6 to enable new services and solve the problems inherent with IPv4 networks. IPv6 offers an enlarged address space, e.g. in IPv4 the address length is only 32 bits whereas IPv6 addresses have a length of 128 bits. These longer addresses enable IPv6 to offer a total of 2128 IP addresses. The IPv6 protocol offers the following to 3G mobile networks, besides the enhanced address range: same IP address wherever you roam, i.e. global reachability multifarious 3G services to the mass market enhanced security through standardized and mandated security features availability of IP addresses for billions of terminals built-in QoS enhancing performance. On the other hand, supporting IPv6 in 3G mobiles implies changes in various elements of the 3G network, and this may not come from all equipment manufacturers at the same time, or may not happen inR99. These changes include: the HLR and SGSN will need SW upgrades to support IPv6 parameters the GGSN will need to support IPv6 protocol stack in its external interfaces (i.e. the MSs will need the IPv6 protocol stack. Since the subscriber's IPv6 traffic would be carried over the 3G backbone within the GTP tunnels, it is not expedient that all the backbone routers, or even the SGSN proto- col stacks, migrate from the start to IPv6. On the other hand, even if IPv6 capable MSs could still use IPv4 backbones because of the GTP Tunneling Protocol that separates the backbone IP layer from the subscriber IP packet payload; IPv6 will still benefit the backbone layer, e.g. from the build in security features. 3.2.6 Coexistence Interoperability Issues Although it seems that only one new interface, i.e. Iu, appears when incorporating the UMTS radio network to the 2G or 2.5G CN, inter-networking impacts spread to all the The UMTS Development Platform integrated network elements as shown in Figure 3.3. In particular, mobility management and call control bring in new interoperability requirements. These requirements are summarized next before we concentrate on describing the different UMTS bui