as the card number is unique, such as the serial number on currency bills, the security is maintained. Manufacturers should all be requested to provide a written statement certifying that their cards are unique and will never be duplicated.
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EXAMPLE H10301: 26-Bit Format Definition
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Standard 26 Bit Format Structure:
The standard 26 bit Wiegand format is H10301. It is binary encoded data. The format consists of 2 parity bits, 8 bit facility code and 16 bit card number fields. The format is shown below.
PAAAAAAAABBBBBBBBBBBBBBBBP
EXXXXXXXXXXXX.............
.............XXXXXXXXXXXXO
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Bit Coding
P = Parity
O = Odd Parity
E = Even Parity
X = Parity mask
A = Facility code, range = 0 to 255
B = Card Number, range = 0 to 65,535
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