he logical network struc- ture. The external view (here NI) of a network interface is identical for both physi- cal nodes. However, the internal views (here NIx and Nly) of a network interface are independent. An internal view of a network interface defines how the proto- cols from the external view are terminated in internal physical nodes. The decision upon how to terminate the network interface inside a physical node is a matter only for the node designer. Figure 4.7 also exemplifies the AML notation for the internal view of a network interface. The external interface is split into a number of communication relations, each relation defining an internal physical node that terminates that part. If the rela- tion terminates a physical media connection, it is drawn with a solid line, otherwise with a dotted line. Each relation is specified with respect to which part of the exter- nal interface it defines. In this example some relations comprise only one protocol. 4 Physical Network Modelling However, a relation can define any part of the disintegration structure of the exter- nal interface, and therefore comprise hierarchical related protocols, as well as inde- pendent protocols. Figure 4.8 shows the internal views of the external network interface defined as partial protocol stacks. SAPIO SAPI16 Q.931 (ni1) (ni2) SAPIO SAPI16 B(ni3) Q.931 I.411 I.411 SAPI16 SAPIO (ni1) (ni1) {Q.931} {Q.931,X.25} {LAPB,IP} {1.411,LAPD,M} {1.411,LAPD,M} {LAPB,IP} {X.25} Fig. 4.8 Protocol stacks for external and internal views of a network interface Note that internal physical nodes must also handle the protocol stacks that are identified by internal network interfaces. 4.3.2 Physical Network Levels and Variants To make and maintain the distinction between external and internal views of a net- work interface is also crucial for the integrity of a system architecture also because external and internal views represent development activities that are performed by different groups, at different points in time. In developing a telecom sy