Standards
Modern proximity cards are covered by the ISO/IEC 14443 (proximity card) standard. It defines two types of card ("A" and "B", with different communications protocols), which typically have a range up to 10 cm (4 inches). There is also a related ISO/IEC 15693 (vicinity card) standard, which typically works up to a longer range of 50 cm (19 inches).
The card readers communicate in various protocols, for example the Wiegand protocol that consists of a data 0 and a data 1 circuit (or binary or simple on/off (digital) type circuit). The earliest cards were 26 bit. As demand has increased bit size has increased to continue to provide unique numbers. Often, the first several bits can be made identical; these are called facility or site code. The idea is that company A has a facility code of xn and a card set of 0001 through 1000 and company B has a facility code of yn and a card set also of 0001 through 1000. Numbering system is internationally harmonized and allocated by Netherlands based NEN (registration authority) as international registration authority according to ISO/IEC 6523 and ISO/IEC 15459 standard.
Read more about this topic: Proximity Card
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