Strowger switch

1. Strowger Switch

The **Strowger switch**, also known as the **step-by-step switch**, was a crucial electromechanical device in the history of telephone exchange technology. Invented by Almon Brown Strowger in 1889, it automated telephone routing, eliminating the need for human telephone operators and fundamentally changing the way telephone calls were connected. This article will provide a detailed explanation of the Strowger switch, its operation, its historical context, its limitations, and its eventual replacement by more modern technologies. We will explore its impact on Telecommunications and its legacy in modern networking.

1. 1. Historical Context: The Problem Strowger Solved

Before the Strowger switch, connecting a telephone call required a human operator at a central exchange. The caller would lift the receiver and signal the operator, stating the number they wished to reach. The operator would then physically connect the caller’s line to the line of the called party using patch cords on a switchboard. While effective, this system had inherent limitations:

• **Scalability:** As telephone networks grew, the number of operators required increased proportionally, becoming a significant cost and logistical challenge.
• **Human Error:** Operators were susceptible to errors, leading to misconnections and frustration for callers.
• **Bias and Discrimination:** Almon Strowger, an undertaker, is famously credited with inventing the switch due to his belief that the local telephone operator, the wife of a rival undertaker, was deliberately misdirecting calls to his competitor. While this story is often romanticized, it highlights a genuine concern about potential bias in the manual system. He sought a system that was impartial and automated.
• **Speed:** Connecting calls manually was relatively slow, particularly during peak hours.

These limitations spurred the search for an automated telephone exchange system, and Strowger’s invention provided the first viable solution. He filed his first patent in 1891 and continued to refine his design over the following years. The initial implementation wasn’t perfect, but it proved the concept of automatic telephone switching was feasible.

1. 1. How the Strowger Switch Works: A Detailed Explanation

The Strowger switch is a complex electromechanical device, but its core principle is relatively straightforward. It utilizes a series of two-motion switches, each controlled by the digits dialed by the caller. Let's break down the process step-by-step:

1. **Dialing:** When the caller dials a digit, the telephone generates a series of electrical pulses corresponding to that digit. For example, the digit '1' might be represented by one pulse, '2' by two pulses, and so on, up to '0' which uses ten pulses. This signaling method is known as Pulse dialing. (Later, DTMF - Dual-Tone Multi-Frequency - dialing was introduced, providing a faster and more reliable signaling method, but the Strowger switch originally used pulse dialing).

2. **First Motion Switch (Units Position):** The pulses received from the dial activate the 'units position' switch. This switch is designed to move one step for each pulse received. The number of steps corresponds to the first digit of the dialed number. This switch selects a specific "bank" of switches. Think of it like selecting a floor in a building.

3. **Second Motion Switch (Tens Position):** After the first digit is completed, the switch waits for the next digit. The second digit activates the 'tens position' switch. This switch also moves one step for each pulse received, but it operates in a perpendicular direction to the units position switch. This selects a specific switch *within* the bank selected by the units position. This is like selecting an office number on that floor.

4. **Progressive Selection:** The Strowger switch uses a "progressive selection" approach. This means that the switches don't know the complete phone number in advance. They only react to each digit as it's dialed, moving incrementally to the correct position.

5. **Finding an Idle Path:** The switch seeks an idle path to the destination line. If the selected switch is already in use (i.e., another call is already connected through it), the switch "hunts" for the next available switch within that bank. This hunting mechanism is crucial for ensuring that calls can still be connected even when some lines are busy. This hunting is a form of Queueing theory in action, albeit a mechanical one.

6. **Establishing the Connection:** Once an idle switch is found, it completes the connection to the destination line. The circuit is then closed, and the call is connected.

7. **Release:** When either party hangs up, the connection is released, and the switches return to their initial positions, ready to handle the next call. This is a form of Circuit switching.

This two-motion principle is the heart of the Strowger switch. More complex switches used three or even four motions to accommodate larger telephone networks and more numerous subscribers. Each additional motion added another level of selectivity, allowing for more destinations to be reached.

