Electronics Guide

The First Exchanges

The first commercial telephone exchange opened in New Haven, Connecticut, in 1878, serving a small number of subscribers through a simple switchboard. An operator answered a calling subscriber, learned which party was wanted, and joined the two lines by inserting the ends of a flexible cord into the appropriate jacks. The principle was elementary, but it established the architecture of the telephone network: a star of lines radiating from a central switching point.

The Cord Switchboard

As exchanges grew, the manual switchboard matured into a standardized instrument. The operator sat before a vertical panel containing a jack for every subscriber line, with a signal lamp beside each jack. Before the operator lay rows of cord pairs, each pair able to connect two jacks, together with keys for talking to and ringing the parties. When a subscriber lifted the receiver, a lamp lit; the operator answered with one cord of a pair, learned the desired number, tested whether that line was free, completed the connection with the other cord, and rang the called party. The common-battery system, introduced in the 1890s, supplied power to subscribers' telephones from the central office and signaled the operator automatically, removing the need for hand-cranked magnetos.

In large cities a single exchange could not hold a jack for every line within reach of one operator, so the multiple switchboard was devised, repeating the field of subscriber jacks at every operator position. In this way any operator could reach any line, and the work of answering calls could be spread across the whole staff.

The Operators Themselves

Telephone operators, very soon predominantly young women, were the living core of the manual system. The work demanded speed, accuracy, a retentive memory, and unfailing courtesy under pressure. By the early twentieth century the occupation employed tens of thousands and represented one of the first large-scale forms of respectable employment for women outside the home, factory, or schoolroom. Yet the manual system had inherent limits. Its cost rose in direct proportion to the volume of traffic, service could degrade at busy hours, and the operator necessarily knew who was calling whom. These limits motivated the search for a machine that could switch calls without human intervention.

Almon Strowger's Invention

Almon Brown Strowger, an undertaker in Kansas City, is said to have been driven to invention by the suspicion that a local operator, connected by marriage to a rival undertaker, was diverting calls away from his business. Whatever the truth of the anecdote, Strowger patented an automatic telephone exchange in 1891 that dispensed with the operator entirely. The first commercial installation opened in La Porte, Indiana, in 1892. Early subscribers signaled the switch with push buttons; the rotary dial, which sent a precise number of electrical pulses for each digit, soon became the standard instrument and remained so for most of the twentieth century.

How Step-by-Step Switching Worked

The Strowger switch was an electromechanical device whose moving contact, called a wiper, could both rise vertically and rotate. Each pulse dialed for a given digit drove the wiper up one step, and the mechanism then rotated to find a free path onward. The defining characteristic of the system was that the dialed digits acted directly upon the switching mechanism, each digit advancing the connection one stage further toward its destination. Because the intelligence was distributed throughout the chain of selectors rather than gathered in any central control, the system was called direct progressive control, or more familiarly, step-by-step.

This directness was both the strength and the weakness of step-by-step switching. It required no complex common control, so a fault affected only the calls passing through the faulty switch. But it used equipment inefficiently, tied the structure of telephone numbers closely to the physical wiring of the exchange, and made advanced features difficult to provide. The many moving parts also demanded constant cleaning and adjustment.

Adoption and the Bell System

Independent telephone companies, competing with the Bell System after Bell's two fundamental telephone patents expired in 1893 and 1894, adopted automatic switching eagerly as a way to cut costs and offer privacy. The Bell System, with its large investment in operators and manual plant, moved more cautiously, but it too embraced automatic step-by-step switching in the early decades of the twentieth century. For many years step-by-step exchanges, in Bell and independent territory alike, carried a large share of the nation's telephone traffic.

The Crossbar Matrix and Common Control

A crossbar switch consists of a grid of horizontal and vertical bars. Electromagnets tilt selected bars so that, at their intersection, a set of contacts closes and joins an incoming path to an outgoing one. The switch itself is merely a fabric of possible connections; the decisions about which connection to make are taken by a separate body of equipment called the common control, or marker. The marker receives the dialed digits, selects a free path through the switching fabric, commands the crossbar switches to close, and then releases itself to serve the next call.

This separation of switching from control, known as common control, brought decisive advantages. The expensive control equipment served only briefly during call setup and was then available for other calls, so a small amount of it could handle a large traffic load. The switching contacts, used only while a path was being set up and held, wore far less than the constantly moving Strowger wipers. The marker could also translate between the dialed number and the physical equipment, loosening the rigid link between telephone numbers and wiring.

Deployment

The Bell System introduced its No. 1 Crossbar system in metropolitan areas in the late 1930s and later developed the widely deployed No. 5 Crossbar, which served communities of many sizes for decades. Crossbar switching, more reliable and more capable than step-by-step, became the mainstay of the telephone network through the middle of the twentieth century and provided the platform on which nationwide direct distance dialing was built, allowing subscribers to place long-distance calls without operator assistance.

