 2.5 Layers that are identified by access-point identifiers (defined as SAPI in LAPD) and actor-layer identifiers (defined as "management entity identifier" (MEI) in LAPD) A second level of function separation is performed within the layer-management layer (see Fig. 2.5). 9 This layer defines a single message protocol and up to 256 different management functions that can operate simultaneously, independent of each other. The model separates the MEI agent layer from the management func- tions themselves. This separation is an example of actor-agent separation: all management functions rely on the same message protocol; an actor-layer identi- fier (here MEI) in the layer protocol provides discrimination between management functions (i.e., between actor layers). Note that although the LAPD standard defines 256 possible actor layers, SO far it only describes one: a function called "TEI assignment," (identified by MEI=15). In the OSI RM tradition, layer management is regarded as part of the layer that is managed, which is why many ITU-T protocol specifications are very complex and comprise several layers. However, since the protocols of the layer-management layer are specified independently from the LAPD transport protocol, it is better (both from a modelling and design maintenance point of view) to always separate the specification of a managed layer from specifications of the layers that manage its resources. This separation of functions is supported by the management dimension (M) and the plane concept defined in AMLn. Read more about this in Chap. 5. Therefore, when we discuss "protocols" in this book, we refer to layers in that sense. Furthermore, we will make a distinction between message protocols (such as the ones that describe the LAPD transport layer and the MEI agent layer), and This layer performs an autonomous management function (AMF) over ISDN accesses. It is therefore part of the ISDN management plane. Note, however that it exists in the ISDN traffic system. 2 Layer Structures abstract protocols (such 