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Q: Car alarm system ( Answered 5 out of 5 stars,   1 Comment )
Subject: Car alarm system
Category: Science > Technology
Asked by: kenn82-ga
List Price: $20.00
Posted: 27 Sep 2006 11:40 PDT
Expires: 27 Oct 2006 11:40 PDT
Question ID: 768952
I need an overview of how a car alarm system works especially on the
signal transmittal for locking and unlocking the car. What kind of
wireless signal involved. For example, does it use 900 MHz frequency
like the wireless home phone and if it does, does the lock and unlock
function uses the same frequency. Lastly, does all car alarms use a
standard wireless frequency for communicating between remote and the
alarm itself?
Subject: Re: Car alarm system
Answered By: sublime1-ga on 28 Sep 2006 01:10 PDT
Rated:5 out of 5 stars

A rough block diagram of the typical RKE (remote keyless entry)
system can be found on the following informative page from the
Maxim Semiconductor website, which notes:

"Remote keyless systems consist of a key fob transmitter and a
 receiver inside the vehicle. They most commonly use a frequency
 of 315MHz in the the U.S. and Japan, and 433.92MHz in Europe.
 Europe has also opened up the 868MHz band to accommodate the
 growing demand for remote keyless entry systems."

The same frequency is used to both lock and unlock the vehicle,
and one might think this provides little security, since, as
noted on the 'numb3rs blog' - a blog where a professor from
Northeastern University's Math department posts mathematical
comments on the television show "Numb3rs", which featured an
episode that discussed the mathematics of RKEs:

"(a) Essentially, all the entry systems use the same frequency,
 so when a controller is operated, all nearby cars get the same

 (b) It is obviously essential that each controller controls
 exactly one car. Otherwise, sending a "door open" command in
 a parking lot would open several cars -- not a satisfactory

 (c) The code that a particular controller sends to a particular
 car can not be the same each time the controller is used.
 Otherwise, thieves could simply record that code and use it the
 next time they encounter that particular car.

 So, the way that a controller can deal with these issues is to
 create or generate a sequence of numbers..."

Much more on the page:

For a more complete comprehension of the process involved in the
functioning of RKEs, see this 5-page article from HowStuffWorks:

The article from the same site provides more details about the 
nature of the random number generators used in the CPUs of the
tiny computers which RKEs actually are:

If anything is unclear, please request a clarification.


Additional information may be found from further exploration
of the links provided above, as well as those resulting from
the Google searches outlined below.

Searches done, via Google:

car "keyless entry" frequency

numb3rs remote car
kenn82-ga rated this answer:5 out of 5 stars
Excellent answer. I am actually just trying this feature from google
and it so happens that the car alarm system fascinates me. The answer
I got addressed a lot of the curiosity I have about the alarm system.

Subject: Re: Car alarm system
From: sublime1-ga on 28 Sep 2006 12:59 PDT

Thanks very much for the rating and the 5 stars!


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