GPRS interception

Hi .. all

Any one can give me some idea. how interception of gprs data is
possible at GGSN node.
Is it can be done by implementing GTP protocol which stablise a
tunnel at GGSN node and user data (packets ) can be received on some
port after completing the signaling. Please explain me in datail how
it can be possible. Design issues...

thanks
manu