hing actors from agent layers and strata indicates what is of importance on the network level (see Fig. 2.44). Call-handling VC/VP Media layer control layer stratum SDH path MTP L2 control layer BRA agent layer I.430 PRA agent layer 1.431 Circuit-switching Circuit- actor layer switching ATM stratum stratum 1.363 Nx64K SDH stratum G.707 PDH multiplexing layer Fig. 2.44 The layer structure of the ISDN circuit-switching stratum. Some relevant ITU-T proto- col standards are shown The model shows all major agent layers that are used in an ISDN circuit-switch- ing stratum (BRA and PRA to connect terminals to the ISDN network; ATM, SDH, and PDH to connect switching nodes within the ISDN). Agent layers and strata (except for ATM) interface physical media strata. 19 ATM and SDH strata sepa- rately controlled: a "connection" through an SDH stratum is called "path;" a "con- nection" through an ATM stratum is called "virtual circuit" (VC). A "virtual path" (VP), is a number of VCs that are switched in parallel. Submodels of packet switching actors (see Fig. 2.45), in particular for datagram switching, are considerably simpler than models for circuit switches. We choose 19 A circuit-switching stratum is therefore often regarded as existing on OSI layer 1. Since the telecom world always tries to align their standards to the OSI RM, this was a problem when SDH became defined since this network system had a layered structure in itself that also included cross connectors (a kind of circuit switches). It was therefore difficult (to say the least) to regard SDH as just an OSI physical layer. This led to an effort to define a separate layered functional structure for transport strata (see ITU-T G.805). G.805 also defines how to describe stratification since an SDH network can carry data for PDH and ATM networks, where the latter is also a kind of transport network. However, not only is the modeling technique suggested by G.805 more complex than AMLn, its applicability is restricted to synchronous transport net- works, such as SDH, P