In a major breakthrough for long-distance call completion time, AT&T launched the world?s first electronic digital switch, the 4ESS switch, in the Chicago network January 16, 1976.

The 4ESS, taking five years and costing over $400 million to design and build, could comfortably handle 500,000 calls per hour - 10 times the capacity of the electro-mechanical switch it replaced.

Customers noticed an immediate boost in service speed. The new switch combined with the related technology of common channel signaling dramatically improved long-distance completion time, which dropped from 10-20 seconds to a mere 1-2 seconds. The 4ESS delivered the intelligence, flexibility, and speed of a special purpose computer to the long-distance network.

The success of the 4ESS relied on a solid foundation going back to the 1930s, when AT&T research director Mervin Kelly challenged his team to find a solid-state replacement for electro-mechanical relays in telephone switches, up through the 1947 invention of the transistor, and the introduction into local service in 1965 of the first analog electronic switch, the 1ESS switch.

In 1999, AT&T installed the 145th and last 4ESS switch in the network.