AT&T Long Lines: The First Continental Data Backbone

From the late 1940s through the 1980s, AT&T’s Long Lines network became the first system to truly link the continent’s growing phone networks. Before this, telephone service was largely local. Cities and towns often operated their own exchanges, and long-distance service was limited, expensive, and unreliable. Connecting the entire country by wire would have required millions of miles of copper and a level of construction that was unrealistic in cost and time. Going wireless presented a way around those constraints.

Long Lines used thousands of high-elevation relay towers spaced twenty to forty miles apart, each passing voice, television, and data traffic through the air at microwave frequencies. It was the first real execution of a coast-to-coast network without laying any hardlines.

This nationwide telephone and later television backbone operated primarily in the four to eight gigahertz range. Although common in consumer electronics today, these frequencies were untouched at the time. Engineers at Bell Labs realized that they offered the perfect balance of repeatability in mixed environments and bandwidth capacity. The wavelengths were short enough for compact, high-gain antennas, yet long enough to propagate reliably through rain, fog, and the complex terrain of North America.

Elevation is leverage in radio, especially in the line-of-sight propagation that occurs in this range of frequencies. Each Long Lines tower was deliberately placed on ridgelines, hills, or purpose-built concrete structures designed to maximize visibility between sites. The engineers also learned a critical lesson about RF behavior. Because of the high-gain transmit systems in use, towers could never be arranged in perfectly straight, three-in-a-row alignments. Doing so would create cascading interference of the RF signals. To prevent this, sites were carefully staggered in a "zig-zag" pattern across the US. The result was a national chain of precision-engineered microwave relays that carried the country’s conversations through the air.

Bell Labs’ TD-2 microwave system, launched commercially in 1950, formed the early backbone. TD-2 began with six RF channels in the 3.7–4.2 GHz band, with frequency-division multiplexing (FDM). Each channel initially carried about 480 telephone circuits or one TV program. Hardware improvements soon pushed per-channel voice capacity to about 600, and later advances raised effective capacities again. To boost throughput, AT&T introduced TH in the 6 GHz band and added dual polarization (H/V), effectively doubling capacity in the same spectrum slice; a single channel could now handle on the order of 1,200 voice circuits.

During the 1960s and 1970s, Long Lines began to shift from purely analog frequency-division multiplexing to digital multiplexing. New microwave platforms like TD-3 and TH-3 started carrying pulse-code modulated traffic and time-division multiplexed signals rather than stacked analog channels. Bell System journals from the era describe this transition as the first major step toward a fully digital nationwide network.

By the late 1970s and into the 1980s, many Long Lines routes were moving entirely digital payloads. DS1 and DS2 circuits became common, and DS3 at 44 megabits per second began running over fixed six and eleven-gigahertz microwave paths. These digital links tied the older radio relay system directly into the early digital telephone network.

Equipment varied between manufacturers, but the industry gradually moved toward more efficient digital radios. Modulations like QPSK and higher-order QAM became standard on long-haul microwave paths. The backbone moved from analog to digital while still using the same towers, the same geometry, and the same reliance on clean line-of-sight paths.

This history sounds complicated, but the core idea is almost identical to what modern DMR radios do. DMR takes voice, samples it, turns it into digital data, and places it into time slots inside a TDM frame. Long Lines did the same thing, just at a much larger scale. Their microwave radios carried early PCM and TDM streams instead of analog audio, and their modulation schemes served the same purpose that 4FSK serves in DMR. The formats and bandwidths were different, but the logic was the same: encode the audio, divide it into time slots, and send it efficiently over the air.

Long Lines was not magic. It was a massive, corporate effort backed by budgets and manpower that only a national monopoly could pull together. They poured concrete on mountaintops, built hardened relay sites, and ran power and maintenance crews to places most people would never see. Ordinary citizens could not replicate anything close to that in the 1950s or 1960s. What makes the story interesting now is that the same basic principles can be used by small, self-funded teams with gear you can order online.

A single Ubiquiti NanoBeam has more throughput and is more power-efficient than any of the early Long Lines radios. For a few hundred dollars, you can build a microwave hop that behaves like a tiny version of the national backbone. Give it some elevation and line of sight, and it will move real data over real distances without a cell tower or an ISP.

At RTO Advanced at the Direct Action Resource Center, we proved this directly. We set up point-to-point microwave links to push ATAK data, camera feeds, and ROIP traffic across the facility. Students had to pick their own sites, perform link analysis, and configure the links within the scenario of the training environment. It did not take long for them to realize they were working with the same fundamentals that once held the national network together, just scaled down to a team level instead of a continental one. The same physics that carried long-distance phone calls across mountains in the 1960s now lets a small group move their own information without relying on commercial infrastructure.

Most of the Long Lines towers are quiet today, but the lessons still matter. Resilience comes from building your own solutions instead of hoping someone else will meet your communications needs. With a bit of knowledge and a small supply of Ubiquiti equipment, a self-funded team can stand up its own network, and a few committed people can now do what once required an entire corporation.