By 1928, NBC was broadcasting radio programs from coast to coast, but nationwide television transmission was still prohibitively expensive. To broadcast television waves, the networks needed a more efficient medium of transmission than dedicated phone lines and radio relay towers. The following year, researchers at the Bell Telephone Laboratories developed the coaxial cable to improve telephone service. The coaxial cable’s high capacity also happened to be ideal for television broadcasting. In 1937, AT&T completed a coaxial cable network between New York and Philadelphia, and was able to broadcast the 1940 Republican National Convention to a few hundred NBC receivers. By 1951, the networks were broadcasting coast to coast, using the new microwave radio relay network.

The first permanent commercial TV network was none of the three major networks. That distinction goes to the DuMont Television Network, which began operations in 1946. Despite several innovations in broadcasting and several hit shows, Dumont was never able to overcome its financial woes. The company simply did not have the large revenue stream and celebrity connections enjoyed by NBC, CBS, and ABC’s radio networks. DuMont ceased broadcasting in 1956.

As the networks expanded, nationwide groups of connected stations, called “affiliates,” formed to carry network programming. This network-affiliate relationship is tightly regulated by the federal goverment. The Federal Communications Commission (FCC) restricts the number of television stations that can be owned by any one company. As a result, most local television station affiliates are independently owned and receive programming from the networks through a franchising arrangement. (In larger cities, the major networks generally own and operate their own stations). At first, many affiliates had relationships with two or more networks, and aired shows from different networks. Eventually, television stations had to be exclusively associated with one network, and only carry that network’s programs.

The Prime Time Access Rule, another FCC regulation, restricts the number of hours of network shows on a local affiliate stations. The purpose of the Prime Time Access Rule was to encourage the development of locally produced programs and give local residents access to broadcast time. However, this rule generally led to programming consisting mostly of old movies and reruns (called “syndicated shows”) of network programs.

Until the mid-1980s, television programming in the United States was dominated by the three major networks. But today, NBC, CBS, and ABC must compete with cable channels (such as Fox and CNN), satellite channels, and Internet websites. The major networks have had to develop new mediums of programming distribution, including promoting shows on airline flights, delivering shows on portable devices such as iPods, and simultaneous and recorded Internet broadcasts. In addition, all three major networks have gone through several ownership changes as these companies adapt to the new mediums of information distribution.