mpler. Let us take IP networks as an example, looking from the application down the stack. The power of TCP/IP applications is that the application generally can assume a simplis- tic view of the network (a basic packet-carrying network). This is because the complexity of the network is hidden from the application: the error-free end-to-end transmission chan- nel offered by TCP and the fast, 'best-effort' packet-sending interface offered by UDP are there for the application to use. The complexity of addressing, routing, re-transmissions, etc. is implemented in the TCP/IP protocol stack. The migration process implies a convergence process, since the target network is quite similar for the various vertical networks. When shifting from vertical to horizontal, we Understanding Changing Telecommunications - Building a Successful Telecom Business Services, today Services in next generation network structure Services Transport Access transport & switching networks Users Figure 3.17 The layering will take time in the control layer because of the existing signalling systems Applications IP multimedia subsystem (IMS) domain server server server server PS domain mobile Control f server Router SGSN MGW functions GGSN MGW Corporate functions Internet Router Multiservices IP network hot spot GSM/GPRS/EDGE UTRAN GERAN Figure 3.18 Example of the new horizontal 'all-access' architecture tend to speak about the main layered functions such as communication control, ser- vices and applications rather than nodes in PSTN, GSM or the Internet. A node can be seen as an implementation option of functionality location, usually comprising more than one layer. Networks and Technologies Service network and IP infrastructure servers Applications Service/User management IP infrastructure Service Portals content AAA, notification etc DHCP, DNS etc enablers HSS/HLR Network support servers traffic Communication control Network management MSC/TSC server SGSN control f. CSCF/BGCF Billing/accounting GMSC server GGSN control f. MRFC/MGCF Customer car