Question:
Just some additional questions: - How does the wireless system behave when and external low power RF transmitter source interfere on the wireless alarm data communication loop frequency? - Do it still function, why? - Do it warn about the eventual inhibition of the system to perform hiss normal operations? How is interference detected and how fast?

Answer: To answer your questions most good wireless systems in security recognise interference and jamming in a number of ways: AGC detect - If the noise floor of the receiver rises above a preset level for more than 30 seconds then the systems can be set to provide first either a local fault indication or directly into alarm depending on the system set-up. Almost all class 3 systems can do this but sometimes this is called carrier detect. Better quality class 3 systems and above have better receivers with SAW filters so that the interference will have to fall dead centre in the channel before this causes problems. Crude regen. receivers can effectively be blocked or desensitised from signals megs away from the spot frequency and therefore are highly susceptible to all manor of transmissions.

During this "attack" the alarm may be compromised as of course real detectors signals may not have sufficient power to punch through this interference. If the system is set to go directly into alarm then the alarm is not really compromised but may be treated as a "false alarm" as a result, but there again if this was a deliberate attempt to block the system then the attacker has not really achieved much as the system responded by going in to alarm. The wired system equivalent of this is someone cutting a wire!

Other forms of attack include deliberate code attacks where one is trying to replicate system unset data. Many advanced systems recognise this form of attack and an alarm is raised within 5 subsequent code attempts i.e. less then 5 seconds.

Some more advanced class 6 systems use spread spectrum or multiple channels so if one frequency is blocked then signals can still be received on other channels or frequencies. In this instance the system is not compromised at all but a local fault indication may still follow to alert this situation. Remember just because 433 Mhz is fairly common place now in the UK as a European standard other frequencies are available for high power commercial systems and some 16 channels are available on UHF with power up to 500 milli watt and spread spectrum at 868 Mhz? (I think).

When systems approach class 5 and above the devices must perform house keeping. For class 6 accurate signal strengths must be recorded in to the control panel memory. This log can be viewed by engineers during commissioning or routine inspection. This means the PIR or whatever will transmit an "I'm ok" signal every 15 mins or so to let the control panel know the device is still in range and functioning perfectly. With class 3 and 4 the devices only transmit for events i.e. alarms or tampers but if the PIR should die then the control panel will have no knowledge and will not report anything is wrong.

With the class 5 and 6 systems the control panel knows to expect signals at regular intervals from every device stored in its memory so if a device fails to report in a set time frame then a fault indication will appear as a result. This time span is variable but is normally 60 mins. For fire alarm systems this verification process does not need to raise a fault from loss of signal from a device for 4 hours 30 mins. This may sound a long time but fire alarms are not normally under deliberate attack as maybe the case with a wireless security system. The current BS5839 standard for this verification time is currently under review and is likely to be reduced to 60 mins. This will place a huge technical burden on large systems as a 1000 smoke detector system for an example will only allow perhaps 6 attempts from each device to reach the control panel before raising a fault indication. This looks to be easy but in this example but there would be 6000 signals flying around all competing for air space and naturally a lot of clashing will occur and thus only one or two of these signals may reach the control panel over a 60 minute period. Increase the number of detectors to say 2000 for a general hospital and the clashing may be so severe that detectors may well fail to report in this time frame. Some clever techniques are now being employed to overcome these problems.

For alarm events the detector then transmits 20 or more repeated alarm signals so reception is then guaranteed.