Mains Wireless Interconnected Smoke Alarms.

some designs of 815MHz kit (a well known type of doorbell and some alarms come to mind, oh and at least one design of car tyre pressure monitor) are pretty poor and have very unselective receivers that are blocked by anything that transmits nearby over a wide range of frequencies, including GSM phones on the 900MHz band.

Some others really well engineered and have tight channel filtering and a high dynamic range and are only affected by things on the same or adjacent channel, or very strong signals indeed out of band, and as none of these low power devices transmit for more than a small percentage of the time, a simple wait and retry protocol will normally get through after a few goes, slotting in the gaps in the other systems transmissions. However, as things get more crowded, then the risk of collision and loss of connection rises sharply.  I'd be wary of a block of flats with a lot of these that were not part of one single synchronous system.


OF course a hostile attacker with a modest transmitter, would find it easy to deny service and cause a fault state.  More likely for a burglar alarm than a fire alarm, but in certain types of building, food for thought perhaps. (A variation on the attack where you reprogram the heating and bake out your enemy,)


Mike.

some designs of 815MHz kit (a well known type of doorbell and some alarms come to mind, oh and at least one design of car tyre pressure monitor) are pretty poor and have very unselective receivers that are blocked by anything that transmits nearby over a wide range of frequencies, including GSM phones on the 900MHz band.

Some others really well engineered and have tight channel filtering and a high dynamic range and are only affected by things on the same or adjacent channel, or very strong signals indeed out of band, and as none of these low power devices transmit for more than a small percentage of the time, a simple wait and retry protocol will normally get through after a few goes, slotting in the gaps in the other systems transmissions. However, as things get more crowded, then the risk of collision and loss of connection rises sharply.  I'd be wary of a block of flats with a lot of these that were not part of one single synchronous system.


OF course a hostile attacker with a modest transmitter, would find it easy to deny service and cause a fault state.  More likely for a burglar alarm than a fire alarm, but in certain types of building, food for thought perhaps. (A variation on the attack where you reprogram the heating and bake out your enemy,)


Mike.