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Jamming: are you at risk?

The South African Intruder Detection Services Association (SAIDSA), has issued a crime trend warning on the ease of jamming of radio signals on equipment operating on the 433 MHz frequency. While this crime trend has been prevalent in many countries for years now, more frequent instances of this method of jamming are appearing in South Africa.

While the public have recently been made aware of the ease of jamming of vehicle locking systems using remote transmitters on the same frequency, this crime trend has now progressed to wireless alarm systems.

Although any wireless frequency can be jammed by using the right equipment, it is easier to jam the 433 MHz frequency due to ease of its off-the-shelf availability. The frequency is acknowledged as the most populated and over-crowded frequency in the world and is used in everything from alarm systems, vehicle locking systems, remote arming devices, panic buttons, gate automation, garage door automation, model cars and baby monitors.

They are freely and legally available as an open, licence free frequency and therefore anyone can use it. They are used to jam another 433 MHz signal if in close proximity. Some of these devices make use of the 100% duty cycle allowed in this band, whereby the user keeps the button depressed, and a constant signal is sent to the receiver, effectively blocking any other signal in the 433 MHz band, operating in close proximity.

While alternate frequencies such as 868 MHz are starting to be used more frequently for alarm systems, no instances of successful jamming have thus far been detected on this frequency.

The 868 MHz band suffers from little congestion, partly due to the complicated band-plan and exclusive sub-bands assigned for specific uses, i.e. alarm systems. The 868 MHz alarm specific sub-band is exclusively regulated world-wide for alarm equipment with a restricted channel spacing and duty cycle. In the 868 MHz band manufacturers have access to two versions of improving data transfer, frequency-hopping spread-spectrum (FHSS) and direct-sequence spread-spectrum (DSSS). Some alarm equipment manufacturers already make use of these technologies for wireless equipment in their range of products.

SAIDSA is obliged in terms of its constitution to advise its members and the public of any information that may affect their safety and security and therefore makes the following recommendations to reduce the possibility of successful jamming.

1. While SAIDSA does not prohibit the use of 433 MHz, customers should be made aware of any possible risks associated with the use of this overcrowded frequency.

2. Where possible, more secure frequencies or proprietary licenced frequencies should be investigated.

3. The use of bi-directional systems are recommended in order to obtain an acknowledgement that a signal has been successfully received.

4. SAIDSA also recommends that no more than 50% of the spread risk area be covered by wireless equipment.

5. Preferably, make use of transmitters offering a short duty cycle, thus reducing the possibility of a signal been sent constantly while the button is depressed.

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