andles an element state machine (ESM) that is virtually contained in the actor. Actor-actor communication can therefore be described as communication between ESMs of the layer. We say that the system of such state machines in a layer is a layer state machine (LSM) (see Fig. 2.21 for our DSS1 L3 example). This is an instance of an ASP (an "event") Terminal user in the abstract protocol Call-handling actor in LE Call-handling actor in TE Call-handling connectRequest(..) Call-handling SETUP This is an DSSI L3 DSSI L3 abstract B-channel agent agent Terminal machine Resource Switching manager resources access actor actor Many LAPD Single LAPD layer interfaces layer interface Fig. 2.21 Actors contain element state machines (ESM) which communicate by instances of abstract service primitives (events) Note that since the actor also has functions that can handle actor-agent inter- faces, we must make a distinction between the actor and the state machine it "con- tains." Any system of state machines can, by using AMLs (see Appendix D or Muth (2001)), be described as an abstract machine system that defines state machines as abstract machines which communicate by instances of abstract serv- ice primitives (ASP; we frequently call an instance of an ASP event). When we 15 We assume that the reader has a general understanding of what a state machine is. If not, see Muth (2001) that gives an in-depth presentation of the state-machine concept in AMLs. 2.3 Agents and Actors describe the communication between abstract machine in terms of ASPs, we describe an abstract protocol. The ESMs are also the elements that handle resources. Since communication over rCPs is independent of DSS1 L3 agents (and the AAIs) as well as how resources are distributed, an ESM sees resources as (local) abstract machines only, as depicted in Fig. 2.22. Note that the ESM in a TE sees the terminal user as an abstract machine as well Terminal user Call-handling DSS1 L3 abstract protocol Call-handling ESM in LE ESM in TE DSS1L3 DSS1 L3 agent agent Terminal Sub