h layer 3 on its way from layer 1 to layer 7. Figure 3.37 shows (in an intuitive notation) the OSI RM view on how data are transported through the OSI protocol stack. Application layer relaying function Transport layer Network layer layer Physical layer Fig. 3.37 Assumed data transport through an OSI network (according to OSI RM) Since, however, the layers are defined only through the protocols they terminate, and relaying is not a protocol function, you cannot allocate relaying to a layer that is defined by a protocol stack. This is a fundamental mistake in the OSI RM. Figure 3.38 shows how the OSI RM model should look, when protocol termination is separated from relaying. Application layer o message originating point T message terminating point Transport layer Network layer relay control relaying relaying relaying discrimination layer Physical layer Fig. 3.38 Modified model of data transport through an OSI-CON network In this model, the OSI RM view is corrected by separating the control flow for network layer messages from the user-data flow. The former originates in the net- 3.4 Modelling Telecom Resource Systems work layer, passes the link layer, the physical layer and terminates in the network layer. The latter originates in the application layer, passes the presentation layer, the session, (OSI)transport, link and physical layers and terminates in the applica- tion layer. Note that the user-data flow has need not pass through the network layer since, after the physical and link layers have done their job on an incoming flow (transmission, error control, fragmentation etc.), there is nothing more to do with the user-data flow than to pass it to the (OSI) transport layer (if in a terminal sys- tem) or relay it to another link (if in a switching system). Thus, as Fig. 3.38 shows, relaying is neither done in the link layer nor in the net- work layer. Relaying is a function that does not belong to the layered structure at all. In case of a connection-oriented network one can say that, in a switching system, rela