DUs. The ICI contains some type of con- nection endpoint identifier (CEI) that identifies different subflows of SDUs. Examples of CSPs that define CON-SAPs are X.25, SCCP and ATM. A CNL-SAP is also a SAP that is based on packet switching. The IDUs of a CNL-SAP contain fixed or variable length SDUs. The ICI contains an address, i.e. some type of service access point identifier (SAPI). Example of CSPs that define CNL-SAPs are X.25, MTP, SCCP, IP and UDP. A CN-SAP is a SAP that is based on on-demand circuit switching. The corre- spondence to an IDU in this case is synchronous streams of bits. Discrimination between different CN connections (in case of multiple connections) is some kind of synchronization mechanism that works over the SAP. Example of CSPs that define CN-SAPs are PSTN and ISDN. A TA-SAP is a SAP that is not based on switching. It has otherwise similar properties as a single connection CN-SAP. A TR-SAP is a SAP that is not based on on-demand switching. It may, however, comprise channels that are circuit switched under the control of the operator of a transport network. Properties of a TR-SAP are similar to the properties of a multiple connection CN-SAP. An undefined-SAP symbol is also given, to be used as a general teleservice-SAP symbol, or in cases when the type of SAP is of no interest, is unknown, is given by the context or is given by a textual, user-defined annotation. There is always a user-provider (or "client-server") relation in a SAP. The rec- tangular part of the symbol belongs to the provider side, the other part to the user side. The user is the one who can send and receive data over the SAP. The provider is a logical node in the CSP that, in cooperation with other logical nodes in the CSP, performs data transfer between sending and receiving SAPs. In telecom systems we talk about interfaces for data flows as simplex, half- duplex or duplex. The SAP symbol covers part of this semantics. There is a symbol for both-way transfer (send and receive operations) that represents both duplex and h