ype of layer. Circuit-switching actors (in PSTN, ISDN and cross-connecting systems) and connection-oriented packet-switching actors (in X.25, FR, and ATM) use a con- trol store table (T-CS) that defines how to connect a particular data stream between AAIs, also considered multichannel AAIs. Information in the T-CS is set and removed by a control layer, either in a management system (for cross- connecting systems) or in the traffic system (all other systems). Connection-less packet-switching actors use routing tables (T-ROs). Informa- tion in the table is either set by a management system (as for X.25 and MTP L3) or managed by special routing layers (as in IP). Asynchronous multiplexing actors use a table (T-L) that translates a link identi- fier (Li) on the multiplexed side to an AAI identifier (AAli) on the non-multi- plexed side, and vice versa. The table is managed by a management system. Synchronous multiplexing actors need no tables at all. How the information in T-CS, T-RO and T-L tables look depends on the type of relaying layer. The model below shows the circuit-switching case. The SAC primi- tives are just conceptual since channels are defined by their positions in frames. Each AAI comprises many channels, and the actor may connect one or several AAIs. Circuit switching: Control layer relayOrd{CHi,...} -implicit Single-AAI T-CS: CHie>CHi Circuit- Multiple-AAI T-CS: {AAli, CHi}<-{AAli, CHi) switching actor termOrd{CHi,..} --implicit The next model shows the connection-less (or "datagram") switching actor. Note that the layer element is normally both relaying and terminating, and that the agent will know its own network address (NA). The agent is also the terminating element in this case. Datagram switching: sendOrd Datagram- recRep switching T-ROs: NA-REi relayOrd{destNA,...} actor T-RE: REi-REdef T-ROs termOrd{destNA, REi, The following model shows the connection-oriented packet-switching actor. The agent may use several link layers (as in X.25) or a single cross-connecting layer (e.g., SDH as in ATM). T