2 The protocols do not correspond to the same OSI layer. Towards IP Based Networks The above includes the interfaces from the GGSN to the HSS (i.e. Gc reference point), from the SGSN to the HSS (i.e. Gr reference point), from the GMSC server to the HSS (i.e. C reference point), and the MSC server to the HSS (i.e. D reference point). We can implement the MAP based interfaces using MAP transported over IP, or MAP over SS7, and we can transport it on the same protocol CAP stacks as illustrated in Table 9.1. 9.4.16 lu Reference Point The Iu remains as the reference point between UTRAN and the ROO core network. We realize this reference point by one or more of the following interfaces [7]: transport of user data between UTRAN and SGSN takes place based on IP; transport of signalling between UTRAN and SGSN takes place based on IP or SS#7; transport of user data between UTRAN and MGW takes place based on different technologies (e.g. IP, AAL2), and includes the relevant bearer control protocol in the interface; transport of signalling between UTRAN and MSC server takes place based on IP or SS#7. When we base the Iu_cs on ATM, then we can apply R99 protocols or an evolving ver- sion, and when we base the Iu_cs on IP, we need to add new IP transport related proto- cols as part of the Iu protocols. On the other hand, it will be possible to have a R99 Iu interface with MSCs compliant with R99 specifications in a ROO network. MOBILITY MANAGEMENT 9.5.1 Address Management We can implement a UMTS network as a number of logically separated IP networks, which contain different parts of the overall system. Here we refer to these elements as an IP Addressing Domain. In an IP addressing domain we expect to have nodes with non-overlapping IP address space and be able to route IP packets from any node in the domain to any other node in the domain by conventional IP routing. An IP addressing domain implementation can take place through a physically separate IP network or an IP VPN. We can interconnect the IP addressing domains at variou