Today I am going to post an article that I’m sure will be interesting and curious to many, especially those of you who are regularly working with GSM devices. Last Thursday, I finally brought a little gadget home that I had bought just a few weeks ago. Nobody has anything bad to say about it, so I wanted to be able to test and compare its ability to Cinterion modems as until now it had not been possible to test it and I had to believe in Jamming detection.

I have based my tests on the official Cinterion application note called “AN_45_Jamming_Detection.pdf”. Here you have all of the documentation on that subject, I2m going to focus on the practical part. As for the modem, I have used a Matrix MTX65i that has a TC65i inside, however I could have used another MTX or Cinterion model and it would have made no difference.

Well yes ok. But what is Jamming detection?

Well to put it simply it is the modem’s ability to detect an intentional interference in communication channels that block them.

You might think that it doesn’t matter and certainly in the vast majority of GSM application this is true, but in safety-related applications (alarms etc.), it’s quite important (or it should be).

Let’s take an example. A house or a car has a typical GSM alarm. If the alarm goes off it must communicate this to a central office of GSM alarm receivers. But, what happens if some thugs use a GSM inhibitor? Could the GSM alarm communicate with the central office? Will the alarm at least be capable to detect interference with a GSM inhibitor and act accordingly?

Obviously, no device (no mobile, GSM/3G module etc.) would be capable of making a GSM communication with a powerful enough GSM inhibitor present. But yes, some devices have (al least) the ability to detect interference from a GSM inhibitor: Cinterion modules. This ability allows them to act accordingly. This means to say that if a device suspects that there is interference, it can try to communicate while the interference is still weak (it can pre-activate warning sound etc.). In short, it’s always best for an alarm device to know what’s happening than to not know, how it acts in these situations will depend on the ability and imagination of whoever developed the alarm.

How to operate Jamming Detection in Cinterion modules:

As I said before, all of the documentation is in Cinterion’s applications 45 note. Basically and quickly summarized, it describes the activation of the communication link’s stability indicator “Ista”, for example with:

AT^SIND=”lsta”,1,5

And to activate URCs with:

AT+CMER=2,,,2

As soon as we activate jamming (interference), we will see how the modem is going to indicate the situation through URCs (sending messages through your serial port). First of all it will send messages like this:

+CIEV: “lsta”, , ,

This indicates that there are errors in the communication link. “lstaEdv” (which is a countdown timer) which will quickly decrease to “0″. Hmm… the suspicion begins…

After this URC, the modem will return another type of message:

+CIEV: “lsta”, , , , , ,

This URC already indicates network coverage loss. According to Cinterion documentation, when you get a URC with “IstaNo” = 40, a low “Istavar” (lower than 10) and “IstaMean” > 40, this is a clear indication of jamming.

As everyone always understands better with pictures, here you have a video which is even better. (If you have received the article via email you will probably have to watch it on (www.blogelectronica.com).

jamming-detection

I hope that you found it interesting. Bye  ;)

 

 


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