ion link that implements the TA-UDT (as, e.g. a modem). Such a physical node is a PM-TS (physical media, PM), see Fig. 3.42. TA-TS TA-TS PM-TS Fig. 3.42 Mapping physical nodes on a transmission link In this figure, the terminal system to the left is mapped on the link in a way that shows that a terminal system may include (i.e., must implement) logical nodes that are part of the link (here the TA-UDT, which OSI RM would call "physical layer entity"). The physical nodes are, however, not part of the link. Note that the transmission link symbol hides the transmission protocol for users. It defines only the boundary of a transmission link and focuses on the transport properties of SAPs. Logical nodes (and physical nodes), on the other hand, depend on the protocols of different kinds of relaying units. This is an important observa- tion: all types of relaying units (and logical networks, in general) hide protocols from their users. This is what makes logical networks such effective modelling ele- ments in logical network modelling. It also makes a logical network structure a very stable part of an architecture. Theoretically, the complete functional structure 3.4 Modelling Telecom Resource Systems of a sysnet can be modelled as a structure of logical networks, before any distribu- tion analysis has been made and any protocols have been developed. A transmission network is a somewhat more complicate type of CSP, see Fig. 3.43. A transmission network TA-OM TA-OM Transmission protocol Physical media network Transmission protocol TA-OM TA-OM The transmission network CSP-symbol Type: Instance: N Number of links C Bitrate per link 2*(N-3) Fig. 3.43 The transmission network A transmission network is constructed by connecting TA-UDTs to the physical media connection endpoints of a physical media network 15 Such a network can appear in many ways. It might include fixed media connections, mobile media con- nections, multiple-access physical media connections (a ring, a bus, shared radio channels), point-to-point media (wires, 