as the ones that describe communication inside the actor layers of the LAPD layer-management layer). A well-formed layer normally has a single, well-defined purpose. The LAPD transport layer in Fig. 2.5 is an example of such a layer: its only purpose is to trans- port data frames between other layer elements in ISDN terminals and in ISDN local exchanges. This basic function can be defined by a single protocol. However, there are innumerous situations where a layer must be described by several proto- cols, in particular for layers that perform some type of control function in a net- work. A typical layer of the kind is the call-handling layer in ISDN. Figure 2.6 gives an example of the intralayer structure for such a layer. INAP in TCAP DSS1 L3 handling handling layer DSS11 L3 MTPL3 TE TE LE/SSP TE TE Fig. 2.6 The call-handling layer in an ISDN. LE=local exchange; N=node dimension; SCP,SSP=node names defined in the "intelligent network" (IN) concept; TE=terminal; TR=transit exchange This layer is defined by at least three different protocol specifications: DSS1 L,3 ISUP (OSI type of message protocols), and INAP (an abstract protocol). Contrary to the LAPD standard, where a single protocol specification defines many layers, in this case we have a single layer that is described by many protocol specifica- tions. Note, however, that there exists no specification in ITU-T standards of a net- work-wide "call-handling layer," as depicted in Fig. 2.6. This brings us to the ques- tion of how to identify layers out of the mass of protocol specification documents that are published by ITU-T (and other standardization bodies). The key to this and other problems in modeling networks is to distinguish between the message han- dling that a layer element performs and the actions it takes as a result of receiving correct messages. This is the actor-agent separation principle (see Sect. 2.3). 2.1.2 Service Types and Layer Types In the OSI RM as well as in AMLn, services are offered over layer interfaces. The OSI RM is, however, so