t tolerance and rapid isolation the possibility of remoting man machine communication terminals and identification of faults. or connecting the exchange to a Network Service Centre (NSC). Since the initial introduction of System 12 in public telephone Figure 32.3 shows the modular structure of System 12. networks, the range of its features and facilities has been signifi- cantly expanded: 32.6.1 System 12 hardware configuration 1. To extend the application range to very small sizes as required for rural networks, a Remote Subscriber Unit (RSU) was de- A System 12 exchange comprises a Digital Switching Network veloped. (DSN) and a range of modules and Auxiliary Control Elements 2. For national and international transit exchanges, a fully fea- (ACEs) which perform all the exchange control and signal process- tured Operator Position System called 12SO, was developed. As required for several advanced features and in view of the Each module comprises a standard Terminal Control Element quickly approaching ISDN in many networks, the CCITT No.7 (TCE) and a terminal which is specific to the function of the module. common channel signalling system was introduced in System Figure 32.4 illustrates the Terminal interface configuration. A further enhancement was the development of a centralised operations and maintenance facility for System 12 networks, 32.6.1.1 Digital Switching Network (DSN) called the Network Service Centre (NSC). The NSC greatly benefits from System 12's CCITT No.7 capability since the The DSN is a combined time and space switching network that No.7 links provide the basic means of communication between provides time and space selection at each of this switching nodes. the System 12 exchanges and the NSC. The DSN comprises standard Digital Switching Elements (DSEs), which are used for all stages of switching. Additional proof of System 12's flexibility is its application in various specialised configurations or entire specialised networks as used by governments and large institutions. Examples are ex- 32.