work conditions. of specific modules, as in Figure 32.6. The AXE system concept 32/13 System level 1 HW unit System level 2 Subsystem Regional SW unit level Programme Function block level Function unit level Central SW unit APT The switching system APZ The control system Programme Figure 32.6 Functional levels in the AXE system Software signals Hardware signals 32.8.3 Functional modularity Figure 32.7 Functional block structure Rapid developments in electronics technologies and of telecom- munication networks have imposed specific requirements on data belonging to other software units. In other words AXE provides switching systems. Functional modularity is a prerequisite if the inherent software security. system is to be introduced easily and effectively in a variety of Hardware units normally exist in numbers of identical devices different and changing environments. AXE is defined in terms of with the exchange. In a similar manner, regional software units are functions; what they do rather than how they are to be implemented. repeated in regional processors controlling a certain type of hard- Functions will not change in the future; implementation techniques ware unit. The central software units are stored in the central processor. The entire AXE system is a set of specified functions, im- Figure 32.8 illustrates the internal structure of a central software plemented in modules, or function blocks. These function blocks are unit. The software unit consists of a programme part, which contains grouped in subsystems. Each function block and subsystem is con- the logic and the ability to receive and transmit software signals, and sidered as a 'black box' at its specific level in the system hierarchy, a data part, which contains the stored information of the unit. This and has a defined interface to other function blocks and subsystems. information may be common or can be individual for the controlled Since each function block is defined by its interface with other hardware units. In the latter case, the current data