The ESS was the first electronic common control (ECC) circuit switch. Developed by AT&T Bell Telephone Laboratories with the assistance of Western Electric, and based on the transistor, invented at Bell Labs in 1948, the ESS involved a development effort that began in earnest in the early 1950s.The first ESS central office (CO) began service in Succasunna, New Jersey, on May 30, 1965, connecting 200 subscribers. By 1974, there were 475 such offices in service, serving 5.6 million subscribers.The development effort was estimated to involve 4,000 man-years and a total cost of $500 million. See also circuit switching, CO, and ECC.