1. 1. Components of a Strowger Switch

The Strowger switch wasn't a single device; it was a complex assembly of numerous components. Key components included:

• **Switches (Step-by-Step):** The core of the system, responsible for physically connecting and disconnecting circuits. They are electromechanical relays with multiple contact points.
• **Magnets:** Used to operate the switches. Electrical pulses energize the magnets, causing them to move the switches.
• **Relays:** Used for various control functions, such as detecting dialed digits and controlling the hunting mechanism.
• **Pulse Receivers:** Detect and count the pulses generated by the dialing mechanism.
• **Hunters:** Mechanisms that search for idle switches within a bank.
• **Hold Magnets:** Keep switches in their selected position during the call.
• **Wiring and Connectors:** The extensive network of wires and connectors that connect all the components together. Maintaining this wiring was a significant challenge in large exchanges.
• **Brush and Spring Assemblies:** These provided the physical contacts within the switches, and were prone to wear and tear.

1. 1. Limitations of the Strowger Switch

While revolutionary for its time, the Strowger switch had several limitations:

• **Slow Call Setup:** The step-by-step switching process was relatively slow, especially for long-distance calls. Each digit dialed required a physical movement of the switches. This contrasts with the speed of modern Packet switching networks.
• **Blocking:** The progressive selection process could lead to "blocking," where a call could not be completed even if the destination line was free. This occurred if all the switches along the path were already in use. This is a fundamental limitation of circuit-switched networks. Erlang B formula is used to calculate the probability of blocking in such networks.
• **Scalability Issues:** As telephone networks grew, the complexity of the switch increased dramatically. Larger exchanges required vast numbers of switches and complex wiring, making them expensive and difficult to maintain.
• **Limited Features:** The Strowger switch offered limited features beyond basic call connection. Features like call waiting, call forwarding, and conference calling were not possible with this technology. These features require more sophisticated Signaling systems.
• **Maintenance:** The electromechanical nature of the switch meant it was prone to wear and tear. Regular maintenance and replacement of parts were essential to keep the system functioning reliably. This required a skilled workforce.
• **Long Call Setup Times for Long Distance:** The more digits in the number, the longer it took to establish a connection. Long distance calls were considerably slower to connect than local calls.

1. 1. Evolution and Replacement

Over time, the Strowger switch underwent several improvements and refinements. Larger switches with more motions were developed to handle more subscribers. However, the fundamental limitations of the technology remained.

The introduction of Crossbar switches in the mid-20th century offered significant advantages over the Strowger switch. Crossbar switches are faster, more reliable, and less prone to blocking. They also allowed for the implementation of more advanced features. The crossbar switch represented a significant leap forward in telephone exchange technology.

Ultimately, the Strowger switch was superseded by Electronic switching systems (ESS), which use digital technology to switch calls. ESS systems are even faster, more reliable, and more flexible than crossbar switches. They also enable a wide range of advanced features, such as integrated services digital network (ISDN) and voice over IP (VoIP). The transition to digital switching marked the end of the era of electromechanical telephone exchanges. The Strowger switch, while obsolete, paved the way for these advancements.

1. 1. Legacy and Modern Relevance

Although no longer in use, the Strowger switch remains a significant historical artifact. It represents a pivotal moment in the development of telecommunications and the automation of complex systems. Its principles laid the foundation for the modern telephone network.

Furthermore, the concepts behind the Strowger switch – progressive selection, hunting, and circuit switching – are still relevant in certain modern applications. For example, some network routing protocols use similar principles to find available paths for data transmission. The underlying concepts of Routing algorithms have roots in the electromechanical solutions pioneered by Strowger.

The story of the Strowger switch also serves as a compelling example of how innovation can be driven by practical problems and even personal grievances. It demonstrates the power of ingenuity and the importance of challenging existing paradigms. Studying its design provides valuable insights into the evolution of engineering and the transition from mechanical to electronic systems. The principles of Reliability engineering were heavily considered during its development.

The switch also influenced the development of early Computer architecture and Boolean logic. The need to reliably control the complex switching mechanisms spurred advancements in relay-based logic circuits, which were precursors to modern digital computers. The concepts of State machines were also implicitly used in the switch's operation. Understanding the Strowger switch offers a unique perspective on the history of computing. The concepts of Network topology can be visualized by studying the physical layout of a Strowger exchange. Its operation also demonstrates early forms of Control theory. The efficiency of the switch was analyzed using principles of Operations research. The economic impact of the switch is a case study in Technological disruption. The development of the switch also spurred advancements in Materials science as engineers sought stronger and more durable materials for the switches and relays. Analyzing the switch's performance provides insights into Performance metrics and Optimization techniques. The switch's design also influenced early concepts of Cybersecurity as engineers considered ways to prevent unauthorized access and manipulation.

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