By: Michael Lawton

Different radio frequencies have different properties, and if you are sending control signals for access over a wireless system you need to find the frequency which is the best one for the job.  In a hotel, for example, you don't want to use frequencies which require more infrastructure, and you certainly don't want your signal to end up blocked by a passing chambermaid!

If you wanted to listen to a radio station from the other side of the world, you probably wouldn't turn your dial in the VHF band. And, you also probably wouldn't try the short wave to find a station which would tell you about the traffic jams in your suburb. And we all know why: different radio frequencies have different properties. VHF (Very High Frequency) doesn't travel very far in geographical terms; short wave (or High Frequency) travels much further, especially because of its ability to conquer the curve of the earth's surface by bouncing between the earth and the ionosphere. Therefore, the general rule is: the higher the frequency, the lower the range.

For local security purposes, of course, no one is thinking of bouncing signals from Europe to Australia.  But people do want to use radio. For example, it may make sense to have one transmitter with radio contact to all the doors on a corridor. That way, you don't need to feed wires to each door, but you can lock and unlock each door centrally, you can control entry, or you can read the identification of who's coming in and going out. The major disadvantage of radio is that it's out there in the open air, ready to be intercepted by anyone.  So security has to lie in adequate encryption.

Selecting the right frequency
But what frequency should you use? ASSA ABLOY's Michel Noxfeld, R&D Manager for Interconnectivity Platforms, says that it all depends on the type of installation you are thinking about. Basically, there are two sets of frequencies which are currently being used: 2.45 Gigahertz, which is an unregulated band currently being explored by many manufacturers, and 868 Megahertz (915 Megahertz in the United States) which is a regulated band already well established for security purposes.

Michel says, "Currently, 868 MHz has a big advantage: the regulations only allow applications to broadcast for either 0.1% or 1% of the time. That means there's always space on the frequency for an emergency signal. So it's often preferred by security applications." The 915 MHz band is only regulated for maximum transmission power.

Applications using 2.45 GHz, on the other hand, could easily find themselves in a radio traffic jam.  "We've heard from burglar alarm manufacturers," says Michel, "that they will never use 2.45 GHz because of all the microwave links and video transmissions which might be on it." Such applications need a lot of space, and it could be that the burglar alarm simply wouldn't get through.

So Michel says they wouldn't recommend putting a lock system on 2.45 GHz if it had to be linked in some way to a burglar alarm, meaning it would have to fulfill the local RF regulations.  But he also says that some ASSA ABLOY companies are working on applications for that frequency range, and one reason for that is ZigBee.

Advantages
ZigBee is created by an alliance of manufacturers who are developing application profiles for use with the IEEE 802.15.4 open standard. It's designed for wireless monitoring and control and will work with 868 (Europe)/915 (Americas) MHz, as well as 2.45 GHz (Global), with focus being placed on the 2.45 GHz frequency. It's currently being explored for everything from wireless keyboards for games applications, via patient monitoring, to the most sensitive security applications. There are several advantages to the ZigBee technology, the major advantage being one which it shares with all open systems: products can be designed to allow different applications to interact with each other without problematic and expensive intrusion into proprietary systems. Another advantage is scale: mass use of a standard brings prices down for such components as chips. In addition, says Michel, "Such a standard produces its own momentum, even when the standard is not optimal." 

Nothing's perfect
But it's not perfect: the higher frequency means range is lower, which means there may have to be more routers in the system. Michel says, "If it's a big building you may have the routers there anyway, but, for a security system, you may want to provide your own routers to avoid someone else routing for free through your system." The frequency is also easily absorbed by human bodies, so signals may get blocked that way too. Michel gives the example of a hotel in Las Vegas with card-controlled room entry where, believe it or not, over 300 guests a day ask to change their room. Each time a radio signal has to be sent to the door lock to change its code so that it will accept the new card. Some of those signals will be stopped by anyone walking by the door, physically blocking the path. The only way around that is to have many routers, allowing several alternative radiation paths. Michel fears that there could be a risk that you'll end up with several wireless ZigBee-based systems all using their own routers. "Buildings could get messed up with proprietary routers for the various private networks around. Is that what we want?" asks Michel rhetorically.

And there's the problem of the uncontrolled quantity of traffic. 2.45 GHz does have the advantage over 868 MHz that, like the US standard 915 MHz, it has sixteen channels instead of only three. That allows more channel hopping, should one channel be blocked. The risk of overcrowding is certainly present. One reason why 868/915 MHz was adopted by security applications was that 433 MHz was already overcrowded. Michel has an open mind. "I think ZigBee at 2.45 GHz has been shown to be a robust network," he says, "but it doesn't matter what I think. The question is what the industry and the regulators want."

And the regulators are certainly not always happy. One ASSA ABLOY company in Norway making wireless sensors for windows wanted to use ZigBee at 2.45 GHz because the chips would be less expensive, but the regulators wouldn't have it.

Issues to be solved
Michel insists that frequency is not the only important issue. Just as important are issues like software and battery life. And in that respect many 868/915 MHz based solutions are up to eighteen months ahead. All the same, the ZigBee standard promises very economical battery usage, with years of life on an AA battery. 

Michel says that, in a system using 2.45 GHz and thus requiring many routers, you could increase efficiency by using your nodes as routers; but nodes can be put to sleep to save battery power, while routers need to be on all the time and have to be mains-powered. "Battery-powered routers are still a vision," he says. 

ZigBee is here, but it's a bit like Bluetooth five years ago. Everyone was saying Bluetooth was ready, but it seemed very shy and never seemed quite ready to leave the laboratory and face the world. Now it's everywhere. It may yet take a while for ZigBee applications to become a part of daily life, but the problems will be worked out in due time. ASSA ABLOY is working on it.