Strowger switching mechanism

Working principle

The Strowger (step-by-step) system uses two-motion selectors that move directly in step with the dial pulses sent by the subscriber (direct, progressive control).

     Bank: 10 levels x 10 contacts
   level 10 o o o o o o o o o o
     ...
   level 2  o o o o o o o o o o
   level 1  o o o o o o o o o o
            ^
     wipers │ vertical step (one per pulse)
            └─> rotary step (hunt or per pulse)

 Caller ─> Uniselector/line finder ─> 1st group
 selector ─> 2nd group selector ─> final selector
 ─> called line

1. The subscriber lifts the handset; a uniselector or line finder connects him to a free first group selector, which sends dial tone.
2. Dialling the first digit sends $n$ pulses; the vertical magnet raises the wiper shaft $n$ steps to level $n$.
3. In a group selector the wipers then rotate automatically across that level, hunting for a free outlet (trunk to the next selector). Each digit thus selects a group of lines.
4. The final selector (connector) uses the last two digits: the tens digit gives vertical steps and the units digit gives rotary steps, reaching the exact called line.
5. The final selector tests the line; if free it rings it, otherwise returns busy tone. On clearing, the release magnet returns all switches to normal.

Why it was the beginning of automatic switching

• It was the first system (patented 1891; first exchange at La Porte, 1892) in which calls were connected without a human operator, using only the pulses from the subscriber's dial.
• It introduced the rotary dial, numbered subscriber lines and selection by digits – the basis of all later numbering schemes.
• It introduced selectors arranged in stages (grouping, hunting, final selection) – the idea of a multistage switching network still used today.
• It gave privacy, 24-hour service and reduced operating cost, which led to world-wide replacement of manual exchanges.