More than a switch
A telephone exchange is a place as well as a machine. It is where the wires from many homes and offices come together, where calls are connected, and where lines link to other exchanges. Switching technology is its own subject; this article is about the exchange as a building and as a node in a network, and how that changed over time.
Rooms above shops
The first exchanges, opened in 1878 and the following years, were small. A company rented a room, often above a store or in an office building, put in a switchboard, and ran wires to the rooftop and then outward to subscribers. In many towns the exchange was one room with a couple of operators, a battery, a table and a stove.
Because the building was ordinary, the interesting feature was outside: the wires. Early cities grew thick with overhead telephone lines, strung on rooftop poles and across streets. Tangles of wire were an eyesore and a hazard, and many cities passed laws or the companies chose to bury them in cable. Bundled cables carrying many pairs of wires underground or on poles replaced the open wires in busy areas from the 1880s and 1890s onward.
Purpose-built central offices
As telephone service grew, companies built large, purpose-designed buildings, and by the early twentieth century they were building substantial structures for their central offices. The term “central office” became the common American name for the exchange building. Designs aimed at conveying permanence and trust. Many were solid brick or stone, with decorative entrances, and some architectural styles of the 1920s and 1930s, including art deco, were used. Others were deliberately plain, built to fit in with houses in a neighborhood.
Inside, a large building was organized around a few main elements:
- The switching floor. In the manual era, this was a long room of switchboards with rows of operators. Later it held racks of automatic switching equipment.
- The main distribution frame. All the outside cables ended at a huge iron frame, where each subscriber’s pair of wires was connected to a matching terminal for the switching equipment. Technicians could change connections with jumper wires, and this is where a customer’s number was in effect assigned to a physical line.
- The cable vault. Underground cables entered the building through a vault, usually in the cellar.
- The power plant. Exchanges ran from large banks of rechargeable lead-acid battery cells charged by generators or rectifiers. Central battery power meant that service could continue through a mains outage, and it helped create the reputation for reliability that telephone networks enjoyed.
- Offices and training rooms. Larger exchanges also had cafeterias, rest rooms and classrooms for operators.
Names, numbers and neighborhoods
For many decades, an exchange was also a local identity. In the United States and other countries, telephone numbers began with an exchange name, such as a word from a neighborhood or a landmark, followed by digits. Customers learned both the name and the number. In the mid-twentieth century, this system gave way to all-number dialing, and the exchange name became a historical detail. In some places an exchange name still lives on in local speech or in the names of old buildings.
Hubs, trunks and the hierarchy
Exchanges did not stand alone. Each local exchange connected to others by trunk lines, and long-distance traffic flowed through higher-level exchanges, often called toll or tandem offices, that connected many local ones. A call between distant cities might pass through several of these in turn, much as a journey across railway networks passes through junctions. Telephone companies designed this hierarchy to reduce the number of lines needed and to provide alternate routes if one failed.
Because the exchange was a hub, it was also a point of risk. A fire or flood at a major central office could disconnect tens of thousands of lines, and some did. This led to attention to fire protection, backup power and, in some cases, hardened construction. Large telephone buildings built in the later twentieth century, including some windowless structures in big cities, were designed to protect equipment from fire, weather and intrusion.
Electronic exchanges and shrinking buildings
Electronic switching, which began entering service in the 1960s, changed the physical scale. A mechanical exchange needed large rooms of relays and switches, which required constant maintenance, produced heat and noise, and used a lot of floor space. An electronic or digital switch, built with transistors and later integrated circuits, handled the same traffic in a fraction of the room. Many exchanges were consolidated, and floors that once held clattering equipment became quiet or empty.
Digital technology also made it possible to move switching closer to customers. Remote switching units and equipment cabinets by the roadside could serve a neighborhood and connect to a larger switch by optical fibre. In recent years, much of the network’s logic runs as software on general-purpose computers, and the old buildings are often reused for data equipment, since they already have power, cooling, cable entrances and secure construction.
Why the exchange matters
The exchange is the reason that every telephone could reach every other. Its form changed from a rented room to a landmark and then to a few racks and a software platform, but its role stayed the same: to be the central point where lines meet and are joined on demand.