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After 10 years research on the electronic switching system, Nippon Telephone and Telegraph (NTT) decided to develop the electronic switching system with stored program control in 1964. The Dendenkosha (NTT) electronic switching system for experiment DEX-I system was designed as the experimental space division unit to study the program control technique, and has the ferreed switch network and the central control of 4-μs memory cycle time. Another test system, the DEX-T1 system was also designed to apply the PCM technique to switching network. Lines are concentrated by the ferreed switch network controlled from the remote parent office and are switched through the pulse code modulation (PCM) switching network at the parent office. Both systems were installed in the Electrical Communication Laboratory, NTT, and tests were completed in 1967. From these experiments the DEX-2 system for the field trial was designed. Small crossbar switches and miniature relays were adopted to make the system smaller, and high-speed logical elements were used to improve the call-processing capability and simplicity of the central control, and the magnetic drum was introduced for the economy of memories. This system occupies only one-third of the space of the existing switching system and has a capability of about 30 000 lines.