ing modules and a digital subscriber lines (2B+D). Special LSMs also exists for alter- number of space switching modules. Time switching is performed 32/16 Telephone exchanges The Charging Subsystem, CHS, performs the task of charging calls or at least to collect raw data needed for charging purposes. The charging system can be used for either pulse metering, itemised billing or both. The Operation and Maintenance Subsystem, OMS, is equipped with a wide range of functions for supervision, fault locating, statistics and administration. AXE is designed to work without any external support system but necessary interfaces to interwork with centralised operation and maintenance systems are also provided. 32.9.2 The control system APZ Directly The control system architecture, with a central processor and a varying number of regional processors, is optimised for real time remotely data processing. The real time environment imposes high demands connected on the control system: calls appear in an irregular manner; a variety to GSS of call models must be supported; very short response times are LSM 1 required; and overload situations must be handled. All the specific requirements and demands of real time data processing are efficiently handled by the AXE distributed control solution; an expandable front end line up of small but fast proces- sors, controlled and co-ordinated by a central processor. Frequent call processing functions such as scanning operations and signal processing present stringent real time requirements. In AXE these functions have been assigned to special purpose regional processors. The central processor is free to handle the higher level processing functions. The two processor levels are shown in Figure LSM15 32.10. LSM Line Switch Module JTC Junctor Terminal Circuit LIC Line Circuit ETC Exchange Terminal Circuit TS Time Switch EMRPExtension Module Regional Processor 32.9.2.1 KRD Key Tone Receiver TSB Time Switch Bus APZ subsystems The Central Processor Subsystem, CPS, consists of a duplicated Figure 3