ted. These serve the same purpose as when OSI RM defined prefix Ph for its physical layer, DI for its data link layer, etc. However, SM&AML defines another set of prefixes, since the OSI RM layer definitions are not embraced by SM&AML. The main reasons are repeated here: The OSI RM does not support the distinction between resource system and proc- ess control system (called "user plane" and "control plane" in ISDN). Among other things, it results in a network layer definition that is not productive for modelling. The OSI RM definition of SAPI as a unique attribute of every SAP does not hold in general. Only SAPs of CNL-CSPs have such individual attributes. The OSI RM seven layer structure is not general enough to be useful for logical network modelling. Above all, it is not recursive 18 and can therefore for describing the stratified nature of logical models for most physical networks. 18 Although not specifically discussed SO far, there are layers in the OSI RM that have frequently been questioned, in particular the presentation and session layers. For example, from the SM&AML point of view, we do not define the OSI RM presentation layer as a layer at all. This layer represents encoding of PDUs, a function that must be solved for every protocol, and therefore must exist in every layer (in SM&AML represented by the encoding function ENC). This and similar problems with the definition of the OSI RM transport, session and application layers all come down to the same basic problem: The lack of recurrency in the OSI RM. 3.5 Logical Network Structures The generic prefixes that are defined in AML represent a completely different logical structure, a logical network structure that, on a high generic level, repre- sents how real networks are built: TA (for transmission) is used as a generic prefix for units in transmission net- works. This prefix corresponds to the physical layer prefix, Ph, in OSI RM. TR (for transport) is used for units in transport networks. The concept "trans- port" is widely used to mean different t