Wireless Technology and security systems: With new advances in wirelesstechnology, what was once thought to be impossible is often affordable andright at hand.
Oct 1, 1997 12:00 PM, Howard E. Friedman
Last August, the third article of our security technology series introducedthe components in wireless subsystems, consisting of several transmittersand a single receiver. It dealt with its operation and ability to enhancehardwired security systems like those mentioned in the first two articles.It showed how pertinent information can be encoded onto a transmittableradio wave and, through digital data, how wireless installations candifferentiate themselves from identical systems and how transmitters can beuniquely identified. Finally, it introduced the concepts of wireless systemsupervision and transmitter battery monitoring.
In this article, we'll continue by introducing transmission range, testingand the do's and don'ts of an installation. We'll include a review of thewide variety of integrated sensor-transmitters now being used, and we'lltake a look at modular wireless systems - those engineered with bothhardwired and wireless components in mind - as well as systems dedicatedentirely to wireless operation.
Transmission rangeThe type of wireless technology we've been describing is known asshort-range wireless, mainly because the power levels used by these kindsof transmitters are limited by the FCC. Nonetheless, the open-field rangeone can expect from a modern security system transmitter can be as much asseveral thousand feet and even greater. Although such range issubstantially reduced in an indoor environment, there aren't many homeswhose size or construction would preclude the use of wireless components.In spite of this, radio signals have always been subject to nulls and deadspots - small regions in the reception area in which transmitted signalswere either not present, too weak or distorted. In the following scenarios,we've supplied a few of the many factors which can attenuate atransmitter's range and the integrity of the received signal:
Concrete and brick: Large amounts of concrete and brick in interior wallsmay cause some deterioration of a transmitted signal. Even outer wallscontaining brick and concrete may be a factor in L-shaped homes if thesignal has to pass through them to get to the receiver.
Wire lath in plaster walls: In areas where plaster walls exist, a finescreen of metal mesh, called lath, may be found within the wall to give itits foundation. If sizable and continuous portions of this material exist,it may reduce the strength of the signal as it tries to reach the receiver.
Insulation: Homes with foil-backed insulation placed in both inner andouter walls may also serve to reduce a transmitter's range. The foilbacking acts as a reflector of radio waves, and if present in large enoughquantities, may manifest the same difficulties as those found withconcrete, brick and wire lath, as described above.
Metallic materials: When there are large enough quantities of foil-backedwallpaper or large mirrors spanning large areas, the same problems mayoccur. Mirrors have reflective coatings that, if sizable enough, act toblock out or redirect radio frequencies. Although kitchen cabinetsconstructed of metal are not especially common, they may have the sameeffect.
Stucco walls: In certain types of construction, exterior walls are formedof stucco, a combination of a cement finish affixed to a base of wire mesh.As with metal lath in plaster walls, the wire mesh can attenuate radiosignals, especially if used in L-shaped buildings.
Metal studs: In newer construction, the use of aluminum studs, placed about16 inches (406 mm) apart, has become popular. A large number of these studswithin a long wall can attenuate some radio frequencies.
In spite of all these potential problems, positioning a transmitter justseveral inches from its original location may cause a stronger (or weaker)signal to be received. The distortion found in some received waves is oftenthe result of the partial cancellation of a direct signal by the samesignal, reflected off a nearby object and received just moments later.Fortunately, problems like these have been minimized through the use of thespatial diversity receiver, which, by using two appropriately spacedantennas, takes reflected signals into account and captures the best signalpossible. Once the receiver locks onto a good signal via one antenna, theother antenna is disabled for the duration of the transmission.
Modular systemsUp to now, we have described our wireless system in the context of asubsystem consisting of several transmitters and a single receiver, capableof tripping one or more protective zones in a hardwired panel. Althoughthis arrangement is still used mostly for emergency and medical purposes,wireless technology in security systems is found far more readily in bothmodular and dedicated systems.
Modular systems have become quite popular and are fashioned around a basichardwired alarm panel whose design permits the addition of several modulesto enhance the panel's capabilities. Among the most popular of these is anRF receiver whose use makes it possible, depending on the manufacturer, toaccommodate dozens of wireless transmitters, making each equivalent to asingle hardwired zone with a unique zone number and a set of programmedparameters.
Dedicated wireless systemsIn control panels dedicated to wireless use, the RF receiver is directlyintegrated into its design with no necessary special accommodations. Suchsystems may also have several hardwired zones that may be desired inapplications where a suitable wireless detector is not available or wouldbe inappropriate. Dedicated wireless systems usually have provisions forboth hardwired and wireless keypads. Besides the obvious differences, thehardwired keypad is capable of receiving and displaying feedback from thesystem: armed or disarmed status, faulted zones, system trou-bles andmemory of prior alarms. Because the typical wireless keypad is just atransmitter, it can only arm and disarm and cause the control panel toproduce audible signals. These signals inform the user that there is a needto examine certain information that can be obtained from the display oneither a hardwired keypad or on the panel itself.
Wireless transmitters revisitedToday, it's quite common for sensors and detectors to be integrated intothe same package as the transmitter. Doing so makes for a neaterappearance, reduced cost, greater convenience and more flexibility.
