 radio resources in uplink and downlink directions are assigned independently. The radio channel is reserved only for the time of transmission although a virtual connection (see Section 6.2) exists at all times for each GPRS subscriber. GPRS users share physical channels allocated for packet-switched serv- ice. It offers a real packet access method and supports charging based on the amount of transferred data. 5.7.1 GPRS Network Structure Because GSM was originally designed for circuit-switched service, the intro- duction of packet-switched transmission requires some significant functional and operational changes. GPRS introduces two new network nodes, GPRS support nodes (GSN) to support end-to-end packet transfer. They are serving GPRS support node (SGSN) and gateway GPRS support node (GGSN), which are shown in the simplified network architecture in Figure 5.16. To keep the figure simple, many GSM network elements, such as EIR and SMSC, have been excluded. Circuit-switched calls are routed from the BSC via the MSC to PSTN. Base station subsystem, GPRS core network Mobile transceiver to PSTN stations, stations station interface Inter-PLMN controllers backbone interface interface interface interface MSC = Mobile services switching center VLR = Visitors location register interface HLR = Home location register interface SGSN = Servicing GPRS support node interface GGSN = Gateway GPRS support node Packet data transfer BG = Border gateway Signaling PCU = Packet control unit PDN = Public data network Figure 5.16 GPRS system architecture. Introduction to Telecommunications Network Engineering For GPRS operation the HLR is enhanced with GPRS subscriber data and routing information. The HLR is to be updated to include GPRS register (GR), which stores packet user related data, such as IP address of the present SGSN. The GR stores routing information (SGSN address) and maps IMSI to one or more Packet Data Protocol (PDP) addresses if addresses are perma- nently assigned to subscribers. Typically, an Internet Protocol (IP) ad