DH, and OTN ("optical transport network"). 2.3 Agents and Actors however to show the slightly more complex submodel for X.25 virtual-circuit (VC) switching in Fig. 2.45 since there are more similarities in this model to circuit switching. VC control actor T-ROs invoke X.25 L3 {ITi, VCi, setOrd(in: REi,VCi; out: REi, VCi) relay {ITi, VCi, ...} terminate {REi, VCi, Agent Switching actor LAPBs An X.25 VC switching node Fig. 2.45 An X.25 VC switching node. CS=control store; T-RE=route endpoint table; T- ROs=routing table The model shows a switching node of an X.25 network. Such a node supports no users. It only relays data between LAPB links. Similar to circuit switching, there is a distinct difference between switching actors and control actors. Also similar is that the switching actor contains a kind of "control store" (CS), set by the VC-con- trol actor. The data in this CS defines how to relay incoming data of a VC on a par- ticular LAPB link to a VC on an outgoing LAPB link. In the actor-agent interface, LAPB layer interfaces are referred to by a neutral route-endpoint identifier (REi). The agent translates this identifier to a LAPB layer interface identifier through a special table called T-RE (more about these tables in Chap. 3). The purpose with REi is to isolate actors from knowing which connectivity layers are actually used by the agent. Specific to X.25 L3 is that this protocol serves both the switching actor and the VC control actor. Thus, the single agent supports two types of actor-agent inter- faces (SAG and SAC), which is why the protocol must include a parameter that identifies if a message contains data for the SAG or the SAC. We call a parameter that is used for discriminating between different interface types interface type identifier (ITi). The ITi in X.25 L3 is called "control bit" in the protocol specifica- tion. Note that X.25 L3 control elements communicate by an (implicit) abstract protocol since the X.25 protocol specification states explicitly all control messages. As a comparison, ATM (se