Step-by-Step Switch

Part of Telephony

The Strowger switching mechanism — the first widely deployed automatic telephone exchange, where each dial pulse directly moves a rotary selector one step at a time.

Why This Matters

The step-by-step (SxS) switch is where automatic telephone exchange begins. Invented by Almon Strowger in 1889 (a Kansas City undertaker who suspected telephone operators of routing calls to his competitor), the Strowger switch replaced human operators with pure electromechanics. A subscriber could dial a number directly and the switch would connect the call without human intervention.

Understanding the step-by-step mechanism is valuable beyond historical interest. It demonstrates the fundamental principle of distributed switching: each switch in the chain acts independently on the dial pulses it receives, moving one position per pulse. This distributed intelligence eliminated the need for any central controller — each switch made its own connection decision based on the local pulse count. The Strowger exchange could scale to hundreds or thousands of lines with no central computer.

For a post-collapse telephone network with resources to build a modest automated exchange, step-by-step switching is the most mechanically constructible option. No electronics are required — only mechanical precision and reliable relay logic.

Strowger Switch Mechanics

The Strowger switch (also called a two-motion selector) moves in two dimensions: up and out. A cylindrical rack of 10 levels × 10 positions provides 100 contact points. An arm rotates around the inside of the cylinder, selecting the level (vertical position) and the arc position (rotational position).

Vertical motion (level hunting): When the subscriber dials the first digit, each dial pulse steps the selector arm upward one level. Dialing “4” raises the arm 4 levels. The arm is now at level 4 of the bank.

Rotational motion (arc hunting): After the vertical motion completes (a timing delay allows the dial to finish), the second digit’s pulses rotate the arm around the arc at level 4. Each pulse rotates to the next contact. Dialing “7” as the second digit rotates the arm 7 positions clockwise to contact 4-7.

The arm’s current path (wipers) makes electrical contact with the selected bank terminal at position 4-7. This terminal connects to the next stage of the switch or to the subscriber line if this is the final selector stage.

A two-stage exchange with two-digit dialing provides 10 × 10 = 100 possible connections. A three-stage exchange with three-digit dialing provides 1,000. Real Strowger exchanges used 2-5 stages depending on network size.

Relay Logic and Timing

The Strowger switch requires no centralized controller because the dial pulses drive the switch directly. Each loop disconnect pulse (described in the signaling article) steps the appropriate selector one position. The mechanism is:

1. Subscriber goes off-hook — first selector hunts for a free path and connects
2. Subscriber dials first digit — first selector steps vertically (one step per pulse)
3. After inter-digit pause — first selector locks on selected level
4. First selector connects to a second selector
5. Subscriber dials second digit — second selector rotates (one pulse per step)
6. After all digits dialed — final selector connects to the called subscriber’s line
7. Exchange rings the called subscriber while completing the voice path
8. Called subscriber answers — exchange detects loop current and connects voice circuit

The timing between vertical (step) and rotational (arc) phases uses a simple ratchet mechanism: the vertical ratchet disengages when the dial pauses (inter-digit gap), and a spring-loaded governor completes the arc motion.

For a relay-based implementation (without the mechanical Strowger housing), each selection level requires one relay per position. A first-selector with 10 vertical positions and 10 arc positions requires 100 contact relays, 10 level selecting relays, and 10 position holding relays — 120 relays for one selector stage, with wiring complexity multiplying accordingly.

Blocking and Busy Signals

In step-by-step exchanges, call blocking occurs at any stage where all outgoing trunks are busy. The selector hunts through the available trunks at the selected level; if all 10 positions at that level are occupied, the selector cannot find a free path and the call is blocked.

The blocked subscriber receives a fast busy tone (all circuits busy) generated when the final selector or an intermediate selector completes its sweep without finding a free trunk. The subscriber must hang up and redial after a delay.

Blocking probability depends on the ratio of traffic to available trunks. Engineering practice for step-by-step systems sizes each stage to keep blocking below 1% during peak hours.

Maintenance Characteristics

Step-by-step switches are maintainable with basic mechanical skills and minimal electronics knowledge. Moving parts wear — the ratchet pawls, the rotating arm contacts, and the bank contacts all experience millions of operations per year and require periodic inspection.

The main failure modes are: worn ratchet teeth (causing missed steps — dialing “7” connects to position 6 or 8), oxidized bank contacts (causing high-resistance connections or no-connections at specific positions), and bent wipers (causing intermittent contact). All are correctable with basic hand tools and replacement parts machined from brass or bronze.

A step-by-step exchange can be maintained by a non-specialist who understands the mechanism, compared to crossbar or electronic exchanges that require more sophisticated test equipment and technical knowledge. This maintenance accessibility is a significant advantage for post-collapse operation.