Physical access control is the system of ID cards, card readers and software that decides which doors open for which person, and at what times. It is also called access control, or electronic access control. The cards are often called badges. Offices, tech parks, hospitals, factories and data centres all use it.
The system has four parts. The credential is what a person carries: a plastic card with a chip, a phone app, or a fingerprint or face. The reader sits next to the door and reads the credential. The controller is a small computer in a panel near the door. It holds the list of allowed cards, decides in a fraction of a second whether to unlock the door, and keeps working even if the network goes down. The software sits on top. In it, the security team groups doors into access levels, such as "Floor 6, all staff, 7 a.m. to 10 p.m." or "Server room, network team only", and gives each person the right levels.
Take, for example, a software company with 4,000 employees on three floors of a tech park in Hyderabad. A new engineer joins on Monday. The HR team adds her to the company's directory, the master list of employees. The access control software reads the directory and gives her card the level for her floor. Her card does not open the server room. The server room also asks for a PIN after the card tap, so a lost card alone cannot open it. On her last working day, three years later, HR enters her exit date. At 6 p.m. that day, her card stops opening doors. Her laptop login through single sign-on (SSO) stops working at the same moment. Running doors and logins from one identity in this way is called converged access control.
The software also keeps a record of every tap, allowed or refused. Each record is linked to the camera at that door, through the video management system (VMS). So a guard who checks a refused tap at 2 a.m. can watch the video of that moment. Some systems also stop a card from being used to enter twice without leaving in between. This rule, called anti-passback, stops one person from passing a card back to a friend.
Older cards are easy to copy. Many offices in India still use 125 kHz proximity cards, which send a fixed number that anyone with a cheap device can read and copy. Newer smart cards use encryption, so the card proves itself to the reader without giving away a secret that can be copied.
Access control also connects to other systems. The door locks release when the controller approves a card. Visitors get temporary cards through a visitor management system. Housing societies use simpler versions at their gates, run from a society management app.