ssion, which are sent to the billing centre for charging. The 2G-SGSN is connected to the BSC via the Gb interface while the 3G-SGSN is connected to the RNC via the Iu-PS interface. The GGSN provides interworking with external packet switched networks and is connected to the SGSN using an IP based packet domain backbone network. The GGSN generates call data records (G-CDRs) for each session, which are sent to the billing centre for charging. The roles of the SGSN and GGSN will be discussed in more detail later in the chapter. A charging gateway collects CDRs (call data records) from the SGSN and GGSN, stores them and forwards them to the billing/mediation server. A mobile station performs a GPRS attach to the SGSN as the first step in order to be known to the packet core network. The SGSN will be the home SGSN for a local subscriber and the visitor SGSN for a roamer. After a successful GPRS attach, the mobile station is available to the packet switched services, like receiving an SMS over the GPRS, paging by the SGSN or receiving notification of incoming data. In order for the MS to be able to send or receive data, it has to create a Packet Data Protocol (PDP) context with a GGSN as the next step. The GGSN may be a home GGSN or a visitor GGSN (for a roamer) depending on the policy of the service operator. If operators want roamers to use the visitor GGSN, it has to be defined in the HLR. Once a PDP context is established, an MS has access to the external packet data networks like the Internet. Thus a GPRS network does not need to understand/interpret a host of Internet protocols, thereby making things simpler. The GPRS packet core uses the Internet protocol (IP) as specified in RFC 791 for communication with external data networks. Figure 4.42 mentions the names of important interfaces used in the GPRS to connect various network elements in the packet core. GPRS Support Nodes (GSN) GPRS functionality in a GSM network is implemented with the help of GPRS support nodes (GSNs). There are two types of GSN, serving GS