ng logical nodes (many different logical nodes). This disintegration of an interface gives rise to the (conceptually) onion-like data structure of messages between physical nodes, that is well-known from the data communication world, see Fig. 2.48. This structure also causes the layered structure in models like the OSI Reference Model, i.e. defines network service interfaces and global SAPs. A useful interpretation of the model in Fig. 2.48 is that 2.3 Architectural Principles it shows a structure of messages that carry other messages: transport-level mes- sages carry CCS messages that carry the MAP messages. Transport data CCS signalling data service data Fig. 2.48 The structure of data in a message between an MSC and other nodes in the core network Thus, we have managed to separate the protocol of a network interface into three different protocols between logical nodes in different physical nodes. Although the OSI RM exhibits seven levels of logical nodes (called "layers"), it is important to remember that the disintegration technique in no way restricts or stipulates how many layers may be created. That is entirely a decision to be made by the architects of the particular sysnet, or (to be more precise) by the designers of protocols in net- work interfaces. Through this disintegration step we have identified logical nodes that are func- tionally less complex than the original physical node. Each logical node belongs to a distributed system of logical nodes, i.e. a logical network, in the D-dimension. A logical network can be handled relatively independently from other logical net- works, both from a specification point of view and from a network configuration point of view, since the only relations between logical networks are the global SAPs they use and offer. As long as the logical nodes of a logical network are not reallocated to other global SAPs, the logical network represents a permanently physically distributed function. In each network interface of a physical network, several logical networks will be 