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Fig. 1 Traffic distribution by distance with the telephone service, where the community of interest centres primarily in the local service area, telex calls are predominantly made over long distances. A marked difference was noted between telephone and telex traffic distribution by distance and is illustrated in Figure 1. Table 1 shows the traffic development by type of call, and several additional characteristics of telex traffic can be noticed from this table. It will be observed that telex traffic is increasing at a very rapid rate. Further, it will be noted that there is a significant traffic flow between the telex service and the public telegram system. This "printergrani" load accounts for half as many switching operations at the present time as the trunk call component, although no longer growing so rapidly as more customers connect to telex. As previously observed, short distance traffic is relatively small. The relationship between subscriber lines and total traffic has also been examined, and it is obvious that the automatic telex network is a very heavily loaded network compared with a typical telephone exchange system; in fact, following studies made on the manual system, the automatic telex network was based on an average subscriber loading in the busy hour of 0.15 erlangs per subscriber. This compares with a typical loading per telephone subscriber in a metropolitan exchange area of 0.08 erlangs per subscriber. Table I. Telex Service Calls Growth After automatisation of the automatic telex network in June 1966, the distinction between "local" and "trunk" disappeared as all calls are metered by periodic pulses applied to the subscriber's meter. 22
In the design of common control switching systems, it is necessary to carefully estimate the number of switching transactions which will take place since this represents the switching load on the common control system. In computing common control equipment quantities, it was assumed that automatisation of the telex system would cause call holding times to decrease, and a call-holding time of effective trunk calls of 2.1 minutes was used. An allowance has, of course, to be made for ineffective calls since these represent a switching load even though the call cannot be successfully connected, usually because the called subscriber is busy. This factor, coupled with the much lower call holding time on printergram traffic, has led to an overall planning average for all classes of traffic, effective and ineffective, of 1.2 minutes. Number and Charging Principles The group charging plans for telephone purposes used in Australia are based on the community of interest characteristics and, because of the preponderance of calls over distances of less than 100 miles, a fairly accurate assessment of the distances involved by these calls is required. Consequently the telephone charging system tends to make charges relate fairly accurately to distance for the shorter distance calls. As shown in Figure 1, telex in Australia has quite different community of interest characteristics. The community of interest now tends to be between business firms in the different business centres, and because the big cities are more than 40 miles apart, most of the traffic occurs in this mileage category. Because there is so little traffic over short distances it is not so necessary to make charges relate accurately to distance for short distance calls. It was decided that advantage should be taken of this characteristic of telex traffic by establishing simple numbering and charging rules, which would lead to useful reductions in the cost of the system. In view of this a 5-digit closed numbering plan providing for the division of Australia into 44 charging areas, with charging discrimination obtained on examination of a maximum of 2 digits, was established. These charging areas, and the first and second Fig. 2 Charging areas and charging centres • Zone centre & charging point 90 Telex numbering prefix Charging area boundary State boundary 23
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