 a distributable system. It is distributable, with respect to a particular type of integration function. It is, however, not distributed, as long as it is not allocated in a logical network that provides such capabilities. An integrator is an element that is common for some or all service elements in a logical node. The effect of allowing a logical network to contain many protocol networks is that a logical node can contain many service elements that execute con- currently. Figure 5.2 shows the basic properties of a logical node with respect to the service elements it may contain. Logical node temporary existence permanent existence Service Service Service Service element, element, hent, Service type B type C element, type A Integration functions: - discrimination - association and Integrator connection handling message handling - relaying - instantiation Fig. 5.2 The functional contents of logical nodes A logical node can support service elements that are of different types (A, B and C in the model), as well as service elements that are of the same type (type A in the model). Furthermore, service elements may have a permanent (C in Fig. 5.2) or a tempo- rary (A and B in Fig. 5.2) existence in a logical node. Service elements in the same logical node are independent of each other, i.e. belong to different protocol networks. Service elements may, however, commu- nicate with other service elements in the same logical node over CSIs. Such 5.1 Introduction CSIs represent a partitioning of a larger protocol network, and should always be of the user-provider (client-server) type. Service elements that handle connectivity services provide the SAPs to other logical nodes, see Fig. 5.3, which shows two logical networks. The lower net- work consists of two protocol networks, interconnected over a CSI in one of the logical nodes. The protocol network in the middle is interconnected to the upper logical network over SAPs. Logical node Service Service element element Service Service Service element element element Service Ser