Telephone Switching
Telephone switching is the machinery that connects the caller's line to the line of the person being called, and for most of the twentieth century it was the largest electronic or electromechanical system in the world. Early telephone exchanges relied on human operators who plugged cords into jacks; automatic exchanges replaced the operator with switches that responded to dialed digits, and later with computers. The scale of this task, millions of lines each needing reliable, constantly running switching hardware, is the central reason the Bell System invested in solid-state research that produced the transistor.
Electromechanical switching began with Almon Strowger and ran for decades on relays and stepping switches. Strowger, an undertaker in Kansas City, patented an automatic telephone exchange in 1891, and the first commercial Strowger exchange opened in La Porte, Indiana in 1892. A caller's dial pulses drove a stepping switch, moving a wiper up and then around a bank of contacts to select the line. Later designs used relays, small electromagnets that open or close contacts, as the basic logic element. The Western Electric crossbar system, introduced in the late 1930s, used a grid of horizontal and vertical bars to close a contact at the crossing point, controlled by relay-based common control circuits that remembered the dialed number. These systems worked, but contacts wore out, needed adjustment, made noise, and consumed power.
The Bell System wanted a better switch and amplifier, which is why it funded the transistor. Bell Labs' research director Mervin Kelly had long argued that reliability and cost in the network would depend on replacing mechanical contacts and short-lived vacuum tubes with solid-state devices; the 1945 reorganization that created the solid-state physics group under William Shockley and Stanley Morgan followed this goal. The transistor itself was first valued as an amplifier for long-distance lines and hearing aids, and then as a switch for logic and memory. A relay takes milliseconds to move; a transistor can change state in nanoseconds, has no moving contacts to wear, and can be made in huge numbers at small cost.
The No. 1 Electronic Switching System put a stored-program computer in charge of the exchange. After years of development at Bell Labs, the first No. 1 ESS entered service in May 1965 in Succasunna, New Jersey. It used a stored-program computer with solid-state logic to control the switching network, whose crosspoints were small sealed reed-type contacts called ferreeds, and its software could be changed without rewiring, so features like call forwarding, speed dialing and three-way calling became programmable services instead of hardware. The system was a milestone in software engineering as well, with demanding reliability goals that pushed ideas of fault detection, redundancy and online recovery. Later switches, such as the No. 4 ESS for long-distance traffic, went fully digital, converting voice to binary samples and switching them as data.
This was also the setting for several ideas that reshaped computing and communication. The same company's research community produced Claude Shannon's 1937 master's thesis, which showed that Boolean algebra describes relay and switching circuits, and Shannon's 1948 information theory, which defined the bit as the unit of information; Unix, written at Bell Labs by Ken Thompson and Dennis Ritchie from 1969, grew out of the same institution's need for software tools. Switching was, in effect, the first industrial application of logic circuits and the reason Bell kept recruiting mathematicians alongside physicists. A common misconception is that the transistor was invented to make computers; its sponsor's concern was a vast and failure-prone telephone network, and computing was largely a by-product.
Modern networks still do switching, but now in packets and silicon. The data center switch and internet router move data in packets, with chips that decide in nanoseconds where each should go, descendants of the crossbar in both structure and name. Cellular networks hand calls between towers under software control, and an old copper telephone exchange has become a software service.
Read telephone switching as the demanding original customer of the transistor and of Boolean logic: the system whose need for tireless, fast and compact on-off elements turned a laboratory curiosity into a technology, and whose ideas of stored-program control now run through every network.