09.613GPPGeneral Packet Radio Service (GPRS)Interworking between the Public Land Mobile Network (PLMN) supporting GPRS and Packet Data Networks (PDN)TS
Figure 2: Transmission Plane shows the relationship of the GPRS Bearer terminating at the SNDCP layer to the rest of the GPRS environment. It is shown for reference purposes only and detailed information can be found in 3GPP TS 03.60, 3GPP TS 04.64  and 3GPP TS 04.65 .
NOTE: In the SGSN and GGSN UDP is mandatory. TCP is optional but recommended for X.25 services.
Figure 2: GPRS Transmission Plane