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Demand for high-speed wide-band telecommunication services is rapidly growing due to the remarkable progress in office, factory, and laboratory automation. These services are expected to be first introduced in leased circuits and then extended, to high-speed switched services. This paper presents the users' demands for such high-speed services and the NTT design concepts for the high-speed digital data transmission system (HDDTS) in NTT to provide such high-speed leased circuit service. The ISDN user-network interface is adopted. The basic high-speed pointto-point transmission service started in October 1984, and enhanced leased circuit service involving multiplexed interfaces and muitipoint circuits will be introduced in the fall of 1985.