er of variants on this theme, includ- ing how to access network elements that cannot themselves afford Q-adapters, as e.g. units of transmission and transport links. The physical model of TMN also dis- tinguishes between TMN nodes for management of physical nodes in the traffic system, and TMN nodes for network management. The former are called Opera- tion and Maintenance Centre (OMC), the latter Network Management Centre (NMC). For more details on TMN, see ITU-T recommendations in the M.3000 series. 3.5.5 Notations for Logical Networks While a collapsed logical network, as well as a logical network structure, only needs to show the type of network service interfaces, in a logical network model, logical nodes and network services interfaces are instances. Depending on the pur- pose of the model, it can be presented i a more or less generic fashion. It may only present a partial view, in which case it is primarily an instrument for functional analysis, for training and for specifying products of the implemented system. 3 Logical Network Modelling It may present one node of each type and all protocols of concern, in which case it is an instrument for design of concrete networks. It may present exact numbers of instances of each type of logical node, in which case it represents a concrete network. Such a model can be an instrument for simulation of network characteristics, or it can act as a resource model over part of a traffic system. In the latter case it is managed by the network manage- ment system (stored in the network MIB). Most models we have discussed are of the first kind. In this section we suggest a notation for logical networks that also covers the other two cases. The notation is based on the route concept, and on the reference point concept. Reference point is a popular concept in communication standards. In SM&AML it is a high level abstraction of communication relations between logical nodes. An extended version of our ISDN model will be used to show the notation for logical networks. The model in 