Agents and Actors Call handling Call handling MTP L2 MTP L2 Termi- nating actor Control store (CS): AAli, CHie AAli, CHi Switching actor PDH transmission BRA & PRA transmission End systems (ES) Local exchange (LE) Transit exchange (TR) Fig. 2.40 Agents and actors in a circuit-switching layer. AAli=actor-agent interface identifier; BRA=basic rate access; CHi=channel identifier; D=D-channel data; PRA=primary rate access; PDH-plesiochronous digital hierarchy; T=actor that terminates the switching layer; ML=media layer element This model shows a part of the circuit-switching layer in an PSTN/ISDN. The layer consists of two types of actors: (common) switching actors and two types of terminating actors (for BRA and PRA). The actors are interconnected over agent layers of the BRA, PRA and PDH types (a number of other types of agent layers may be used between switching actors as well, e.g., an SDH layer). The switching actors in local exchanges (LE) and in transit exchanges (TR) relay 64 kbps syn- chronous bit streams between any of their actor-agent interfaces. Switching actors in LEs are designed SO that they can connect a large number of agents on their "end-system side." These agents are either of the BRA or PRA types. LE switching actors also terminate signaling channels (called "D channels," used by the LAPD layer in DSS1) that are carried over BRAs and PRAs. On its "network side" an LE switching actor connects a smaller number of PDH agents, for example. It also ter- minates signaling channels (called "data signaling links," used by the MTP L2 layer in SS7) to other switching actors of LEs and TRs. TR switching actors are basically identical, except that they do not connect BRAs and PRAs. A terminating actor (T) of the switching layer exists in an end- system and is connected to a BRA or a PRA. From the ISDN point of view, these are not switching elements. They just terminate ISDN user-data (at media-layer elements) and D channels (at MTP L2-layer elements). Note, however, that a termi- nating actor may be realize