 for later Releases CS Domain IP Multimedia Subsystem PS Domain External Release 99 IP Networks Figure 9.1 Release 99 and medium term architecture. The service drivers for R99 based on Ref. [1] include: compatibility with GSM, access to high-speed data services, and managed QoS. The CS domain provides circuit- oriented services based on nodal MSCs (an evolved GSM), while the PS domain pro- vides IP-connectivity between the mobiles and IP networks (an evolved GPRS). 9.1.1.2 Release R4 and R5 The medium term vision (starting R4 and R5) has the added feature of IP multimedia as illustrated in Figure 9.1. The service drivers include: compatibility with Release 99, addition of IP-based multimedia services, and an efficient support of voice-over-IP- over-radio for the multimedia service, but not necessarily fully compatible with the te- lephony service and its supplementary services. The UMTS Network and Radio Access Technology The CS domain retains and provides 100% backwards compatibility for R99 CS domain services. We can implement this domain through the evolution of MSCs, or MSC servers and a packet backbone. The PS domain also retains and provides IP connectivity. It gets upgraded to sup- port QoS for IP-multimedia services. The added IP-multimedia subsystem provides new IP multimedia services that comple- ment the services provided by the CS domain. These services will not necessarily align with the CS domain in the medium term. 9.1.2 The Long Term UMTS Architecture Vision After the evolution of R99 culminating with ROO (R4 and R5) we aim to have an inte- grated platform based entirely on a packet switched system. The service drivers for the long term include: migration of many users to IP-multimedia services and wide-spread adoption of IP-multimedia outside UMTS. IP Multimedia Subsystem PS Domain External IP Networks Long-term UMTS Architecture Figure 9.2 Long term UMTS architecture. By this time, we assume that the IP multimedia subsystem has evolved to the degree that it can practically stand as a substitute 