 in the route symbols that are defined here. Criterion 1: If we had modeled an OSI system, it would have been natural to define a specific route type for every one of the seven layers. However, we are not pursu- ing such a model (the author regards the fixed number of layers and the position of layers in this layer structure as a severe mistake of OSI RM). As we have seen, real networks exhibit very complex interlayer structures and no network system is com- parable as far as their layer structures are concerned. There is actually no upper limit to how many layers may exist. A lower boundary exists in every network, however, and that is the interface towards a physical-media stratum. We therefore introduce only one special route symbol within this criterion the physical route, which is a route that interconnects 3.3 Route Properties and Symbols logical nodes over such strata. Since nodes in such a network model are special compared to every other logical node type, we say that physical routes interconnect physical nodes in physical networks. This "physical" picture of a network is important for several reasons: Through the depicted physical connections, the model can be used as a basis for describing all details of how to connect nodes of the operator network by physi- cal media. Physical media and the electromechanical design of the nodes constitute a major part of the costs for implementing and managing an operator network. The model of a physical network is therefore important for analyzing the cost aspects of an operator network, and for developing a plan for installation and manage- ment of physical elements. Note, however, that physical routes are abstractions, as any other type of route. A physical route abstracts a path between two or more physical interfaces to a physical-media stratum. Such a stratum is built by electromechanical devices, such as physical connectors, antennas, cables, cable concentrators, and manual cross- connecting devices, that in wide-area networks constitute a major part of the tot