th functional elements (such as layer elements), and route symbols are chosen SO that routes cannot be confused with interfaces and protocols. 2.3.3.2 Submodels of Switching Actors A common actor relays data either between AAIs, or between a layer interface and an AAI. To show how these two forms of relaying are controlled requires a com- 2 Layer Structures mon actor submodel. Such submodels also reveal the difference between circuit switching and packet-switching actors. For example, the circuit switching ISDN- actor submodel in an LE may look like Fig. 2.43. Note that this kind of model exists on a lower network level than the one of Fig. 2.42, and is therefore normally first defined by implementors. Internal protocols External protocol handling (proprietary) MTP L2 (standardized) sac21 2x64K SW AG SW AG BRA AG Multi- plexing Switch- layer sac22 SW AG SW AG MTP LI 31x64K sac23 Nx64K SW AG SW AG PDH AG 31x64K Local switching node Switch interfaces Terminating node External NIs (internal NIs) Circuit switching actor Agent layers/strata Fig. 2.43 A circuit switching actor in an ISDN local exchange. AC=actor; AG=agent; D=channel for signaling data; K=kbps; NI=network interface; SAC,sac...=specified actor interfaces The model depicts a switching actor in a local exchange (LE) since it terminates a BRA agent layer (used for access; comprises two channels of 64 kbps) and a PDH agent layer (used for interconnecting switching nodes in the network core). In reality, an LE may connect a large number of BRA agent layers, many PRA agent layers (30 channels of 64 kbps each), and probably more than one core-network agent layer. All LE switching actors also connect thousands of PSTN access agent layers (one channel of 64 kbps each). The circuit-switching actor may also be used for terminating 64-kbps channels. Figure 2.43 shows one example: the MTP L1 actor. It terminates channels that are used by MTP L2-layer elements of the of SS7 stratum. Similar actors (not shown here) provide layer interfaces to layer elements of the medi