rds by NI names or spec- ifications. 3.2 Logical Networks Logical Networks 3.2.1 Definitions, Parameters, and Tables The concept of "logical network" has been used from time to time in network standards, as well as in implementor's solution models. There exists, however, no formal definition of this concept (it is also not included in the OSI RM). For exam- ple, why is it "logical" and not physical? What then is a "physical" network? Why do we need to define logical networks? We spend a considerably amount of pages in this book to shed some light on these issues. When we discuss or describe a network system, we often refer to only a part of all functions that exist in an operator network. For example, the network system ATM is defined by the ATM switching layer and a number of ATM adaptation lay- ers (AAL). An operator who operates an ATM network must also implement a number of layers that together can serve the ATM part with transmission functions. Such parts could be a number of SDH layers (another network system) and, in addition, different types of transmission systems for access transmission. Since a particular transmission technology can be used over different kinds of physical media, the system of physical media on which the operator network relies consti- tutes another "layer of functions," that is not necessarily an instantiation of any of the other network systems. Another well-known example is the Internet: if we regard all specifications that are published as Internet standards, we have the standardized version of the Inter- net network system. The Internet consists of much more than that, however. For example, without all the networks that are used for connecting IP nodes to each other (PSTN, ISDN, FR, ATM, SDH, IEEE LANs, DSL technologies, etc.) and used as transmission systems, there would not be an Internet. All these supporting networks are instantiations of other network systems. Furthermore, what users nor- mally perceive as Internet functions are a very large. number of services which are not 