GPRS is defined by 3GPP ( Third Generation Partnership Project) and is employed to connect mobile cellular users to PDN (Public Data Network). Within the GSM network it shares the network databases and radio access network and employs functions known as the PCU (Packet Control Unit), SGSN (Serving GPRS Support Node) and GGSN (Gateway GPRS Support Node) to provide packet switching data services across the fixed and radio network. GPRS offers packet data bearers within 3G networks. The functions of the SGSN and GGSN are the same as those in the GSM network. The difference is the capability of the radio access network. The RNC (Radio Network Controller) supports RAB (Radio Access Bearer) that can support real time bearer services in the region of 100Kbps to 300Kbps. Furthermore, with the introduction of HSPA , this figure can approach 3Mbps. Billing for GPRS services is typically based on volumes, although a number of service providers around the globe have adopted a fixed fee, unlimited download model.