 smaller then on the incoming side). Modelling Telecom Resource Systems 3.4.1 Introduction The only formal symbols used in this (and following) sections are different types of interface symbols. The type of unit in a model is defined indirectly by the type of interfaces it supports. As you may recall, AML only defines symbols for inter- faces, not for units. So, if you are more comfortable denoting, e.g. CSPs as ellipses and other types of units as circular shapes, or if you want to show physical nodes in white and logical nodes shaded, feel free to do SO. It is, of course, recommended that you use a single convention, at least within a particular sysnet model. In this chapter we will present models of different types of resources that are needed to model logical networks that provide connectivity services. We know some of the generic unit concepts for logical network modelling already (UDT, TPC, CSP, etc.). What is missing are resources that can transfer data flows, like links, multiplexers and switches. The important difference between units like UDTs and units that relay data is that the former terminate protocols, the latter do not. Relaying elements just take the bits on a channel in one link and relay them to a channel in another link, without interpreting what the bits mean. Protocol ele- ments, on the other hand, terminate messages, which means that they interpret all or some of the bits of a messages with respect to what the parameters of the mes- sage mean, and then act accordingly. Thus, units that relay data are not protocol elements. They belong to a different class of units, called relaying units. As background to the sections that follow, let's discuss how relaying was pre- sented in the OSI RM. When OSI RM defined the functions that were performed in 3 Logical Network Modelling the OSI network layer for data communication networks, it defined relaying as one of the functions of this layer. This implies, e.g. that (according to OSI RM) user data that terminate in a terminal system, must pass throug