going junction are released. 32.5.2 System 12 evolution 32.4.3.4 Supervision The studies, which lead to the System 12 configuration, were based Once a connection has been set up, the MCU is released for the on two main premises: setting up of other calls and the established connection is monitored and supervised by the supervisory processing unit (SPU) in con- Digital systems should from the start be designed not just for junction with either a bridge link circuit for locally terminating calls voice traffic but also for an ISDN environment. or an outgoing junction circuit for outgoing calls. The bridge link 2. New semiconductor technology (LSI, VLSI) became available circuits apply the necessary ringing current and tones required. during the 1980s providing ample computing power and massive memory storage capability. 32.4.3.5 Feature evolution During this period also the end controlled digital switching net- work (DSN) was conceived and its basic LSI component (switch The TXE4 system provides service to some eight million lines, port) designed. approximately a third of the U.K. network and it has been necessary By the end of 1981 a first system was installed in the Belgian over the years to enhance the features provided to match those network and the next year 4 additional System 12 exchanges were provided by the newer digital systems. This has been achieved by installed in Germany. the addition of specific functional processors interconnected by an Subsequently, System 12 product versions were quickly intro- Ethernet local area network and connected to the TXE4 MCU and duced in several countries in parallel, first in various Alcatel n.v. home markets, then in export. The System 12's novel attractive This arrangement provides call logging facilities including both architecture aroused interest with many telephone companies and bulk and itemised billing; fast call set up using Common Channel administrations throughout the world. By 1991 over 15 million Signalling System No.7 to other TXE4 or digital exchanges; and