ata in both the D- and B-channels. Note that, besides other functions, the I.411 and the LAPD units perform a discrimination function. The 1.411 unit discriminates between the bits of the D-channel and the bits of the two B-channels. The LAPD unit discriminates between SDUs that are identified with different SAPIs. Discrimination supports independence between the users of different data flows. Discrimination may be used to separate logical nodes that belong to different logical networks (i.e. network functions and services), but it may also be used to allocate users to separate physical nodes (the effect of the latter 4.3 Physical Network Structures on interface specification is discussed in Sect. 4.3.2). We will use this insight to create a physical subnetwork where we can allocate the logical nodes of the NNF. Q.931 SAPIO SAPI16 Discr. I.411 Discriminator Fig. 4.5 Step 2: apply a logical node structure to the physical node, according to the logical net- work structure The analysis that led to the structure in Fig. 4.5 was not difficult to perform, since ITU-T put quite an effort into describing the logical architecture of ISDN (see recommendations ITU-T 1.320 and 1.324), and into structuring the NNF protocol into a set of protocols for an ISDN access. Although concepts like logical networks and logical network structures are not mentioned in ISDN standards, it is not too difficult to redesign the ISDN architecture in that respect. The reader should under- stand the model in Fig. 4.5 after studying Sects. 3.2.1 and 3.4.2, where the ISDN architecture is described. However, sometimes one is faced with network interfaces that show little aware- ness of the need to separate network interface functionality into logical networks before specifying protocols. There exist cases where the functions of a complete network access interface have been specified in a single protocol specification (an "NNF protocol"). In these cases, one must first "reverse engineer" the protocol specification to extract suitable logical network