Sometimes, because of either cost or appearance, a single transmittermonitors a bank of windows. Violations of any of these windows would all bereported as the same zone number. Were the transmitter a multipoint unit,it would have the capability of reporting, as unique points, each of thecontacts wired to it. Multipoint transmitters currently come with two-,three- and four-zone capabilities.
Many transmitters are also equipped with a tamper switch, which is a smallmagnetic reed or microswitch that produces a tamper transmission wheneverthe unit's cover is either opened or removed. Some wireless smoke detectorsare also tamper-protected and will report such tampering if the detector isever removed from its base; such an annunciation might be desirable incertain institutional settings. During system programming, tamper alarmscan be made to sound an audible alert heard only at the system's wiredkeypads, a full-fledged intrusion alarm annunciated by the siren, ornothing at all. To simplify the changing of batteries by the user, aspecial battery change mode permits the opening of a transmitter and thereplacement of its battery without causing a tamper transmission. Asexpected, a transmitter's tamper alarm is reported by zone number and isnot treated in the same way as if the sensor associated with thetransmitter were triggered.
The learn modeIn August, I explained how DIP switches are set to identify thetransmitter's house code, its zone or ID number and its function in thesystem. As you might expect, the latest technology has all but done awaywith these tiny switches because they're prone to installer error and tofailure over time.
To eliminate DIP switches, manufacturers had to incorporate creativetechniques into the circuitry of both their transmitters and receivers,which has made programming considerably easier than it was in the past.First, though, it's important to know that whenever they broadcast, thesetransmitters have been hardcoded to produce a unique multidigit serialnumber and, depending on their role in the system (whether they function asa smoke detector or a PIR), may automatically provide a function code. Onceit's set up to do so, the panel can easily "learn in" the relevant detailsof each transmitter and assign it to the desired zone in the system. Here'sa generic look at how it works from the installer's point-of-view:
Either before beginning the installation or at a suitable time while it'sin progress, use the proper keypad command to place the panel in theprogramming mode.
Enter the zone programming area and select, from the keypad's display, thefirst zone to which a wireless device will be assigned.
Trigger the transmitter to be assigned to that zone and wait for the anaudible acknowledgment from the keypad. If properly received, thetransmitter's serial number and system function, if transmitted, will beassociated with the selected zone.
Perform any additional programming for that zone (e.g. delay before alarm,supervision factors, perimeter and interior designation, alphanumericdescription) that might be required.
Once all the transmitters are learned in, they should be positioned attheir respective locations and tested according to the manufacturer'sinstructions.
More on supervisionIn August, we learned how a wireless system supervises its transmitters.However, suppose the system contains one or more portable panic or medicaltransmitters. These units by their very nature are often carried offpremises, where their supervisory signals would be far out of range. Ifaway from the premises long enough, such transmitters would be responsiblefor the resulting no check-ins which would be in store for anyonesubsequently using the system.
To avoid this problem, zone supervision for individual transmitters may beselectable and should be programmed accordingly. Obviously, any permanentlymounted transmitter should be supervised, and any transmitter likely to beremoved for a period longer than the selected supervisory interval shouldbe programmed as unsupervised.
Transmitter testingVirtually all modular and dedicated wireless systems, along with standalonereceivers, are capable of testing the quality of a received signal. Beforeany wireless installation is completed, each transmitter in the systemshould be subject to such examination. Often called a go/no-go test, itusually cuts in half the sensitivity of the RF receiver, pretty muchassuring that any transmitters that pass the test should operatesufficiently well when full sensitivity is restored during the actualoperation of the system. Transmitters that fail the go/no-go test shouldeither be repositioned or relocated until the desired results are obtained.
Using two receiversIn installations that cover several thousand square feet or in homes thathave several floors or present construction factors that may impede systemperformance, it might be wise to consider the use of one or two remotelymounted receiving modules. In such cases, each module can be strategicallypositioned to be close to a group of transmitters so as to optimize thesystem's performance. For all these reasons, this technique is alsoapplicable in some commercial installations. (See figures.)
Spread spectrumPerhaps the newest trend in wireless security is the use of spread spectrumtransmissions to convey data. Spread spectrum technology, which has been inuse for many years, was developed decades ago to resist attempts at jammingmilitary communications. The use of spread spectrum transmissions in acommunications system greatly increases its immunity to noise andinterference. Basically, the technology spreads what would normally beinformation confined to one frequency (narrow-band) over a relatively wideband of many frequencies. The spread spectrum receiver that captures thesesignals correlates them to reassemble the original data. Up to now, thecost of incorporating this technology into wireless security systems wasprohibitive. However, these days, as with most advances in electronics,what was once thought to be impossible is often affordable and right athand.
The future holds that residential and commercial security systems will relymore and more upon wireless techniques. Not only do they decreaseinstallation time, provide annunciation by specific point, and work quitereliably, they do what hardwired installations have failed to do all theseyears: They offer the only practical way to bridge brick walls and concretefloors, deal with smoke detectors on cathedral ceilings, and keep delicatemolding, wallpaper, wall-to-wall mirrors and other fragile furnishings fromthe installer's drill.