e con- trol point) which channels to connect. This information is stored in a control store (CS) in the switch-core actor. The functions of the model in Fig. 2.43 are otherwise: The BRA agent layer carries two types of channels, B (there are two of them: B1 and B2, both 64 kbps) and D (16 kbps). The BRA agent (BRA AG) terminates the protocol and delivers all channels to the BRA actor (BRA AC). The BRA AC decides on how to handle the channels, depending on the application. When the BRA AC is a part of a circuit-switching actor, D-channel data are extracted and delivered over a layer interface to the LAPD layer element, while B chan- nels are delivered to its local switching agent (SW AG). The same principle goes for the 2-Mbps PDH ("plesiochronous digital hierar- chy" which means "almost synchronous") agent layer. The PDH AC does not do much (besides handling synchronization and management functions). It delivers its thirty one 64-kbps channels to its SW AG. Some of these channels are con- nected dynamically through the switching actor to BRA or PRA AGs, or to other PDH (and SDH, "synchronous digital hierarchy") AGs. Other channels are connected semipermanently to the local MTP L1 AC. These channels are used for signaling purposes, similar to D channels. The reason for not letting the PDH layer element extract those channels is that a more flexible SS7 network results from controlling these channels over the local switching network. Furthermore, it is not necessary to define a specific channel in the PDH frame that must always be the signalling channel (as in BRA and PRA). Signal- ing capacity between two network elements can also be increased simply by using more than one channel for signaling. The MTP L1 AC terminates many streams (called "signaling data links" in SS7) over a layer interface (to the MTP L2 layer). 2 Layer Structures Realizations of circuit-switching actors are obviously complicated systems. On the standardized ISDN network level we should however not be concerned by that. The separation of switc