 a local call, the MCU stores the directory number of the called control and to supervise the call during set up. Both 10 pps dial line. It then inspects the output of the cyclic store highways until by pulses and MF key signals can be accepted. The registers are comparison it sees the same digits that it has stored. System 12 overview 32/7 At the same time, the state-of-line information indicates whether enhancement process, which will make TXE4 exchanges almost the wanted line is free or busy. If it is free its equipment number and indistinguishable to customers from fully digital systems, is still in class of service are stored by the MCU, which will then apply with progress and is likely to continue for the life of the system which is the equipment numbers of both calling and called lines to the expected to continue well into the twenty-first century. interrogator markers for final path selection. This is done in the same way as for the connection from calling line to register. Again, the successful completion of the path will be detected by the regis- 32.5 System 12 overview ter, after which the original path between calling line and register is released. 32.5.1 Introduction Should the called line appear busy when it is scanned, the MCU immediately hunts for a free tone circuit and connects this to the calling line via the network to return busy tone. The System 12 functions are concentrated in different module types which are connected to a digital switching network. Each modular function is controlled by its own processor. Therefore the System 12 32.4.3.3 Outgoing call switching system is a modular system with fully distributed control, using the latest microprocessor technology and large memory capa- When the MCU examines the dialled digits received by the register cities. The switching network control functions in System 12 are and identifies that the call is outgoing from the exchange, it will, dispersed throughout the network. This results in an end-controlled where appropriate, apply with the initial 