dvanced charging method than regular telephone calls. One of the basic ideas behind UPT is that an UPT user will, regardless of loca- tion, have access to the same services that a fixed access subscriber has over his or her own access. The only restriction is that services that require special capa- bilities that the actual terminal does not have cannot be provided. This also implies that the UPT user (as well as the network) must be allowed to modify the user service profile from any suitable terminal. Realization of UPT services in WANs affect only the call-handling layer of the network. Handling the additional translation tables, registration, charging, and authentication procedures requires centralized solutions, such as home location register nodes (HLR) and service-control point nodes (SCP). 4 Modeling Vertical and Horizontal Partitions Introduction Particular layer structures and network structures are the result of how a given functionality is partitioned and how distribution is to be supported. Both activities result in refinements of layer and network structures. In existing network systems, the dominating approach to functional partitioning has been vertical partitions. By introducing the logical-network structure concept, we have indicated how a net- work can be viewed as a system of horizontal partitions. In this chapter we will take a closer look at how these two partitioning approaches are made visible by dif- ferent types of relations in layer and node structure models. We will find that applying vertical partitioning to a given layer structure implies: Logical network structures are not defined (in other words, all functions exist in a single logical network). Layer interfaces are not refined. They should always be defined by abstract service primitives (ASP) and, if relevant, by a layer interface behavior descrip- tion in order to facilitate the understanding of how the layer service must be used. Such a specification also provides an opportunity for later horizontal partitioning. Standards norm