Chapter 8
I Part 1: Communications
i Before the Telegraph
ii Electrical Communication Before Federation
iii Federation to the End of the Second World War
iv Post-war and on to 1975
v 1975 ONWARDS
II Epilogue
III Part 2: Early Australian Computers And Computing
IV Acknowledgements
References
Index
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Post-war and on to 1975 (continued)
In the years from 1945 to 1963 the capital city networks were extended in step by step switching equipment and grew to about four times their pre-war size, requiring major changes in the networks. The plan for Sydney, evolved pre-war, soon encountered numbering problems and various ways of growing to seven digits were examined and rejected in favour of a re-arrangement of main exchange areas to extend the life of a six digit system. The new numbering and switching plan, evolved under the leadership of E. Sawkins, had nine main exchange areas instead of eight and required changes in the switching and numbering of some 20 exchanges as well as the setting up of two new main exchanges at Redfern and Homebush, and with the CBD served by five exchanges in a co-main configuration. As under the plan level 0 was required for the Homebush main exchange area and therefore no longer available for the trunk exchange code, a study was made of the false traffic on level 1 and it was concluded that codes starting with 11 would be usable for trunk exchange use. A revised plan was produced for Melbourne in 1944[24] with provision for 57 exchanges and involved replacing three of the earlier main exchanges which had, of necessity, been placed in less than ideal locations. As with the Sydney scheme, it could be implemented gradually as need or opportunity dictated and was still incomplete by 1960. A feature was extensive use of alternative routing from DSRs to get more efficient trunking.
When the SE50 selector, an improved version of the 2000 type, was chosen for Australian manufacture the APO played a large part in circuit designs and, in particular, in the impulsing and testing circuits which incorporated refinements developed in the Circuit Laboratories.[25] The attention given to impulsing arose from the size and complexity of the Sydney and Melbourne networks in particular. Features of these networks were:
- They were physically larger than other step by step networks in the world
- Because of this, junction loop resistances were higher
- The proportion of calls which used three junctions in tandem was usually high
- There was a large amount of old equipment in Sydney
- Post-war plans envisaged extensive satellite working with up to five junctions in some calls
Loop resistance limits for acceptable signalling performance were established in the Circuit Labs, then under J. Harwood, for the planned network configurations, including satellite working, allowing the post-war network re-designs to be implemented and were a key factor in stretching the life of step by step in these networks to 1963, when crossbar was introduced. The trunking limitations of step by step were a continual irritation in this period and a variety of more flexible and/or economical discriminating switches were suggested and investigated and a number found limited use in the networks.
After the war carrier systems were used in local networks as short term relief, particularly where there were delays in providing a major conduit route, producing a requirement for special signalling relay sets. The introduction of negative impedance repeaters resulted in higher loop resistances on junctions and the BPO SCDC signalling system was adapted to Australian needs and other line signalling relay sets were designed for country networks where phantom circuits were used. In 1961 and 1962 there were trial installations of Siemens No. 17 and Siemens/Halske EMD direct control equipment, which were under consideration as the next type of equipment. Both used motor uni -selectors as 200 outlet group selectors and had some advantage over bi-motional systems but they lacked the versatility of crossbar, also under trial at that time, and no further installations were made. Since 1961 there has been very little change made to the existing step by step equipment and these have been concerned with interfacing to crossbar.
Organisations in Australian Science at Work - Australian Post Office (A.P.O.); L. M. Ericsson
People in Bright Sparcs - Harwood, J.; Sawkins, E.
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Australian Academy of Technological Sciences and Engineering |  |
© 1988 Print Edition pages 561 - 562, Online Edition 2000
Published by Australian Science and Technology Heritage Centre, using the Web Academic Resource Publisher
http://www.austehc.unimelb.edu.au/tia/546.html