Physical security describes measures that prevent or deter attackers from accessing a facility, resource, or information stored on physical media. It can be as simple as a locked door or as elaborate as multiple layers of armed guardposts.
The field of security engineering has identified three elements to physical security:
* obstacles, to frustrate trivial attackers and delay serious ones;
* alarms, security lighting, security guard patrols or closed-circuit television cameras, to make it likely that attacks will be noticed; and
* security response, to repel, catch or frustrate attackers when an attack is detected.
In a well designed system, these features must complement each other. There are four layers of physical security:
* Environmental design
* Mechanical and electronic access control
* Intrusion detection
* Video monitoring
The initial layer of security for a campus, building, office, or physical space uses environmental design to deter threats. Some of the most common examples are also the most basic - barbed wire, warning signs and fencing, concrete bollards, metal barriers, vehicle height-restrictors, site lighting and trenches.