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Q: Car alarm system ( Answered ,   1 Comment )
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 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?```
 ```ken... 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." http://www.maxim-ic.com/appnotes.cfm/appnote_number/1774 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 message. (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 outcome. (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: http://www.atsweb.neu.edu/math/cp/blog/?action=get_events_for_date&date=2005-10-02 For a more complete comprehension of the process involved in the functioning of RKEs, see this 5-page article from HowStuffWorks: http://auto.howstuffworks.com/remote-entry1.htm 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: http://computer.howstuffworks.com/question697.htm If anything is unclear, please request a clarification. sublime1-ga 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 ://www.google.com/search?q=car+%22keyless+entry%22+frequency numb3rs remote car ://www.google.com/search?q=numb3rs+remote+car```
 kenn82-ga rated this answer: ```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. Thanks!```
 ```ken... Thanks very much for the rating and the 5 stars! sublime1-ga```