Strowger switches

Strowger Switches: The Mechanical Marvel of Early Telephone Routing

Introduction

The Strowger switch, also known as the step-by-step switch, was a groundbreaking electromechanical device that automated telephone exchange routing. Developed by Almon Brown Strowger in 1889, it represented a pivotal moment in the history of telecommunications, shifting control of call connections from human operators to a mechanical system. This article provides a comprehensive overview of the Strowger switch, its underlying principles, evolution, impact, and eventual obsolescence, geared towards beginners with little to no prior knowledge of telephony. Understanding the Strowger switch offers valuable insight into the foundational technologies that underpin modern communication networks. It is a beautiful example of ingenious engineering solving a very real problem – and a problem born of distrust!

The Problem Strowger Faced: Operator Bias

Almon Strowger, an undertaker in Kansas City, Missouri, believed that the local telephone operators were deliberately misdirecting calls to his competitors. The story goes that his competitor's son was a telephone operator, and she was actively steering business away from Strowger. While the veracity of this specific claim is debated, it highlighted a genuine issue: the potential for human bias and error in manually operated telephone exchanges. In the late 19th century, telephone exchanges relied heavily on operators who physically connected calls by plugging wires into switchboards. This system, while functional, was susceptible to both intentional and unintentional manipulation. Strowger envisioned a system where the subscriber *directly* controlled the routing of their calls, eliminating the operator as an intermediary and therefore, the potential for interference. This is where the concept of automatic telephone exchanges began.

The Core Principle: Step-by-Step Switching

The Strowger switch operates on the principle of *step-by-step switching*. This means the call connection is established in stages, with each stage selecting a new level of the switching system until the desired destination is reached. Imagine a multi-level building. To get to a specific office, you wouldn't go directly from the lobby; you'd first take an elevator to the correct floor, then walk down a hallway to the specific office. The Strowger switch works similarly.

The key component of the Strowger switch is a series of electrically operated “selectors.” Each selector is essentially a rotary switch with multiple contacts. These selectors are arranged in stages - *first selectors*, *second selectors*, and so on – corresponding to the digits dialed by the subscriber. Each digit dialed causes the selector to step mechanically, seeking a free path to the next stage.

How a Strowger Switch Works: A Detailed Walkthrough

Let's break down the process of a call using a Strowger switch, step-by-step:

1. **Dialing the First Digit:** When a subscriber dials the first digit of a telephone number, it sends a series of electrical pulses corresponding to that digit. In the original Strowger system, these pulses were generated by a rotary dial. Each pulse would briefly energize an electromagnet in the first selector. 2. **First Selector Operation:** The electromagnet in the first selector would pull down a lever, causing the selector to rotate one step. The selector continues to rotate, step-by-step, for each pulse corresponding to the dialed digit. For example, dialing '3' would cause the selector to rotate three steps. 3. **Finding an Idle Path:** The selector’s rotation continues until it finds an *idle* contact – a contact that isn't currently occupied by another call. When an idle contact is found, the lever locks into position, completing the electrical circuit and signaling that the first digit has been successfully processed. 4. **Second Selector and Subsequent Stages:** The completed circuit then activates the next selector in the sequence (the second selector). This selector operates in the same manner, responding to the second digit dialed. This process repeats for each digit in the telephone number. The number of selectors required depended on the number of subscribers in the exchange. Larger exchanges required more selectors and more stages. 5. **Final Selector & Connection:** The final selector in the sequence connects the caller's line to the line of the dialed party. This establishes the complete telephone connection. 6. **Talk Path Establishment:** Once the final selector is locked, a separate set of contacts establishes the *talk path* – the circuit that carries the actual voice signal between the two parties.

This entire process happens relatively quickly, although it's significantly slower than modern digital switching. The 'clicking' sound heard during dialing was the sound of the selectors stepping. This sound became iconic of the era.

