ations of Sysnet Modelling There is a single interface between the message handler and the event handler in a protocol element, called event interface. Contrary to SAPs and connection end- points, this interface is not a data flow interface. It is an interface where the mes- sage handler offers a number of other services to the event handler, such as encoding, message formatting and reliable transport service. The view in the D-dimension of an event handler is an abstract machine. Inter- connected abstract machines constitute an abstract network. Communication in an abstract network is message transparent, which implies that the whole abstract net- work is described in a distribution-transparent way, as if it executed on a single processor and had no network communication problems whatsoever. This implies that an abstract network can be integrated into protocol networks in many ways, to suit a number of different distribution solutions. Disintegration step 4, abstract networks -> service networks: Abstract networks may still have a functional complexity that matches the com- plexity of logical networks. This is why we need an additional disintegration step. In order to understand step 4, we need a basic understanding of the difference between events and operations, depicted by Fig. 2.51. The model also introduces the AML symbol for relations between service objects, called service relation, and the user and provider roles in such a relation. A service object that connectRequest(SAPI,SAPI) A service object that requests the operation performs the operation connectPort connectConfirm(-) connectPort (and other operations) (and other operations) service relation event provider Fig. 2.51 Service object, service relation, event and operation Figure 2.51 shows that the previously mentioned events connec- tRequest(SAPI,SAPI) and connectConfirm(-), both refer to the same operation, con- nect. This operation is performed by the provider, who also responds to the user. Thus, if we focus on the operations that are the effect