# Perimeter fence

Demarcation of a perimeter, when the protection of assets, personnel or buildings is required, is normally affected by the building of a perimeter fence system. The level of protection offered varies according to the threat level to the perimeter. Different types of perimeter fencing include:
* Chain-link fencing
* Metal railings
* Palisade fencing
* Rolled mesh fencing
* Timber fencing
* Welded wire mesh fence fencing
Vertical bar fencing has been the most popular form of perimeter security fence since the 1980s. Since the 2000s, welded wire mesh and acoustic barriers have also become popular types of perimeter fence around the world. Vertical bar, acoustic barriers and welded mesh are used in commercial projects and landmarks, and transport hubs.
In high-security applications, sensors may be attached to the fence that generate alarms when they detect someone cutting, climbing, or lifting the fence fabric.

## Purpose

The purpose of perimeter fence is to stop or prevent the incident and reduce the level of risk. Doing so discourages the perpetrator from completing a harmful event. For example, a high wall with fence posts that protects a property discourages criminals from intruding and, when an incident occurs, increases the time it takes to carry it out.
To meet the security requirements of an area, a thorough analysis must be conducted to determine the level of risk. The greater the probability of the threat occurring, the greater the level of risk. Being able to assess this risk allows you to gather all the data to study and solve a security problem. Then It will be necessary to consider all types of threats that may occur such as: theft, intrusion, robbery, kidnapping, and vandalism.

## Security systems

The perimeter security system is the set of all the methods, procedures, services, which allow, by the adoption of deterrent and protection methods, to reduce the risk.
Security systems are of two types: passive, and active.
The passive security system consists of all the physical methods that interpose themselves between possible threats and what needs to be protected. The purpose of these methods is to discourage attempts to overcome, knock down or climb over them as much as possible. Obstacles are the more efficient the longer it takes to knock them down or climb over them.
The passive safety subsystem consists of the following elements:
* armored doors
* armored drawers
* reinforced doors
* metal fences
* reinforced walls
* gates
* turnstiles
* vehicle collision barriers
* shatterproof glass
* bulletproof glass
* general reinforcement
* security locks
The active security system, on the other hand, consists of the electrical, electronic, and telephone methods involved in:
* monitoring the area and facility for the presence of criminal activity through event detection;
* evaluate the information provided by the area through a peripheral alarm box;
* deterring the commissioning of criminal activities by means of deterrents;
* generate a response through local and remote communication methods;
* evaluate the information provided by the peripheral alarm control unit and present it to the operator of a remote alarm receiving center.
The elements that constitute the active security system are:
* detectors
* peripheral alarm control unit
* means of deterrence (local)
* means for communicating information remotely
* remote alarm receiving unit
* accessories
* local electrical interconnection network
* primary and secondary group power supplies
* means of activating and deactivating the subsystem
* means of event recording
* means of image acquisition and recording
* means of human/machine interface
* telecommunication network for sending data to a remote-control center
To move from a high-risk situation to a lower-risk, and therefore more secure, situation, it is necessary to set up all physical obstacles to prevent criminal actions by implementing a passive security system. The latter, however, is not enough; in fact, to have optimal protection, it must be integrated with an active security system that can monitor the structure of the passive system and generate adequate alarm information to verify breakthrough or overrun attempts. Alarm information must be transmitted rapidly to a remote alarm receiving center and presented to an operator who, using the prescribed procedures, activates the appropriate response/intervention service.
It is essential to have a proper balance between the two systems of passive and active security so that the time between detection of criminal activity and intervention is reduced. The active security system consists of the following types of technology, which make up the fundamental architecture:
* detectors
* control of the peripheral alarms
* local deterrent methods
* means of communicating the information over a distance
* remote alarm receiving centre
Supporting these basic components, or forming a specialised part of them are the following accessories:
* local electrical interconnection network
* primary and secondary group power supplies
* means of activation and de-activation of the subsystem
* means of recording events
* means for capture and recording of images
* means for the man/machine interface
* a telecommunications network for exchanging data with the remote control centre.

## Sensors

The sensors are the means by which all of the initial information on the threat status is collected and on which the operation of the active security subsystem is based. They analyse what happens in the environment and on the protected structures, they analyse of the threat level according to various types of criteria, from the simplest to the most sophisticated, producing the alarm information that the remaining parts of the subsystem will use. Therefore, in nearly every case, the sensors are not limited to converting the signals from the environment and/or the structure and sending them to the centre for analysis.The sensors, based on efficient processing, become more and more effective, called distributed intelligence, and are true sensors, their own evaluation points, dedicated to the analysis of specific physical phenomenon, in a specific part of the site and cooperating in the performance of the entire protection system. An example of detector applied on a perimeter fence is the mems accelerometer.
The advantages of having a perimeter intrusion detection system are various, and of great importance, for example we obtain a remarkable capacity to analyse the information produced by each sensor, without the need to increase the resources at the centre. Moreover, we obtain a faster analysis speed since it is made by each sensor, only for the part of the site that it is designed to protect. The sensors transform the physical effect into an electrical signal, the evaluation of which provides information on the surrounding environment. To summarise, the sensors are the metaphorical sense organs of the active security subsystem.

### Classification

The detectors can be classified in different ways, the most meaningful which are:
* as a function of the physical effect analysed
* according to what generates the physical effect to be analysed (Active, Passive)
* as a function of the type of monitoring provided
* as a function of the monitored environment
The sensors are essentially made up of two parts: the first, called the transducer and the second, the analyser or processor. The transducer transforms the physical effect into an electrical signal. To each physical effect there is associated a different form of energy, so we can have transducers capable of transforming light energy that strikes them into electrical current, others which transform sound energy, others thermal energy, still others for mechanical energy, those for electromagnetic and so on. So the detectors can be distinguished based on the technology employed, which is based on the physical effect that is used to obtain the information from the environment. There are detectors for:
* Infrared
* sound
* ultrasound
* pressure
* accelaration
* electrical inductance
* electric field
Domestic and residential projects have since the 1980s featured timber fencing as the perimeter to houses and gardens. Ornamental metal railings have also been employed.
In the United Kingdom, perimeter fencing is banned at any sporting stadium after the death of 97 people at Hillsborough Stadium.