Selecting a Wireless Security System

08/01/2008 | By Fred Katz

There are many factors to consider when deciding whether wireless security systems will work well in your facilities

The major benefit of using a wireless system is that the bulk of the connections to the sensors are done via radio links between the security control panel and the individual sensors (below), keypads (above), and annunciators instead of through physical wiring.

A wireless security system is an excellent approach to protecting older residential and commercial properties in which the cost of running wire can oftentimes be excessive. There are many specific elements to keep in mind when designing a wireless security system, including the size of the building, the type of protection needed, and the budget. In order to select a wireless security system that will work for your building, it's important to know about the different system components and how they all work.

The System's Components
The basic components of a security system are the keypads, which operators use to arm and disarm the system and check the status; the main security panel, which is situated in an equipment room or other non-frequented location; and an array of different types of wireless sensors. The security panel has both a built-in radio receiver and transmitter, or is wired to separate modules that fulfill those functions. In addition, it has provisions to interface with a telephone line, which has been the standard for most security systems. Sometimes, a long-range radio link is used, which requires wiring for an external antenna. Recently, Internet-capable interfaces, which allow for operator interaction from a remote location via a computer or mobile device, have started to be used.

Typically, perimeter protection involves installing sensors on every door and window susceptible to intrusion. The most frequently used sensors are entrance sensors that detect an open or closed state for a doorway or window. Doorways are protected by either a mechanical plunger switch or a magnetic reed switch that will typically open, from a closed-switch state, when the doorway is opened. The magnetic sensor can be wired to the wireless sensor, or some sensors come with the reed switch built in as a part of the electronics and use a separate magnet mounted in a plastic case that works with the sensor to determine whether the doorway is open or closed. Window sensors are similar, and some products on the market can actually be embedded within the window frame for a nearly invisible installation.

In addition to perimeter detection, internal protection is also important. Wireless motion sensors can detect an intruder walking through the premises. In the event of an intrusion attempt, the alarm sounds with an audible alert and sends a message to a central station that will either notify the police or notify the contact person; however, this will not guarantee an instant response to the premises. After a preset amount of time, the audible annunciator (siren) will time out and stop, and that particular entrance point will be ignored if, for instance, the door that was opened during the intrusion is not closed; however, internal motion sensors will detect if the intrusion is still in progress or has reoccurred, and will retrigger the alarm circuitry, which will reactivate the siren and again notify the central station. The motion sensors are usually passive infrared (PIR) sensors: This means that they're triggered by sensing the body heat radiated by the moving intruder crossing the passive beams defined by the lens of the PIR. When used in a residential location, the resident has the choice of arming the system in the "stay" mode, which deactivates the motion sensors, or in the "away" mode, which incorporates the motion sensors. These modes are sometimes referred to as "day" and "night" modes.

In addition to perimeter intrusion sensors and internal motion sensors, there is another class of wireless devices used for ensuring environmental safety for both the premises and tenants/occupants. These devices include smoke detectors, heat sensors, carbon-monoxide and carbon-dioxide sensors, low- or high-temperature detectors, and flood sensors. Most of these devices are available as wireless devices and can be easily connected to a wireless module that activates on a switch contact's change of state, either opening or closing.

In some systems, each wireless module has a uniquely defined numeric "address" that is stored in the security control panel during the sensor installation phase. These wireless modules periodically send out a wireless "housekeeping" message to indicate that they're operational, meaning that they're still present and that their battery does not have to be changed. If a few supervision cycles have been missed, as evidenced by the absence of "housekeeping" messages received from the wireless unit, a "trouble" message is sent to inform the contact person that there's a problem. Problems can be due to tampering or electronic failure. Many security modules have a built-in tamper switch, which activates and sends a message to the control panel if someone attempts to open a module.

Keyfob devices, similar to remotes for automobile alarms, can be bidirectional, which means that there is a feedback to the operator as to whether or not the command was effectively received and acted upon. This wireless remote control is embedded within a wristwatch.

The System's Benefits and Drawbacks
The major benefit of using a wireless system is that the bulk of the connections to the sensors are done via radio links between the security control panel and the individual sensors, keypads, and annunciators instead of through physical wiring; however, there is some wiring involved, including a power line to the control panel and a communications link, which can be a long-range radio transmitter, an Internet interface, or a telephone line.

