s and to specify the part of the logi- cal network structure that is visible in the access interface. Then one can partition the original protocol accordingly and start developing a physical network structure. We will study such a case in detail in Sect. 5.5 (EIA 553, the protocol that specifies the radio access interface for analog accesses in TDMA/136-networks). The net- work architect who intends to specify a new network concept is in a more favoura- ble position: he can (and should) start by defining the logical network structure of the new concept, before he develops protocol solutions and physical network struc- tures. Although the logical node structure can be expected to be similar in physical nodes X and Y (see Fig. 4.4), the physical structures are most likely quite different. The actual substructure depends on many design decisions, including considera- 4 Physical Network Modelling tions of an implementational nature (hardware or software implementation of logi- cal nodes). The deciding factor is how the P-dimension, i.e. the node platform, will be implemented in a physical node. If we focus only on the communication aspect of a node platform, we might decide to split the NPL of X into two different plat- forms (NPL1 and NPL2) and allocate logical nodes to three internal physical nodes, according to Fig. 4.6. Q.931 SAPIO) SAPI16 Discr. Discriminator I.411 Fig. 4.6 Step 3: identify the structure in the P-dimension (i.e. the NPL architecture) and decide on internal physical nodes Note that, since a physical node can have any lateral extension (a whole operator network can be regarded as a physical node), the concept node platform must not be taken as synonymous with a computer platform for executing software (whether the computer platform is based on a single computer or a network platform concept like CORBA¹. Wide area physical nodes (as, e.g. a whole operator network) will most likely have no single node platform that comprises all physical nodes. It will, however, have one node platform that provides 