Number 4 electronic switching system
Also known as: No. 4 ESS
Quick Summary
The "No. 4 ESS" was a toll telephone switching system developed by AT&T and placed into service in 1976. It utilized time-division switching, a digital method where digitized voice signals are divided into time slots. These slots are then multiplexed, allowing multiple voice circuits to share a single data stream. Switching occurs by interchanging the time positions of these user slots. The No. 4 ESS was designed to handle a large capacity of 53,760 two-way trunk circuits, significantly advancing telephone network capabilities.1
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