three levels: the transmission, transport and connection level. A transmission CSP (TA-CSP) is defined only by TA-SAPs. Transport and connection CSPs (TR-CSP and CN- CSP) comprise both SAPs and CSIs, since these networks contain circuit switches that are controlled indirectly from CSIs on the boundary. The owners of transport CSIs are transport network operators, which means that users of TE-, CN- and PS- CSPs do not have access to these CSIs. The owners of the CSIs of a CN-CSP are logical nodes of teleservice logical networks. Connection-oriented packet-switched connectivity implies that each user has access to both a SAP and a CSI, see Fig. 3.13. This CSP model looks similar to the TE-CSP model, but is quite different from a functional point of view. In addition, the users and the CSP are named differently. The users of CON services are called CONU, the CSP is a CON-CSP. There are two main differences: each SAP is asso- ciated with a unique address (SAPI), and the CON-CSP supports discrimination of data over the SAP in terms of subflows, identified by a connection endpoint identi- fier (CEI). Data over the SAP come in SDUs or "packets"; each SDU is identified with a CEI. The end-to-end path of a subflow is regarded as a connection. (Not to 3 Logical Network Modelling be confused with circuit-switched connections. When in doubt, use the terms CN connection versus CON connection.) User protocol Contr. Contr. a SAPI b SAPI CON-CSI CON-CSP Fig. 3.13 The CSP model for connection-oriented packet services CON connections must be ordered before they can be used, which is the func- tion of the CON-CSI Another CSI is indicated between the controlling unit and the using unit of the CONU in Fig. 3.13. This CSI is used by the user to request con- nections, and by the controller to inform the user that a connection request has arrived from a remote CONU. The reason for separating the CSP user in a control units and using units is the same as for TE-CSP. The procedure for ordering CON connections is described in Sect. 3.3.2. 