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The majority of digital communications are now carried on the teletypewriter networks. New modulation techniques, higher transmission speeds, and more effective error control schemes are being provided in systems now being introduced in the digital communications area. A considerable body of theoretical work has been completed which provides a base for most of the activity in the field; however, application of much of this work, until recently, has often been uneconomical in the current state of the art. This is most notably true in the case of error control and the practical achievement of theoretical channel capacities, but the problem also arises in other areas. The work towards a truly automatic central data processing system has been slow, but a number of projects are now showing the way in this area. These include reservation systems, centralized banking, central inventory control, air traffic control, and, above all, centralized military communications and control systems.