d as a switching actor locally (i.e., as a private switch). Circuit-switching actors do not themselves decide on how to switch streams. A separate call-handling layer (e.g., the one described in Sect. 2.3.2.4) decides that for every path through the switching layer, before the path can be used. Once the decision is taken, however, a call-handling-laye element stores its decision in the control store (Cs) of the actor as a mapping of an inlet to an outlet. Since a switch- ing actor has no knowledge of which agent layer is actually connected to an AAI, it defines an inlet or an outlet as a particular channel in a particular actor-agent inter- face, identified by an AAli and a CHi. 2 Layer Structures Since the specified actor interfaces (SAC) for circuit switching actors are syn- chronous, the relayOrder and terminateOrder primitives we defined in the previous multiplexing example, are implicit in this case. Since connectivity layer elements are separated into actor and agents, we can create an LPM representation of such a layer. The model in Fig. 2.41 shows the LPM of an ISDN switching layer. We have assumed that both PDH and SDH agent layers are used in the core of this layer. The PDH layers are assumed to be 1.5 and 2 Mbps point-to-point links (T1 or E1), while the SDH layer is supposed to be a cross-connecting layer (note the different symbols used to denote circuit switching and cross connection respectively). BRA or PRA PDH agent layer agent layer route BRA or PRA transmission transmission BRA or PRA agent layer transmission layer transmission BRA or PRA transmission routes SDH transmission TR LE Fig. 2.41 The circuit-switching LPM in an ISDN. Cs=control store From a structural point of view, this LPM model looks similar to the ISDN call- handling layer according to Fig. 2.36. There are, however, considerable differences supported by notations and annotations: The graphical symbol for actor-agent interfaces (the SAC symbol) is what for- mally defines this model as a model of a connectivity layer. Annotations b