Components of a Strowger Switch System

Several key components worked together to create a functional Strowger switch system:

**Selectors:** As described above, the core switching element. They come in different levels (first, second, etc.) and sizes based on the exchange's capacity.
**Rotary Dial:** The user interface for inputting the telephone number. It generated the pulses that controlled the selectors. Pulse dialing was the standard method.
**Holders:** Mechanical devices used to hold the selectors in position once they had locked onto an idle path.
**Relays:** Electrically operated switches used to control the flow of current to various parts of the system.
**Supervisory Circuits:** Circuits that monitored the status of calls (on-hook, off-hook, ringing, etc.) and controlled the ringing signal.
**Line Finders:** First-stage selectors responsible for finding an idle line to connect the caller to the switching network.
**Talk Batteries:** Provided the DC power necessary for speech transmission.
**Switching Frames:** The physical structure that housed all the selectors, relays, and other components. These were massive, room-sized structures.

Evolution of the Strowger Switch: From Mechanical to Electromechanical

The initial Strowger switch was purely mechanical. However, over time, improvements were made, incorporating electrical components to enhance reliability and speed. These improvements led to the development of *electromechanical* Strowger switches.

**Step-by-Step (SX) Switches:** The original mechanical design.
**Crossbar Switching:** A later development that replaced mechanical selectors with a grid of electrical switches. While still electromechanical, it was significantly faster and more efficient than the original Strowger design. This was a migration path toward fully electronic systems.
**Panel Switching:** Another electromechanical system that improved upon the Strowger design, offering increased capacity and flexibility.
**Electronic Switching Systems:** The eventual replacement for Strowger switches, utilizing transistors and integrated circuits for vastly improved performance. Time-Division Multiplexing (TDM) and other digital technologies revolutionized telephone networks.

Advantages and Disadvantages of Strowger Switches

Like any technology, the Strowger switch had its strengths and weaknesses:

- Advantages:**

**Subscriber Control:** Eliminated the need for operator intervention, addressing Strowger’s original concern.
**Reliability:** The mechanical nature of the switch made it relatively robust and reliable for its time.
**Scalability (to a point):** Exchanges could be expanded by adding more selectors, although scalability was limited.
**Simplicity (conceptually):** The step-by-step principle was relatively easy to understand.

- Disadvantages:**

**Slow Speed:** The mechanical movement of the selectors was slow, resulting in delays in establishing connections.
**Limited Capacity:** The number of subscribers that could be supported by an exchange was limited by the physical size and complexity of the switching equipment.
**Maintenance:** Mechanical components were prone to wear and tear, requiring frequent maintenance and adjustments.
**Blocking:** In busy periods, calls could be *blocked* if all available paths were occupied. This meant a temporary inability to connect. Erlang B formula was used to calculate blocking probabilities.
**Complexity (physically):** While conceptually simple, the physical implementation was incredibly complex, requiring large amounts of space and skilled technicians.
**Inefficient Use of Paths:** The step-by-step nature often meant that calls took longer paths than necessary, reducing overall network efficiency.

Impact and Legacy

The Strowger switch had a profound impact on the development of telecommunications. It paved the way for automated telephone networks, enabling the rapid growth of the telephone system and connecting people across vast distances. It represented a fundamental shift from human-operated to machine-operated switching, a crucial step toward the modern communication infrastructure we rely on today. The principles learned from the Strowger switch influenced the design of subsequent switching technologies, including crossbar and electronic switching systems.

While largely obsolete today, the Strowger switch remains a significant historical artifact, representing a remarkable feat of engineering ingenuity. It serves as a reminder of the challenges faced by early telecommunications engineers and the innovative solutions they developed. Museums and collectors often preserve working examples of Strowger switches, allowing visitors to witness this fascinating technology in action.

Obsolescence and Replacement

By the latter half of the 20th century, Strowger switches were gradually replaced by more advanced electronic switching systems. The advantages of electronic switching – speed, capacity, reliability, and flexibility – were simply too significant to ignore. Stored-program control (SPC) became the dominant paradigm, allowing for more sophisticated call routing and features.

