provider). The concepts IDU, ICI and SDU are seldom used in modelling logical networks. The concept PDU is, however, often used, at least in modelling data communica- tion networks. In other contexts, the term message or "signal" is more common. Since it is commonly used in network modelling, AML allows the term "message" as a synonym to PDU. The term "signal" is not allowed in sysnet modelling. Since this term is frequently used in other modelling and implementation languages (e.g. in SDL, in UML and in my company's in-house language, Plex), AML regards "signal" as a concept of concrete languages. 3.2.3 The CSP in AML In this section we introduce the notations that are used for different types of net- work service interfaces. Sect. 3.2.4 summarizes all notations for network service interfaces. A CSP is a black-box model of a system that provides communication services. The black box only defines which type of network service interfaces are supported by the CSP. A CSP can be instantiated, in which case it defines all individual net- work service interfaces that are available to using logical nodes. A CSP has an 3 Logical Network Modelling internal structure that can be described in terms of internal CSPs and logical net- works, as shown in Fig 3.4. When the internal structure of logical networks is not revealed, protocols are not defined in the CSP model, since all protocols are defined as parts of the internal structure of logical networks, see e.g. Figs. 3.5-3.6. A CSP offers one or several types of communication services. In cases when the service must be ordered before it can be used, we separate the communication serv- ice in two components: the CSP access service, provided in a control service inter- face (CSI), and the connectivity service, provided in a service access point (SAP). This is, e.g the case for a CSP that provides teleservices, as shown in Fig. 3.11. Teleservice user Teleservice user User protocol access point for ordering a connectivity service access points for using the connectivity service