ol layers and con- nectivity layers by providing different layer interface symbols for these layer types, as depicted in Fig. 1.14. The layer interfaces represented in these models are of two classes: connectivity service points (...SP) and control points ( (...CP). A reasonably large set of symbols for denoting different types of layer interfaces within these classes are suggested in Sect. 2.1.1. A notation for denoting layers that combine control and transport func- tions is also included. 1.4.2.2 The Actor-Agent Separation A protocol in the OSI RM is defined as an unstructured description of what a layer element does when receiving a message. This may be a reasonable approach for protocols of non-switching connectivity layers (transmission and link layers), but not for switching and control layers. AMLn therefore separates layer elements in one part that handles message reception and sending (the agent part), and another part (the actor part) that takes actions as a result of users requests and reception of a correct message, as depicted in Fig. 1.15. Through this separation, agents handle interfaces to connectivity layers and actors handle interfaces to using layer ele- ments. Connectivity layers: Layer Layer Actor Agent Agent Actor element element This actor relays and- or terminates streams Control layers: Layer Layer Actor Agent Agent Actor element element This actor acts on received This actor acts on messages and on user events received messages Fig. 1.15 The actor-agent separation is equally applied on connectivity and control layers. AAI=actor-agent interface; SAC=specified actor interface; SAG=specified agent interface Messages carry data that are partly handled by actors, but actors are completely independent of how agents format and encode messages. The coupling between agents and actors is therefore very weak. The same actor could be used over differ- ent protocols, provided that the actor-agent interface (AAI) is kept intact, which is why agents and actors should always be modeled separately. AAIs