r and other mobile networks). Many types of connectivity services support only one SAPI per network ter- mination. Network terminations can, however, be designed to support concur- rently and alternatively used TEU terminals, and in that case, it may make sense for the network to support multiple SAPIs per network termination as well. Typ- ical such cases are network terminations that are private automatic branch exchanges (PABX), ISDN network terminations (NT) or gateways to IP net- works. An important modelling aspect for communicating systems is the distinction between the communication dialogue (e.g. the teleservice protocol between TE- UDTs) and the bits that carry the dialogue. This distinction was first made in the OSI Reference Model, where the interaction elements of the dialogue were called PDU (protocol data unit), and the set of bits that represented a PDU was called SDU (service data unit). When this distinction is made, the communication dia- logue is described as a message protocol. This distinction is valid wherever two entities that must communicate are remote to each other. Remote often implies that there is a physical separation between the entities, e.g. because they reside in dif- ferent physical nodes in a network. To be correct, however, "remote" only means that the two entities communicate according to a message protocol, and thereby have the ability to be physically separated. Therefore, if two entities communi- cate by a message protocol, they can be allocated in the same or in different physi- cal nodes. The actual choice may be just a network configuration issue. The whole idea with a telecom system is to allow TEUs that are physically remote from each other, and which can connect to TE-SAPs, to communicate. This also implies that there will be a lot of entities inside the telecom system itself that need to communicate, since they are remote from each other. It is therefore neces- sary to also build communication systems inside the telecom system that can sup- port those communication 