f discrimination. An exceptional example is the parameter "payload type identification parameter" (PTI) in ATM messages, described in ITU-T I.361 (see Sect. 6.4.3 for an explanation). This parameter defines four types of discrimination in a single parameter of three bits. Such smart- coded parameters may save some bits in transmission, but they also cause severe interpretation problems on the part of the human interpreter. Agents and Actors 2.3.1 Introduction As a general principle (in AMLn as well in most supplier solutions), a layer ele- ment can always be separated into one or several actor elements and one or several (protocol) agent elements. In Fig. 2.14 a layer element of a simple control layer is depicted in this fashion. Using layer element Layer element Actor-agent interface (AAI) Message protocol Agent Layer Actor element Used layer element Fig. 2.14 Every layer element can be separated into one (or several) agents and one (or several) actors The interface between an actor and an agent is called actor-agent interface (AAI) and is invisible outside the very layer element. We use a special graphical symbol for this interface in order to distinguish it from layer interfaces. An agent element deals only with message reception and sending, which prima- rily implies formatting and encoding. Consequently, agents are the elements that use layer interfaces offering connectivity services. An agent may also deal with other message-oriented functions, such as message error control, segmentation, flow control, etc. Beyond that, an agent does nothing but informing its actor (over the AAI) about what a received message tells (in case of control layers) or hand some user data to the actor (in case of connectivity layers). Since this interface is local within the layer element, any suitable language can be used inside a layer ele- ment. Thus, which language actors and agents use over an AAI is independent of 2 Layer Structures the format and encoding of a received or sent message, i.e., actors are not con- cerned with 