For many years, all long distance calls began with a call to an operator sitting at a toll (long-distance) switchboard. Until the 1920s, that operator wrote down the calling information provided by the customer, and then told the customer that he or she would be called back once the party was on the line. The operator then passed the information to another operator, who would look up the route that the call should take, and then build up the circuit one link at a time by connecting to operators at switchboards along the route. A typical call took seven minutes to set up. Once operators established a circuit, it was dedicated to that conversation until the end of the call.
1920s: Switching improvements begin
The first major improvement was the Combined Line and Recording (CLR) method, introduced in 1926. Now, the first operator the customer spoke to was the one who built up the circuit and placed the call. For the first time, most calls were completed while the customer stayed on line. The average call completion time dropped to 2.1 minutes.
International operator, New York, 1936. This operator was in charge of all calls intended for ships at sea.
In 1929, AT&T network engineers implemented the first national General Toll Switching Plan. It established a hierarchical, national network with eight interconnected regional centers across the country. More than 140 primary centers, at least one in each state, connected to the regional centers. More than 2,000 toll offices throughout the country provided connections between the primary centers and every local exchange in the country. Additional circuits provided direct connections between centers with substantial direct traffic. These additional circuits also provided alternate back-up routes. Operators no longer had to rely on massive route books to determine call paths. Now operators sent calls up (and then back down) an established hierarchical chain. A single, nationwide structure provided a solid basis for future planning and expansion as traffic grew. With some modifications, notably the addition of sectional centers in the 1950s, AT&T continued to rely on this hierarchical network until the 1980s.
1940s & 1950s: Automated switching
Automation came to long distance switching when AT&T installed the first No. 4 crossbar switch in Philadelphia in 1943. Now a single operator built up the needed circuit by dialing a series of routing codes to instruct this automatic electromechanical switch. Dialed routing codes soon gave way to the familiar area codes, which the switch itself could translate into the needed routing information. AT&T soon modified the switch to handle customer-dialed long distance calls; the modified design became the No. 4A crossbar switch. No. 4A crossbar switches and direct-distance dialing spread to subscribers across the country during through the 1950s. Call-completion time dropped to 10-20 seconds.
1970s: Digital electronic switching
Switching technology leapt forward in 1976 with the installation of the initial AT&T 4ESS switch in Chicago. The 4ESS was simultaneously the world’s first digital electronic switch and a powerful computer. Common channel signaling followed in 1977. Now the information needed to set up a call traveled over a separate digital network from the call itself. With a separate signaling network, call completion time dropped to 1-2 seconds.
Electronic switching and common channel signaling brought new flexibility to the network, and made possible advanced services such as enhanced 800 service and software-defined networks (in the latter, customers could use a portion of the AT&T network as the virtual equivalent of a private line network).
Additional 4ESS installations followed rapidly. By 1980, AT&T had 53 of the new switches in the network, and by 1984, 100. Few of the older 4A crossbar switches remained. Through the separate signaling network, all of the 4ESS switches were in direct contact with one another, and could search for available circuits in real time. Hierarchical routing gave way to dynamic non-hierarchical routing, where the 4ESS switches themselves could choose the best path between two points.
An AT&T technician replaces a circuit board in a digital interface frame of a 4ESS switch, Los Angeles, 1980.
As a digital switch, the 4ESS was ready when the rise of fiber-optic transmission transformed the AT&T network into an all-digital system between the mid-1980s and the early 1990s. Calling volumes climbed steadily, increasing from 37.5 million calls per average business day in 1984 to 300 million 15 years later.
1990s: The rise of data
Over those 15 years the character of the traffic began to evolve from voice to data. In the 1990s, data traffic increasingly adhered to new formats — packet switching, frame relay, asynchronous transfer mode (ATM) and Internet protocol — routed via newer technologies rather than over the switched voice network. In 1999, AT&T installed its 145th and last 4ESS switch in Atlanta.
Today, data services drive the network. In 2000, the volume of data traffic on the network surpassed the volume of voice traffic. Even the word “switching” is giving way, to the broader term “connectivity,” in recognition that much traffic today gets routed over systems designed to direct data flows rather than voice. And this data travels in packets, mixed with other traffic, rather than over the dedicated circuits that have carried voices since the 19th century.