a stratum. The element of the internal structure that actually switches anything is the switch-core actor. The basic switching rate in ISDN is 64 kbps, intended for on- demand traditional telecom services and for residential use of datacom applica- tions. However, a number of 64-kbps channels can be switched in parallel, thereby providing higher bit rates (denoted as Nx64 kbps in the model). The standard defines N=2, 6, 24, and 30, i.e., 128, 384, 1536, and 1920 kbps switched channels. These channels provide unrestricted bit services and were intended primarily for video applications (e.g. video conferences) and bulk data transport between com- puters. 2.3 Agents and Actors The internal structure is (in this case) designed so that the local switching node can be reused in all types of switching nodes, and SO that it can connect several types of agent layers. The switch-core actor therefore defines a single type of (internal) specified actor interface, sac1, to which an (internal) multiplexing layer and network is connected. A sac1 can comprise up to several thousands of 64 kbps channels. Larger switching capacity can be achieved by adding more core switch- ing actors. The multiplexing layer/network creates a small set of suitable actor- agent interface types (sac21, sac22, etc.), to which terminating actors and external agent layers can be connected. Note that all actor-agent interfaces and internal net- work interfaces are designed according to suppliers proprietary specifications, i.e., only external network interfaces are standardized. The switch-core actor does not know if a stream goes out over an external agent layer or is terminated by the circuit-switching actor. The only element that knows that is the call-handling actor that orders the switch-core actor how to switch streams between channels in sac1. That is also why routing is performed in the call-handling actor, not in the switching layer. Once the call-handling actor has decided how to route a stream, it tells the switch-core actor (over the resourc