f packet-handling functions and inter- working needs with X.25 networks. As Fig. 2.51 shows, packet services are offered to ISDN users both over LAPD links (SAPI= 16) and over switched channels. Let's now summarize our findings on control and connectivity structures: By applying the actor-agent separation approach, any OSI layer structure dis- solves into a single control structure (over at least two levels of control) and a number of mutually independent connectivity structure, which may run on a switching stratum. Conceptually we say that the L dimension dissolves into an Lac dimension and one or several Lco dimensions. The relations between control and connectivity structures are fully defined by SAGs (in ISDN only provided for TCAP) and SACs. Control structures are strictly hierarchical, while connectivity structures nor- mally are not (as the seven-layer structure of OSI RM makes us believe). As mentioned before, the ISDN standard describes ISDN only as an OSI layer structure. Actor functions are only indicated by including actor description frag- ments in message protocol specifications. This standard therefore (more or less) hides the distinction between the control structure the connectivity structures, which certainly does not facilitate the understanding of ISDN. Stratum Levels We have seen that functional structures of real network systems have very little resemblance with the OSI RM (seven layers in a hierarchical layer structure). Their functional structures are not strictly hierarchical, and consist often of more than seven layers (see, e.g., the ISDN layer structure depicted in Fig. 2.51). Parts of a layer structure are also frequently grouped or aggregated into larger functional ele- ments for different reasons. For example the SS7 connectivity system (MTP L2, MTP L3 and SCCP), which is defined as a part of the PSTN/ISDN layer structure, could be treated as such a group in ITU-T standards. When a group of layers is specified in a way that makes it possible to handle it as a black box in a network,