SXS
The Strowger switch was the first example of an electromechanical stepping switch telephone exchange system. It was invented by Almon Brown Strowger, and first patented in 1891. Because of its operational characteristics it is also known as a step-by-step (SXS) switch.
Read more about SXS: History, Patent Details, Development of The Strowger System, British Deployment, See Also