- tocol between core actors and actors in terminals. The DSS1 L3 agent layers will still use LAPD for their connectivity needs. The problem of distinguishing between the two DSS1 L3 agent layers is solved by using different LAPD service-access points (there are still many unused). ISDN services ISDN services users requesters DSSI L3/S- agent layer Service-control- ISUP/S- actor layer agent layer Packet- common handling SAG lagent layer stratum Connection-control actor layer DSSI L3/C T-ROs agent layer Media Media agent actor Other resources MTP L3 Fig. 2.52 A solution for separating call handling into service control and connection control. SW=switching stratum The ISDN structure according to Figs. 2.51 and 2.52 still exhibits a rather com- plex network system, especially when compared to most packet-switching network systems. The corresponding X.25 model looks much simpler (see Fig. 2.53). VC users (user data) clear receive VC control actor layer X.25 L3 agent Switching actor layer X.21bis Fig. 2.53 The functional structure of the X.25 VC network system, separated into its control struc- ture (LAC) and connectivity structures (LCO) 2.4 Stratum Levels Note, however, that packet-switched network systems may have very complex functional structures as well, e.g., as the broadband ISDN which is based on ATM (we describe that structure in Sect. 6.4). The reason for the simplicity of X.25 is that it does not offer anything other than a raw packet service. The complex ISDN structure, on the other hand, was designed to become an evolution from the PSTN with the same (or even improved) requirements on isochronous services (telephony, etc.) as in PSTN. Consequently it was based on the same circuit-switching stratum as the PSTN. Since signaling within the call-handling layer requires asynchronous data transfer, however, one had to make all the layers of the DSS1 and SS7 strata part of ISDN as well. At the same time, the growing need for data communication over ISDN accesses was met by adding additional complexity in terms o