The Idea of Stored-Program Control

Under stored-program control, the call-processing logic that had been frozen into the wiring of relay markers was instead expressed as software. To add a feature or correct an error, engineers could modify the program rather than rewire the exchange. This flexibility opened the way to services that the older systems could scarcely provide, including abbreviated dialing, call forwarding, call waiting, and three-way calling. The exchange became, in effect, a special-purpose computer attached to a switching fabric.

The First Stored-Program Exchanges

The Bell System placed the first large-scale stored-program-controlled exchange into commercial service at Succasunna, New Jersey, in 1965, designating it the No. 1 Electronic Switching System, or 1ESS. Although its central control was electronic and programmable, the 1ESS still switched calls through a fabric of small sealed reed relays known as ferreed switches; that is, its talking paths remained analog and the connection itself was still made by metallic contacts. The novelty lay in the computer that governed it. Stored-program control demanded extraordinary reliability, since an exchange must run continuously for years, and so these systems pioneered techniques of duplicated processors and automatic fault recovery that influenced computing more broadly.

Pulse-Code Modulation and the Digital Voice

The foundation of digital switching was pulse-code modulation, a method of representing a sound wave by sampling it many thousands of times each second and expressing each sample as a binary number. The relevant theory reached back to the work of Harry Nyquist and Claude Shannon and to Alec Reeves, who patented pulse-code modulation in the 1930s. By the early 1960s the Bell System was already carrying digital voice between exchanges over the T-carrier system, which combined many telephone conversations onto a single pair of wires as interleaved digital samples. Once the voice existed as a stream of numbers, it became natural to switch it digitally as well.

Time-Division Switching

Digital switching connects calls not by joining physical paths in space but by rearranging samples in time. Many conversations share a single high-speed digital path, each occupying a recurring time slot. A digital switch moves the samples belonging to one call from their incoming time slot into the outgoing time slot of the other party, using fast electronic memory to hold and reorder the samples. This technique, called time-division switching, requires no moving parts at all; the entire switching fabric is built from digital logic and memory.

The Bell System introduced digital switching into the long-distance network with the No. 4 ESS in the mid-1970s and, somewhat later, brought it to local exchanges with the No. 5 ESS, while comparable systems appeared around the world. Digital switching was smaller, cheaper, more reliable, and more capable than anything that preceded it, and it merged naturally with the digital transmission already in use, allowing a call to remain in digital form from one exchange to the next. By the closing decades of the twentieth century, digital stored-program exchanges had largely replaced the electromechanical plant in the developed world.

A Vertically Integrated Monopoly

The Bell System united, under one corporate roof, the regional operating companies that served subscribers, the Long Lines department that carried calls between cities, the Western Electric Company that manufactured the equipment, and Bell Telephone Laboratories that conducted the research and development. This integration gave Bell the resources to undertake sustained, long-range engineering, and many of the switching systems described here, along with foundational work in information theory, the transistor, and much else, emerged from Bell Laboratories. Operated as a regulated monopoly under the principle of universal service, the system achieved a telephone network of great reach and reliability.

The Divestiture of 1984

Concern that this concentration of power stifled competition led the United States Department of Justice to bring an antitrust suit against AT&T. The case was settled by a consent decree, and on January 1, 1984, the Bell System was broken up. AT&T divested the local operating companies, which were reorganized into seven independent regional holding companies, popularly called the Baby Bells, while AT&T retained long-distance service, Western Electric, and Bell Laboratories. The breakup opened the long-distance and equipment markets to competition and fundamentally altered the industry. It also dispersed the engineering culture that had produced a long line of switching systems, even as the technology itself continued its rapid advance.

Circuit Switching Versus Packet Switching

Traditional telephony is circuit-switched: a path is reserved end to end and remains the caller's alone whether or not anyone is speaking. Data networks, by contrast, are packet-switched: information is divided into small packets, each labeled with its destination, and the packets are sent independently across shared links and reassembled at the far end. Packet switching uses network capacity far more efficiently, since the links carry packets from many sources as needed rather than holding idle channels open. The internet protocol provided a common, universal way to move such packets between any two points.

Voice over IP

Voice over internet protocol, or VoIP, carries telephone conversations as streams of packets over data networks rather than over dedicated circuits. The voice is digitized and compressed, divided into packets, and transmitted across the network to be reconstructed at the destination. Doing so well required the network to limit the delay and loss that packets suffer, since a conversation is sensitive to interruption in a way that a file transfer is not. As broadband networks improved through the late 1990s and the 2000s, VoIP grew from a curiosity into a mainstream service offered by telephone companies, cable operators, and internet-based providers alike.

The consequence was a convergence of voice and data onto a single infrastructure. The specialized circuit switch, the culmination of more than a century of development from the cord switchboard through Strowger, crossbar, and the digital exchange, gradually gave way to general-purpose routers and servers running telephony as software. In this convergence the long history of telephone switching arrives at its present chapter, in which the telephone call has become one service among many carried by the universal packet network.