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has a more even quality and is more consistently reproducible; this is of importancehaving regard to the stringent requirements which have been placedon the individual apparatus units. The work of assembly, especially soldering,is greatly simplified, as all soldering points lie in one plane easily accessiblefor the soldering tool and clearly visible when inspection is carried out. Inthis way, an appreciable improvement in the quality of soldered joints can beexpected.The etched wiring board places completely new mechanical demands onthe electrical components. For example all the inductor and transformer typeshave been redesigned so that they are suitable for this new wiring technique.The plug and socket connexion between the wiring of the apparatus unit, viz.the etched wiring board and the bay cabling has also had to be redesignedowing to the introduction of the etched wiring board. In short, it can be saidthat starting from the etched wiring board, the design of the apparatus unitshas been completely changed.The miniaturization which is provided by the transistor, and to which ofcourse contributions have also been obtained from other steps forward inthe field of components-for example, better ferrite material-has meant a reductionof space requirement to a third of that for the earlier method of construction.This means that the optimum bay is no longer a tall double-sidedbay but is a single-sided bay of approximately half the depth. A logical treatmentof all the new points of view coupled with the experience obtained, forexample as regards maintenance, from the earlier method of construction hasled to a completely new design of all the mechanical details, even as regardsbay design, which are used in carrier systems.We are convinced that by dealing with problems consistently and by avoidingunsatisfactory compromises between earlier and new methods, a designof transmission material has been successfully obtained which does justice toall the demands placed by the customer and by a rational production on amodern method of construction.II. Mechanical DesignGeneralFor a layman, the mechanical design problems associated with the developmentof a new construction technique for transmission equipment may appearrelatively simple compared with, say, those in selector switch design, wherethe mechanical parts, mostly moving parts at that, are directly included in thesignal path, whereas in transmission equipment their main function is only tosupport and contain the electrical components and their wiring.That is correct as far as it goes, but the design problems are of quite adifferent kind. Transmission technique uses considerably higher frequenciesthan exchange technique, so that in the mechanical design much greater accounthas to be taken of electrical requirements, such as for instance screeningbetween different pieces of equipment or units. A further example is thatby its nature transmission equipment is composed of a great number of differenttypes of products, so that for a design to be economically competitiveit must fulfil stringent requirements for flexibility and universal applicabilityof its component parts.Previous Mechanical Design PrinciplesThe earlier construction practice for transmission equipment was to use adouble-sided 19-inch rack with a maximum height of 2 590 mm (8' 6"). The