at all part of the standardized Internet. The functions that produce such services exist as application layers on top of standardized Internet layers. Altogether, these scenarios reveal that real networks are built by at number of "groups of layers" that each is specified in, and an instantiation of, a particular net- work system. It is therefore essential that layer elements belonging to such a "group of layers" can be isolated in some kind of logical nodes that can be inter- connected into a logical network. In doing so, one creates a model of a real network that consists of a number of logical networks that rely on each other, that are con- figurated on top of each other, and that may be managed separately. The main pur- pose of this is to provide maintainable relationships between what is standardized in a particular network system, and one or several interdependent logical networks in a real network. By these relations, any changes in functional requirements that are specified in a network system standard can be traced to a particular logical net- 3 Node Structures work, and from there to components in operator networks. Logical networks are also excellent targets for network configuration, derived networks for function test- ing and for fault tracing, provided that they are adequately defined, and that layer interfaces between logical networks are identifiable and manageable in system solutions. Furthermore, most networks are in constant evolution, which implies that parts may be replaced by other parts that are instantiations of newer network systems (that offer more attractive technologies). For example, ATM networks that rely on SDH networks have been built so far, but the advances in optical switching tech- nologies means that, in the near future, it will be possible to replace existing SDH networks with optical transmission networks. How well logical-network definitions serve these purposes is, however, a matter of how forward-thinking network system designers have been. It is also not neces- sarily wi