TD-2 was a microwave relay system developed by Bell Labs and used by AT&T to build a cross-country network of repeaters for telephone and television transmission. The same system was also used to build the Canadian Trans-Canada Skyway system by Bell Canada, and later, many other companies in many countries to build similar networks for both civilian and military communications.

The system began with the experimental TDX, completed in November 1947, carrying television and telephone between Boston and New York City. TD-2 was a minor improvement on TDX, moving to the 3.7 to 4.2 GHz band set aside in 1947 for common carrier use. The system had six channels, and using frequency-division multiplexing, each could carry up to 480 telephone calls or a television signal. The first TD-2 link between New York and Chicago opened on 1 September 1950, followed by a Los Angeles-San Francisco link on 1 September. The two coasts were linked in 1951.

Equipment improvements in 1953 increased capacity to 600 calls per channel. Looking to further improve throughput, Bell Labs introduced the TH system, which operated in a higher band, around 6 GHz. It also added polarization to the signals allowing two channels per band. This allowed it to carry 1,200 calls per channel, but required the use of horn antennas to retain polarization. After considerable research, Bell developed an antenna that worked for both TD-2 and TH, but these improvements also helped TD-2 and increased its capacity again to 900 calls, delaying a widespread rollout of TH which was added only to the busiest links.

By the late 1960s, almost all of the population of North America was linked using TD-2 and TH. Television signals moved to satellite distribution in the 1970s and 80s, and the network was mostly used for telephone from that time. During the late 1980s and especially 1990s, the installation of fiber optic lines replaced the microwave networks. Some of the towers are in use today for other purposes, but the majority of the sites are abandoned.

Radio telephone systems had been experimented with as early as 1915, the year after AT&T bought Lee de Forest's patents on the audion vacuum tube. Experiments were carried out between Arlington, Virginia, Hawaii and Paris. After being interrupted by World War I, such experiments began again and led to the creation of a permanent link between New York City and London in 1927. This system operated at 60 kHz, using the behavior of lower-frequency radio waves to follow the curvature of the Earth to provide over-the-horizon performance.

Around the same time, the first experiments with megahertz frequency radios were showing the ability to use ionospheric scatter to provide long-distance radio propagation at these higher frequencies. A new link between New York and London started in 1928, and was quickly followed by other users around the world. The main problem with this system is that the scattering meant the ultimate range of the signals could not be predicted, which made it difficult to ensure that any two stations could use the same frequencies and be safe from interference. Research continued on moving to ever-higher frequencies in an effort to avoid interference as well as expand bandwidth.

A single-line link between Boston and Cape Cod was set up in 1934 at 60 MHz, moving to what was then relatively unused spectrum. A more advanced system was set up across the entrance of Chesapeake Bay in 1941, operating at 150 MHz. This system had enough bandwidth to allow 12 telephone calls to be sent on the single connection using the same multiplexing system used on long-distance calling wires.

It was already clear that moving to the gigahertz range would offer far more bandwidth and allow hundreds of calls on a single link. Bell went so far as to show illustrations of what such a system might look like, the illustration using long horn antennas. The opening of World War II ended these experiments.