r although they are not directly interconnected over physical media connections. Messages are, in that case, relayed over physical nodes to which they have physical media connections. Network interfaces are of two kinds: either they comprise a direct physical media connection, or they don't. In both cases, a network interface comprises one or several protocols in a discrimination structure (this concept was dis- cussed in Sect. 2.3.4). The protocols are terminated by the NNFs of the physical nodes, since, by definition, an NPL does not communicate outside a physical node. The symbol for network interface is introduce for the same reason as the refer- ence point symbol in Chap. 3, i.e. to simplify the model and, at the same time, retain a model that allows access to all other model fragments, on all integration levels. However, a reference point is not the same as a network interface without 4.2 Properties of Physical Networks a physical medium. The former is confined to a single logical network, while the latter may comprise protocols that belong to several logical networks. Simplified model through network interfaces: Network interface Network interface NNF Node network function (medium included) (medium not included) NPL Node platform Fig. 4.3 The network interface notation The definitions of physical media connection, physical network, physical node and network interface in no way restrict the lateral extension of a physical network. A physical media connection may be a wide area connection, like a connection to a satellite in geosynchronous orbit (distance: 36 000 km) or a ten kilometre long optical fibre, or it can be the physical connection between two cells on an inte- grated circuit (micrometer distance). Consequently, a physical node can be an inte- grated circuit, a printed circuit board, a network of boards in a magazine, a whole exchange or a router, as well as a network of exchanges or routers that cooperate with other networks. Thus, the physical network portion of a sysnet model is useful for descr