Following diagram shows the GPRS protocol stack and end-to-end message flows from the MS to the GGSN. The protocol between the SGSN and GGSN using the Gn interface is GTP. This is a Layer 3 tunneling protocol.
One of the most important things to note here is that the application communicates via standard IP, which is carried through the GPRS network and out through the gateway GPRS looks like a normal IP sub-network to users both inside and outside the network.
Also notice that packets travelling between the GGSN and the SGSN use the GPRS tunnelling protocol, so the internal backbone network does not have to deal with IP addresses outside the GPRS network. This GTP is run over UDP and IP.
Between the SGSN and the MS a combination of SubNetwork Dependent Convergence Protocol and Logical Link Control is used. SNDCP compresses data to minimize the load on the radio channel. The LLC provides a safe logical link by encrypting packets. The same LLC link is used as long as a mobile is under a single SGSN.
When the mobile moves to a routing area that lies under a different SGSN, the LLC link is removed and a new link is established with the new Serving GSN X.25. Services are provided by running X.25 on top of TCP/IP in the internal backbone.