rom the point where it originally attached to network, the handover has to be performed between higher level network elements. For instance, in 3G-GPRS the next level handover would be between two RNCs. If the continues to move further, the next handover level is between two SGSNs. The only network element that does not change during a session is the network anchor point - the GGSN in GPRS architecture, and the PGW in the EPS architecture. These levels of mobility management create a hierarchy. The most handovers happen between the network elements closest to the UE, and the fewest the furthest from the UE. This hierarchy is also noticeable in the number of network elements in a deployed network. There can be thousands, or even tens of thousands of BTSs, NodeBs, or eNodeBs, hundreds of BSCs, and RNCs, and tens of SGSNs, GGSNs, MMEs, SGWs, and PGWs. (This is dependent on the network operator size, the geographic and population coverage, and the capacity needs of the network.) Figure 2.20 depicts the mobility management hierarchy for GPRS, using 3G-GPRS as an example, and for EPS. A UE would be in the bottom of the picture moving horizontally. The levels of hierarchy are shown from top to bottom, where the anchor point is at the top, and the NodeBs and the eNodeBs are at the bottom. In addition to the intra radio access technology handovers, the 3GPP system specifies handover between the different 3GPP radio access technologies. Thus, when a UE is capable of multiple radio technologies, it can also move between networks depending on the coverage. Deploying IPv6 in 3GPP Networks eNodeB eNodeB eNodeB eNodeB eNodeB NodeB NodeB NodeB NodeB NodeB NodeB NodeB NodeB Figure 2.20 Mobility management hierarchy in GPRS and EPS architectures. 2.4.2 Roaming In the section above, we discussed the mobility management within one operator's network. Additionally, the 3GPP system is capable of enabling a subscriber to move between networks - to roam. Roaming means that the UE moves to a different network from the one to which they s