The main purpose of this security type is to prevent inmates from escaping. This can include things such as fencing, walls, doors, gates, fixed CCTV cameras, alarms, metal detectors, stationary dogs, x-ray machines and patrols. Most of the physical security is used at perimeters and boundaries as well as entry and exit points. Physical security can also be found at cells and gates, like locks, doors and also staff. Without this inclusion of security, inmates would have a unsuitable amount of freedom. The exterior of prisons tend to include fencing near the building and a tall, thick outer wall. The gap between the outer wall and fence is called the sterile zone. Close to the building within the fencing is where dogs patrol. The presence of physical obstacles and security can act as a good deterrent against inmates who are considering escape. The outer walls also prevents and contact from those that are outside the prison area, acting as a high barrier. Prisons tend to be well lit and architecturally open, so there is no obscurity or grey areas that are hidden from sight. Physical obstacles are the initial part of security that keeps inmate secure and positioned in the right area. Equipment moving in and out must also be searched thoroughly, like the housing of inmates in cells.