on using a number of external transport networks for that, similar to the case for IP-networks. Figure 4.13 shows a topological model over a GPRS network, where the external transport and transmission networks (shown as CSPs) are sepa- rated from the GPRS logical nodes that use these networks. The model is drawn in two visualization modes: the relations between logical nodes and external transport 4 Physical Network Modelling networks are shown in a flat mode, while the relations between physical nodes and external transport networks are shown in an overlay mode. SCCP-CSP physical router (CNL only) RF-CSP logical External CSP (ATM or ISDN router FR-CSP router Internal IP-CSP FR-CSP IP router GGSNXISCCP External CSP (ATM or ISDN) Fig. 4.13 Transport networks used in GPRS networks A GPRS network relies on five different types of transport networks: The GSM RF transmission CSP. This CSP is common for both GPRS and circuit switched services. 2. The Frame Relay (FR) CSP for interconnecting SGSNs with their BSSs. The connectivity service used is permanent virtual circuits, set up by the FR network operator. Inside the FR-CSP there may exist special FR routers. The connection-less service of SCCP (part of CCS) for interconnecting SGSNs and GGSNs to MSCs, HLRs and other nodes in the existing GSM-network. Only the connection-less service of SCCP is used. Inside the SCCP-CSP there may exist special CCS routers, so-called signal transfer points (STP). 4. A transport network that interconnects SGSNs and GGSNs. This network must be a high-capacity network. An SGSN may handle 100 000 GPRS-using MSs, not all of them active at the same time. Still, the flow of packages between GPRS users and PDNs will be very large. The solution recommended in the GPRS standard is to create an internal IP network (most likely based on ATM and SDH for transport). Inside this internal IP-CSP there may exist special, high-capacity IP routers. 4.4 General Packet Radio Service, a Case Study 5. Transport networks of WAN type (ATM+AAL and ISDN) that ar