26 September 2008
Flexible and reliable wireless technology in the 433 MHz Vs 868 MHz bands
Wireless security systems are designed to provide convenience, flexibility, reliability and also to reduce cost significantly. These wireless systems have taken the security industry by storm, as they are ideal for home and business security solutions, while also offering the freedom to expand in the future. Radio frequency (RF) bands have played an important role in the introduction of wireless systems. The market has availed two sets of RF controls, those which used the frequency band around 433 Megahertz (MHz) and those which operate around the frequency of 868 MHz.
With the introduction of wireless security, it has been the 433 MHz frequency band that is used for most security systems. It is a frequency band that has generally been used for equipment that does not require any licensing, and it has become known in common parlance as the ‘garage opener frequency’. The 433 MHz band can be used for a range of low rate data applications, including door bells, baby alarms, heating controls and, of course, garage doors.
As wireless technology advanced, it brought about a new frequency range, found at 868 MHz. “This frequency is more restrictive that the 433 MHz, as it is governed by a set of rules and regulations that are put in place to administer the volume of electric power that is transmitted over the frequency, as well as the frequency’s off and on times,” says Zane Greeff, Elvey Security Technologies’ Technical Director.
This frequency band is ideal for portable battery-powered devices such as vehicle alarms, domestic and commercial security systems, process monitoring schemes, computer networking applications and so forth.
As far as the 433 MHz band goes, it has no additional limitations concerning the allocation of the available bandwidth or time resources. This means that you are allowed to perform continuous transmission over the frequency, although you will have to deal with high noise levels caused by other systems operating on the same frequency, resulting in missed or false alarms. The band uses a coding system to prevent instances of ‘cross-talk’; in other words, even though the transmitter is operating on a common frequency, it can be programmed to only recognise its legitimate ‘partner’, the receiver.
To avoid interference, the 868 MHz frequency band employs an advanced duty cycle of one percent, meaning that 99% of the time there is no transmission. This allows other systems that use radio frequencies to fully operate without any interruption or ‘cross-talk’, which is an important function when locating radio frequency controls within buildings or homes. Tim Myers DSC Product Manager says: “Our systems have been designed with special software protocols that will assist in avoiding the interference effects on the system, in the event that the frequency is in high demand.” DSC has recently launched a new self-contained security system which operates around the 868MHZ frequency.
The 433 frequency is licensed for low power domestic controls, however, due to the lack of limitation that has played a large role in making the radio environment seem disorganised. The frequency has a transmission power up to 10 dBm, allowing one to achieve a cover range of 100 meters in open air conditions.
Unlike its predecessor, the newer 868 MHz band has a licensing protocol that limits transmission over the frequency to 1%. “The idea behind this is to ensure that 99% of the time there is no transmission, thereby allowing any other systems to operate without interference. The band offers one channel for all communication at a maximum rate of 20Kbps,” says Greeff.
Assisting in the transfer of data, the 433 MHz transmitter uses a long antenna that draws large volumes of electrical power from the system/controls. Data can therefore be safely transferred over small or medium distances using powerful channel coding techniques.
A few years ago, many installers had doubts about wireless security, as they had little knowledge about the wireless systems themselves. However, the use of these systems has grown significantly. This has also been the case for the 868 MHz frequency band, and as time goes by any remaining uncertainty will disappear.