There are some system limitations that can affect the feasibility of a wireless installation, such as building construction and physical distances. If the basic security panel type is chosen properly, the premises can be divided into multiple independent zones so that different individuals can control particular sections of a property, with arm and disarm privileges independent of the other security zones. The control can be tiered to allow more privileged users access to multiple zones. In addition, some security systems facilitate fire detection and fire alarms.

One downside of a wireless security system is the batteries: All wireless sensors, keyfobs, and keypads require batteries. When wireless products first came out, the batteries lasted only 1 or 2 years. Some of the new products on the market have projected battery lifespans of between 10 and 15 years. At this point, many products use so little battery current that the defining factor for battery life is the battery's physical lifetime, as specified by the manufacturer, rather than its energy-storage capacity. Most sensor batteries use lithium-based chemistry to get the shelf life and operating life required for this type of application. One of the most commonly used batteries is the CR123A, followed by the AAA, the AA, and coin cells. When the battery is changed on a sensor, a tamper signal may be generated and the operator will have to send a "disarm" message to cancel the effect of the tamper signal.

Where These Systems Work (and Where They Don't)
Not all buildings are suitable for wireless security systems. The presence of very intense electromagnetic noise may mask wireless transmissions; therefore, if the building is near a radio transmitter or high-current machinery, a wireless system may not be the best option; however, this is not a common problem.

The more common factor that affects whether a building is suitable for a wireless installation is the physical distance between transmitters and receivers, and the construction material of barriers, such as walls, furniture, and/or other large objects. The maximum distance over which a radio transmitter in a sensor module can communicate is determined by the antenna design and the transmitting power. The transmitting power has to be limited to make sure the battery has an acceptable operating life and to be in compliance with FCC requirements. The maximum range, or open-field range, is often quite different from the actual distance within the building. Each wall or object that the signal penetrates weakens the radio wave. The higher the transmitting frequency, the more the radio wave is affected by obstructions. The presence of steel or other metal will warp and weaken the field of the radio wave and, in some cases, effectively screen any radio transmission from a particular location. Prior to an installation, it's a good idea to perform a survey on the property to determine if a wireless solution is suitable.

The physical distance over which the wireless signal must carry is a significant variable. The radio wave's field strength expands almost spherically and follows a square law, meaning that, if you double the distance, only one-quarter of the signal strength is transmitted. The receiving antenna has the same square-law effect, so the bottom line for distance is actually 0.25 x 0.25 = 0.0625 the field strength for doubling the distance. One method of enabling installation in an area that might present some difficulties due to distance or obstruction is through the use of a device called a "repeater," which receives a wireless message and re-transmits it after a time delay. This is a working solution in some troublesome environments, but the response to commands will be more sluggish due to the repeater delay.

Repeaters are often used in residential facilities, which may feature double-layered aluminum insulation. The keyfobs run on small batteries and have a poor range; if the receiver is at the other end of the building, some command messages can be missed. To resolve this issue, a repeater can be mounted on an inside wall at the front of the building, which works quite well.

Wireless sensors usually have a built-in transmitting antenna. The antenna is quite small; if it's a well-designed sensor, the antenna field should be almost omni-directional so that it doesn't require any special aiming or directional alignment during the installation.

The security system can be controlled by keypads situated in different fixed physical locations. Keypads can be wireless or hardwired. In addition, there are keyfob devices similar to remotes for automobile alarms. Some of these keyfobs are bidirectional, which means that there is a feedback to the operator as to whether or not the command was effectively received and acted upon. This is a safety feature that can warn the operator if there was a previous intrusion. There is even a wireless remote control embedded within a wristwatch for the "James Bond" type of user.

Maintenance of the System
Security systems should be periodically inspected. Batteries need to be replenished, smoke detectors need to be cleaned, and a good visual inspection should be made to verify that the integrity of the system has not been compromised. In the event of a keyfob being lost, it can be uninstalled so that it can no longer be used for access. Codes can be changed so that employees or tenants who no longer have authorized access can be effectively screened out.

The wireless approach is a very cost-effective solution for establishing a new or upgraded security system in an existing building. Installation is rapid and doesn't require as much intrusive drilling and snaking of wires. The expression in the industry for installing wireless sensors is to "lick and stick," which expresses how quick the installation process is.

When wireless security systems first originated, they were considered to be cutting-edge technology, but they are more widely recognized now as being well proven, accessible, cost effective, and adaptable. Wireless security systems can be used in numerous building types and to facilitate various security needs.

Fred Katz is president of Fred Katz Consulting Inc., Hauppauge, NY.

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