ther physical nodes in the GSM core network3. The model to the left shows the NNF of the physical node MSC in the DC-plane and one of its net- work interfaces. GSM is one of the world-wide standards for mobile, cellular networks 2 Foundations of Sysnet Modelling service handling global SAP The NNF disintegration step 1 of the signalling physical global SAP Transport & integration step 4 relaying Network Physical media Message protocol interface con. endpoint (carries user-data) (carries no user-data) Fig. 2.47 Disintegration: physical node -> logical nodes of the NNF This interface is defined by the two major interface parts: one or several physical media connection endpoints and a message protocol. The messages that are sent and received by the MSC are extremely complex since they must carry information used to control low-level transmission functions (operating over the physical medium), as well as high-level service control. In between, the MSC must be able to distinguish between message data that are to be processed in the resource system part of the MSC, and data that are to be processed by the process control system part of the MSC. The only way to handle this complexity is to disintegrate the messages of the network interface into their data elements and allocate the handling of different data elements to different logical nodes, see the simplified model in Fig. 2.47, right. The real structure of logical nodes in an MSC in GSM is much more complex than can be shown in this figure. Basically, the data that are carried by the messages are disintegrated into three subsets: Data that control data transport between the MSC and other physical nodes in the Core Network. These data are created, terminated and relayed in the Trans- port&relaying logical node. Data that control transport of process control data between the MSC and its environment. These data are created, terminated and relayed in the CCS signal- ling logical node. Process control data; these data are created and terminated in the TCAP service handli