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A New Method of Construction forTransmission EquipmentE J ERIKSEN & K A X E L S O N, T EL E FON A KTI E BOL AG ET LM ERICSSON, STOCKHOLMUDC 621.395.44:62.002.2LME 102, 84The article and the subsequent one in this series will deal with L M Ericsson'snew method of construction for transmission equipment, A short surveyis given in this introductory article of the crucial factors in the decision to carryout a new design. Primarily, the desire has been to obtain an efficientof construction for the new transistorized group and supergroupmethodequipment.Furthermore, the article describes the general mechanical features of LMEricsson's new transmission equipment: design details, however, are only goneinto where these are of importance for the mechanical or electrical functioningor formaintenance.I. Background to DesignThe development of the toll telephone network which has taken place inthe last decade has everywhere been characterized by a violent and continuouslyintensified expansion. As an example of this, it can be mentioned thatfor the Swedish national network during the decade 1948-1958, the numberof circuit kilometres has increased threefold. It should here be added that noless than 50 % of the increase of the line network occurred in the latterthird of the period and that the proportion of carrier lines in the increasewas over 60 %.Fig. 1x 7786Channel amplifier of earlier construction (left)and new constructionAnother sign of the development which has taken place as regards thevolume in the field of telecommunications is that carrier systems for up to2 700 telephone circuits per system have been standardized by CCITT andthese have in fact been found by telephone administrations to be economi-114
cally attractive. This means that much carrier equipment has shown the samelarge increase. In this way it has become more and more necessary for telephoneadministrations to demand that equipment, while retaining its performanceas far as possible with reasonable technical outlay, fulfils the followingrequirements:1. Occupying small space2. Consuming low power3. Needing little maintenancePoint 3 has gained further current interest due to the ever-increasing lackof technically qualified staff.In 1958, the main features of the mechanical framework of L M Ericsson'snew method of construction for transmission equipment were ready for production.The method of construction is primarily intended for those parts ofthe carrier terminals where telephone channels are assembled into groups andsupergroups, i.e. for transmission equipment whose frequency does not exceed0.5 to 1.0 Mc/s.The reason why L M Ericsson have decided on carrying out the work ofa comprehensive new design only 10 years after the previous revision of themethod of construction took place, is to be found in the very great advanceswhich have occurred in the last decade, especially in the field of components.If these improvements were to be exploited to the full, this exploitationhad to occur in a rational and methodical manner. Only in this way couldfull justice be given to the requirements laid down for space occupied, powerconsumption and maintenance. The desired result would not have been obtainedby merely building in new components and using new manufacturingtechniques in the earlier method of construction.As a crude simplification, it can be said that two components especiallyhave been decisive in determining the commencement of the work of newdesign; these are the transistor and the etched wiring board.As far as the transistor is concerned, this component offers many advantageswhen compared with the electron tube. The transistor has small dimensionsand low power consumption, which means a correspondingly low dissipationand thereby provides the possibility of having a more compact methodof construction. It is a fact that in designs containing electron tubes, it is notalways possible to utilize the miniaturization which has occurred generally inthe field of components, just because the heat dissipation per bay side in sucha case would exceed the permissible value.It should be remembered that the transistor could not be considered as a designelement in complex telephone systems before the problem of operationalreliability and effective life was solved in a satisfactory manner. So far, notheoretical limits to the life of the transistor or the semiconductor diode havebeen found. With progress in the field of semiconductors, this should lead tothe possibility of producing transistors whose life is of the same order ofmagnitude as that of passive components.Progress has not yet gone so far. The arrival of types of transistors whichare more suitable to mass production such as transistors of the alloy type ordiffused type, together with an ever-increasing experience in the various manufacturingprocesses, has however already resulted in long-life transistors havinga better average life than that normally obtained with electron tubes. Thusthe way is open to make full use of all the advantages of the transistor in thefield of telecommunications.As mentioned previously, the demand for carrier equipment is ever increasing.This also naturally affects the production of this material whichmust be put into larger and larger production runs. The introduction ofetched wiring for the apparatus units is a necessary step to a more rationalmanufacture. As a by-product of this technique, a product is obtained which115
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