From Circuit Switched to IP-Based Networks

Historical Perspective

The origins of circuit switching have been well documented (AT&T, 2006). A year after successfully patenting the telephone in 1876, Alexander Graham Bell, with Gardiner Hubbard and Thomas Sanders, formed the Bell Telephone Company. In 1878, the first telephone exchange was opened in New Haven, Connecticut. For a long time, long distance switching was carried out by manual operators at a switchboard. Until the 1920s, the operator wrote down the number requested by the customer, and called the customer back when the other party was on the line. The route for a call was built link by link, by each operator passing the information to another operator who looked up the route for the call. Once a circuit was established, it was dedicated to that conversation for the duration of the call.

The national General Toll Switching Plan was put into effect in 1929. This established a hierarchical, national circuit switching network. Calls went to local offices connected to more than 2,000 toll offices and 140 primary centers, and up to eight interconnected regional centers. Sectional centers were added in the 1950s. This hierarchical network, augmented with direct links between the busiest offices, continued up to the 1980s.

In the 1960s, transmission facilities were converted from analog to digital, such as the T-1 carrier (a time-division multiplexed system allowing 24 voice channels to share a 1.5-Mbps digital transmission link). Digital transmission offers the advantages of less interference and easier regeneration. Digital carriers used time division multiplexing (TDM) instead of frequency division multiplexing (FDM). In FDM, each call is filtered to 4 kHz and modulated to a different frequency to share a physical link. In TDM, sharing is done in the time domain rather than frequency domain. Time is divided into repeating frames. Each frame includes a number of fixed time slots. For example, 24 voice calls share the T-1 carrier which is 1.544 Mb/s. Each frame is 193 bits and takes 0.125 ms. In a frame, one bit is used for framing while the remainder is divided into 24 time slots of eight bits each. The first time slot is used by the first voice call, the second time slot by the second call, and so forth.

Automated electromechanical circuit switches appeared in the 1940s starting with a No. 4 crossbar switch in Philadelphia. These switches were able to understand operator-dialed routing codes or customer-dialed numbers to automatically route calls. In 1976, the first No. 4 electronic switching system (ESS) was installed in Chicago. Electronic switches were special-purpose computers with programmability.

Advances in digital circuits and computers not only improved the performance of telephone switches, but also changed the nature of network traffic. Starting in 1958, modems allowed computers to transmit digital data over voice-grade analog telephone circuits. In the 1960s, computers were still large, and personal computers would not appear until the late 1970s. However, there was an evident need for computers to share data over distances. Digital data increased dramatically in the late 1970s with the proliferation of Ethernet local area networks and personal computers (e.g., Apple II in 1977, IBM PC in 1981).