The transition was a slow process, often occurring incrementally. Exchanges were upgraded one stage at a time, integrating electronic components alongside the existing Strowger switches. Eventually, entire exchanges were replaced with fully electronic systems. The last Strowger exchange in North America was retired in 1983, marking the end of an era.

Further Exploration & Resources

**The Telephone History Website:** [1](https://www.telephonhistory.org/strowger.html)
**YouTube - Strowger Switch Demonstration:** [2](https://m.youtube.com/watch?v=J-i6x2pA15g)
**Bell Labs - Strowger Switch:** [3](https://www.bell-labs.com/news/story/strowger-switch/)
**Antique Telephone Collectors Association:** [4](https://www.atca.org/)

Related Concepts and Technologies

Trading & Financial Concepts (For Educational Purposes - Relate to System Reliability & Network Effects)

While seemingly unrelated, principles observable in the Strowger switch's evolution can be metaphorically applied to financial markets.

**Network Effects:** The value of the telephone network (and thus, the Strowger switch) increased exponentially with each new subscriber. Similar network effects are seen in social media platforms and cryptocurrency adoption.
**Technological Disruption:** Electronic switching disrupted the Strowger switch market. Similarly, fintech innovations disrupt traditional banking. Understanding disruptive innovation is vital.
**Risk Management:** The blocking problem in Strowger switches highlighted the need for redundancy and capacity planning. In trading, diversification and position sizing are risk management techniques.
**Efficiency & Optimization:** The transition from mechanical to electronic switching aimed for greater efficiency. Traders constantly seek to optimize their strategies using algorithmic trading and backtesting.
**Trend Following:** The shift *away* from Strowger switches was a clear trend. Identifying and following market trends is a core trend trading strategy.
**Mean Reversion:** While the overall trend was towards electronic systems, short-term fluctuations in demand for Strowger switch maintenance might have presented opportunities for "mean reversion" trading strategies in related industries.
**Volatility:** The introduction of new switching technologies (and the associated disruptions) likely created periods of volatility in the telecommunications industry. Volatility trading strategies capitalize on price swings.
**Correlation:** The demand for telephone lines and the growth of businesses were likely positively correlated. Understanding correlation analysis is crucial for portfolio construction.
**Fibonacci Retracements:** While not directly applicable, the concept of retracements can be metaphorically linked to temporary setbacks in the adoption of new technologies (like electronic switching) before the ultimate trend continues.
**Moving Averages:** Smoothing out the data of switch adoption rates could be visualized using moving averages to identify the underlying trend.
**MACD (Moving Average Convergence Divergence):** A technical indicator that shows the relationship between two moving averages of a security's price. Could conceptually be applied to track the momentum of switching technology adoption.
**RSI (Relative Strength Index):** Measures the magnitude of recent price changes to evaluate overbought or oversold conditions. Could metaphorically be used to assess the "strength" of the old technology versus the new.
**Bollinger Bands:** Plot bands around a moving average to indicate volatility. Useful for visualizing the range of adoption rates.
**Elliott Wave Theory:** Suggests that market prices move in specific patterns called "waves." Could be applied metaphorically to the cycles of technological innovation.
**Ichimoku Cloud:** A comprehensive indicator that defines support and resistance levels, momentum, and trend direction.
**Candlestick Patterns:** Identifying patterns in adoption rates using a "candlestick" representation of gains and losses in market share.
**Support and Resistance Levels:** Identifying key levels where demand for the old technology (Strowger switches) met resistance, or where demand for the new technology found support.
**Head and Shoulders Pattern:** A bearish reversal pattern, potentially applicable to the decline of the Strowger switch.
**Double Top/Bottom:** Another reversal pattern, indicating a potential change in the trend of technology adoption.
**Golden Cross/Death Cross:** Moving average crossovers that signal potential trend changes.

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