How do I sniff GSM and GPRS traffic -SMS ??? please assist

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I am interested in sniffing cell traffic 
and decoding SMS 

I am blind to this as to what device do i need
to do this 

I am running ethereal over a wifi card to attempt it
but get no results ... 

Is there someone who can shed some light on this topic
for me. To requirements and setup

It would be most appreciated.

thank you,


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