Access control is the ability to permit or deny the use of a particular resource by a particular entity. Access control mechanisms can be used in managing physical resources (such as a movie theater, to which only ticket holders should be admitted), logical resources (a bank account, with a limited number of people authorized to make a withdrawal), or digital resources (for example, a private text document on a computer, which only certain users should be able to read).

When a credential is presented to a reader, the reader sends the credential's information, usually a number, to a control panel, a highly reliable processor. The control panel compares the credential's number to an access control list, grants or denies the presented request, and sends a transaction log to a database. When access is denied based on the access control list the door remains locked. If there is a match between the credential and the access control list, the control panel operates a relay that in turn, unlocks the door. The control panel also ignores a door open signal to prevent an alarm. Often the reader provides feedback, such as a flashing r ed LED for an access denied and a flashing green LED for an access granted.

A credential is something you know, such as number or PIN, something you have, such as an access badge, something you are, such as a biometric feature, or some combination of these. The typical credential is an access card, key fob, or other key. There are many card technologies including magnetic stripe, bar code, Wiegand, 125 kHz proximity, contact smart cards, and contactless smart cards. Typical biometric technologies include fingerprint, facial recognition, iris recognition, retinal scan, voice, and hand geometry.

A bar code is a series of alternating dark and light stripes that are read by an optical scanner. The organization and width of the lines is determined by the bar code protocol selected. There are many different protocols but code 39 is the most popular in the security industry. Sometimes the digits represented by the dark and light bars are also printed to allow people to read the number without an optical reader. The advantage of using bar code technology is that it is cheap and easy to generate the credential and it can easily be applied to cards or other items.

Magnetic stripe technology, usually called mag-stripe, is so named because of the stripe of magnetic oxide tape that is laminated on a card. There are three tracks of data on the magnetic stripe. Typically the data on each of the tracks follows a specific encoding standard, but it is possible to encode any format on any track. A mag-stripe card is cheap compared to other card technologies and is easy to program. The magnetic stripe holds more data than a bar code can in the same space. While a mag-stripe is more difficult to generate than a bar code, the technology for reading and encoding data on a mag-stripe is widespread and easy to acquire. Magnetic stripe technology is also susceptible to misreads, card wear, and data corruption.

Cards use a simple LC circuit. When a card is presented to the reader, the reader's electrical field excites a coil in the card. The coil charges a capacitor and in turn powers a integrated circuit. The integrated circuit outputs the card number to the coil which transmits it to the reader.

The format uses a facility code or sometimes called a site code. The facility code is unique number common to all of the cards in a particular set. The idea is an organization will have there own facility code and then numbered cards incrementing from 1. Another organization has a different facility code and their card set also increments from 1. Thus different organizations can have card sets with the same card numbers but since the facility codes differ, the cards only work at one organization. This idea worked fine for a while but there is no governing body controlling card numbers, different manufacturers can supply cards with identical facility codes and identical card numbers to different organizations. Thus there is a problem of duplicate cards. To counteract this problem some manufacturers have created formats beyond 26 bit Wiegand that they control and issue to organization.

There are two types of smart cards: contact and contactless. Both have an embedded microprocessor and memory. The smart card differs from the card typically called a proximity card in that the microchip in the proximity card has only one function: to provide the reader with the card’s identification number. The processor on the smart card has an operating system and can handle multiple applications such as a cash card, a pre-paid membership card, and even an access control card. The difference between the two types of smart cards is found in the manner with which the microprocessor on the card communicates with the outside world. A contact smart card has eight contacts, which must physically touch contacts on the reader to convey information between them. A contactless smart card uses the same radio-based technology as the proximity card with the exception of the frequency band used. Smart cards allow the access control system to save user information on a credential carried by the user rather than requiring more memory on each controller.