Abbreviated as SxS.

"An automatic switching system in which a call is extended progressively step-by-step to the desired terminal under direct control of pulses from a customer's dial or from a sender."
-Engineering and Operations in the Bell System, 1977

A Step by Step System is an outdated type of telephone routing. As of 1977, about 34% of all telephone lines in the US were served by this type of switching. Since then, the percentage has dropped closer to zero. This is largely due to the fact that Step by Step switching works only with pulse dialing and does not handle things like tone dialing, which are basic requirements these days.

Step by Step switching is a direct progressive control system. This means that the dialing pulses generated by a telephone directly control the stepping switches, and each successive signal affects the route through the system. A Step by Step switch is an electromechanical device, meaning that connections are made using rotary stepping motors and contact brushes. This makes them cheap to manufacture and easy to expand, but very hard to upgrade for new functions.

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