ded that layer interfaces used by its users are kept intact. Part of the evolution of PSTN/ISDN is also to replace this stratum with IP- and ATM- ("asyn- chronous transfer mode") based connectivity strata. Thus, layer structures can be modeled and viewed on several stratum levels, as soon as you introduce stratum definitions in the layer structure of a network sys- tem. Fig. 2.56 shows the top level (called stratum level 0) of ISDN strata we have discussed (compare with Fig. 2.54). 2.4 Stratum Levels ISDN services ISDN services requesters users Packet-handling stratum Media-services stratum X. and V. ISDN circuit-switching stratum strata Fig. 2.56 ISDN stratum level 0 Each of these strata can now be assigned an internal structure of strata and lay- ers. In the previous models we have used an intuitive framing notation for this. In a correct AMLn model we use the substructure symbol to identify stratum levels. Fig. 2.57 shows the internal structure of the media-services stratum (on stratum- level 1). ISDN services ISDN services Packet- users requesters handling stratum substructure relations ISDN call-handling layer Media stratum SS7 connectivity stratum LAPD stratum ISDN circuit-switching stratum X. and V. Fig. 2.57 The ISDN media-services stratum is modeled on stratum level 1 This means that the ISDN call-handling layer will be modeled on stratum-level 2 (see Fig. 2.58). ISDN services requesters DSSI L3 Call-handling agent layer actor layer agent layer T-ROs common agent layer Media layers SS7 connectivity stratum LAPD stratum Fig. 2.58 The ISDN call-handling layer is modeled on stratum level 2. 3 Node Structures Introduction Node structures represent a completely different view on network systems. While layer structures describe how layers are related over functional interfaces called "layer interfaces," a node structure shows some kind of "nodes," interconnected by some kind of "connections." The intuitive understanding of how nodes and layer elements are related is that a node is some kind of "package" for la