face Fig. 2.2 Symbols for connectivity service points (CSP) and control points (...CP) Some important comments on these symbols: The author has experienced that too many symbol types in a model do not help us understand the model. Therefore, we do not use the specialized CSP symbols in this book, only the general symbol at the top of the list. Note, however, that this symbol cannot tell which part is the user or which is the provider. In this book we use primarily the OSI RM convention to always place the user of layer's services above the provider. Also, to keep models as clean as possible, we do not suggest any special symbol for information services. The uCP symbol is used both as a control point and as A CSP is an interface where the only thing a user can do is to send and/or receive data. CPs are interfaces where a user might control or access any type of service, except sending and receiving data. This implies that, for example, a layer interface that provides a connection-ori- ented package service must be modeled as both a CSP and a CP (see the TCP in Fig. 2.2). 2.1 Concepts Based on the OSI RM a point where an information service is accessed (specification methods are the same in both cases). An rCP is an internal control point in a layer structure that is used to denote con- trol of resources that are used to provide on-demand services. For example, when an external user requests a connection through a switching network, the call-handling layer must use switching elements. Layer interfaces between a call-handling layer and a switching layer are then modeled as rCPs. Management control points (mrCP, mpCP, msCP) are used for managing resources in network systems. For example, in circuit-switching networks, a management system maintains routing tables that are used by the call-handling layer. The interface between the management layer and the routing table is then modeled as a management control point. We make a distinction between control points that exist in the managed system (mrCP), in between the mana