and these concepts provide the basis of a very powerful the destination subscriber. In selecting the destination subscriber, tool for defining network capability in a rigorous manner to enable however, it may be necessary to select the destination network. To interworking, quality of service and performance and routing accomplish this selection, the national (significant) number code studies to continue. field can be seen as comprising a national destination code (NDC) 44/6 The Integrated Services Digital Network Application Processes System Control Management Physical media Figure 44.3 Generic protocol block followed by a subscriber's number. The NDC code field will be User/network interfaces variable in length dependent upon the requirements of the destina- tion address. It can be used to select a destination network serving From a user's perspective, an ISDN is completely characterised by the destination subscriber or used in a trunk code format to route the the attributes that can be observed at an ISDN user-network inter- call over the destination network in the called country. The ISDN face, including physical, electromagnetic, protocol, service capa- address can be further expanded by the addition of an ISDN sub-ad- bility, maintenance and operation and performance characteristics. dress but this is not part of the network numbering plan. The full Recommendation 1.410 lists the requirements of the user-network structure of the ISDN address is shown in Figure 44.4. interface and outlines the scope to be covered in defining the interface characteristics and capabilities. A key objective in the definition of ISDN has been that a small set of compatible user/net- Country National work interfaces can economically support a wide range of user destination subscriber subaddress applications, equipment and configurations. To assist the definition number of ISDN user/network interfaces, the CCITT has produced a refer- ence model for user/network terminal arrangements, shown in Fig- ure 44.5, which is described in Re