tecture. BSC, and A between the BSC and the MSC. The interface between the MSC and the HLR is labeled with two names: C and D. The reason for this is the C-interface is used when the MS is in the home network, and the D-interface is used when the MS is visiting another network - roaming. Thus, this interface is between two operators when the subscriber is roaming. In the roaming situation, the HLR remains always in the home network (hence, the name), and the MSC is in the visited network. 2.2.2 General Packet Radio Service GPRS was first introduced as the packet service for the GSM network. In addition, 3G is based on the same architecture. Therefore, the name 'GPRS' is used in both networks for the packet network. We will describe both flavors in this section. 2G-GPRS As described earlier, GPRS was introduced to the circuit switched GSM system in ETSI Release-1997. Figure 2.4 shows the GSM-based GPRS architecture. When comparing Figure 2.4 with Figure 2.3 it is clear that the RAN (sometimes also called the Base Station System (BSS) as the architectural combination of BSC and BTS) are the same, but the Core Network (CN) differs. We will now focus on the parts that are different in the PS domain from the CS domain. The GPRS uses many of the elements from the GSM telephony system. The BTS, BSC, and HLR are the same in both and shared between both of the domains. However, there are also two new elements that are specific to GPRS - the Serving Gateway Support Node (SGSN) and the Gateway GPRS Support Node (GGSN): SGSN is mainly responsible for authentication, authorization, and mobility management. In addition, it gathers charging information. It is connected to the HLR for the subscriber profile information. The SGSN is an element responsible for both mobility manage- ment signaling, and user packet forwarding. Hence, SGSN is both a user-plane and Basics of the 3GPP Technologies a control-plane element. On the user plane, the SGSN terminates the radio protocols, and thus it is the first element to see the user IP pac