Post-war and on to 1975 (continued)

The introduction of crossbar into the existing mixed step by step and manual system necessarily involved a re-design of the network and a re-evaluation of many existing concepts and practices. One controlling factor in the process was the speed with which the Australian manufacturers of exchange equipment could move to production of the new type of equipment under licence from L. M. Ericsson and the latter company could establish its own manufacturing facilities, issues which were the subject of extensive negotiations and included a decision to phase out step by step manufacturing within three years.

One key requirement for each of the capital cities was a numbering scheme, not only for the next twenty years, including the ELSA extension, but also with a capacity to evolve to satisfy whatever growth patterns might actually emerge. Another was that any subscriber could initiate and receive calls to and from any other subscriber or, put more formally, it was necessary to have a switching network for step by step origins, with access to both step and crossbar destinations and a switching network for crossbar origins with access to both step and crossbar destinations. Thus tandem switching, which had been an important factor since the development of the early manual networks, remained a pivotal consideration in developing register controlled ones. Australia had no relevant experience in register controlled switching, nor were there overseas networks sufficiently similar to serve as useful models and, after extended analysis, the structures adopted included elements from both the hierarchical trunk network and from step by step practices. It soon became clear that the early establishment of crossbar tandem networks was required, as it was inefficient to carry most of the tandem switched traffic via the step by step tandems.

The three stage group selector which was specifically designed to give the greater availability needed in large exchanges and to which the APO made a significant contribution, was quite extensively used in both terminal exchanges and tandems.[38] REG-LP originally proposed as the provincial register was found to offer better facilities at lower cost than REG-LM and became the preferred metropolitan register.

Development of STD from metropolitan exchanges was inhibited by the presence of considerable quantities of pre-2000 type switching equipment in some of the capitals and this eventually lead to plans for accelerated replacement as the economics offered by STD grew and public demand for the facility increased.[39] Initially STD was limited to crossbar and to step by step main exchanges, as DSRs had been designed to give access to a manual exchange on level 0 and could neither repeat dialling on this level nor pass multi-metering signals. Modifications were therefore developed by the Circuit Laboratories and all DSRs in capital cities were upgraded to give STD capabilities.

Australian Academy of Technological Sciences and Engineering