ss control points to nw2 and nw4 nodes (see Figs. 3.33. and 3.34). Since routes say nothing about control points, the only way this is reflected in this logical-net- work structure is by the fact that, e.g., n21 and n31 are both hosted in the same node (n11) of another network (nw1 here). Thus, to reveal if there exist relationships, and what type of relationships such collocated nodes have, one has to consult the corresponding layer structure. Figure 3.36 shows the effect on a layer structure of extracting the T->REdef functionality of two layers (L2 and L4) and allocate it into a common layer (DNS), thus creating three layers instead (L2', L4', and DNS). Model without DNS: Model with DNS: TCP/UDP/IP TCP/UDP/IP Diverse connectivity strata Fig. 3.36 The effect of DNS on a given layer structure 3.4.5.4 Global Routes for Mobile Services Since global routes are insensitive to where in a switching network the logical node resides, they imply that some kind of global route handling may also be performed for networks where terminating nodes and/or users of terminating nodes roam 3 Node Structures around. Such networks are cellular networks and the fixed access, wide-area net- works PSTN and ISDN. The two levels of mobility that are supported by these net- works are normally called terminal mobility and personal mobility. Terminal mobility. This must be supported by all cellular networks. In these net- works, terminals are roaming around. The identification of the terminal is stored in the terminal, and is checked by the network both when a call is made from the ter- minal and when the terminal is called. Since a terminal can be accessible from any fixed access point (i.e., base station) of the network when the call is made, this identifier corresponds to a global title. Note, however, that it identifies the terminal and not the terminal user. Thus, a global route is used by a terminating node (i.e., mobile phone) of the mobile switching network. The distinction between a global title and a network address (NA) is uphel