s a signaling transfer point (STP). The signaling links, STPs, and signaling (originating or destination) points may be combined in many different ways to form a signaling network. 14.8.2 International and National Signaling Networks The worldwide signaling network is structured into two functionally independent levels: international and national as shown in Figure 14.7. Such a structure allows a clear divi- sion of responsibility for signaling network management and permits numbering plans of signaling points of the international network and the different national networks to be independent of one another. An SP including an STP may be assigned to one of three categories: 1. National signaling point (NSP) (an STP), which belongs to the national signaling network (e.g., NSP1) and is identified by a signaling point code (OPC or DPC) according to the national numbering plan for signaling points. 14.9 SIGNALING PERFORMANCE - MESSAGE TRANSFER PART ISP, (NSP3) ISP2 (NSP4) NSP3 (ISP) (ISP2)NSP, Country 1 Country 2 International hierarchical level National hierarchical level International signalling point (ISP) National signalling point (NSP) Figure 14.7 International and national signaling networks. [From ITU-T Rec. Q.705, Figure 1/Q.705, page 2 (Ref. 9).] 2. International signaling point (ISP) (an STP), which belongs to the international signaling network (e.g., ISP3) and is identified by a signaling point code (OPC or DPC) according to the international numbering plan for signaling points. 3. A node that functions both as an international signaling point (STP) and a national signaling point (STP), and therefore belongs to both the international signaling network and a national signaling network and accordingly is identified by a specific signaling point code (OPC or DPC) in each of the signaling networks.³ If discrimination between international and nation signaling point codes is necessary at a signaling point, the network indicator is used. 14.9 SIGNALING PERFORMANCE - MESSAGE TRANSFER PART 14.9.1 Basic Performance