al cost for building the network. In this book, however, we do not deal with how to model and design physical-media strata. Instead we hide such strata behind physi- cal routes. The symbol for physical route has a black square at its endpoints. It is also the only route symbol that reveals something about its properties (wireline or wireless) through the symbol, as depicted in Fig. 3.16. Physical Common Realization of physical route is by: 1. Wireline, point-to-point medium 2. Wireline, multiaccess medium (e.g., a bus medium) 3. Wireless, point-to-point medium (e.g., a radio link) 4. Wireless, point-to-multipoint (a cell) Physical media stratum (not modeled) 5. Wireless, broadcast (e.g., a wireless LAN) Fig. 3.16 Symbols for physical routes. Such a route is realized by a physical-media stratum Also shown to the left in this figure is the special layer-interface symbol that is recommended for denoting interfaces to physical media in layer structures. Note that a physical node is the lowest hosting level that can exist in a logical-network structure. Criterion 2: There are many ways a route may be addressed, as well as many dif- ferent addressing systems. We therefore base route-endpoint symbols only on whether a T-RE table is required, and, if so, if a T-RO table is needed. This results 3 Node Structures in five types of route endpoints which, by their symbols, indicate how the route is addressed (see Fig. 3.17). Global route REdef = REi, i.e., addressed by GT Switched route REdef # REi, network address is needed Socket route REdef # REi, network address and APi are needed Link route REdef # REi, network address is not needed Physical route No REi, no REdef are needed Fig. 3.17 Route-endpoint symbols distinguish addressing methods 1. Global route: this is a route that requires nothing but a global title for address- ing (implies that the route endpoint need no REdef specification in the hosted node). 2. Switched route: switched-route endpoints require an REdef specification that contains only some kind of hosting-