Chapter 21. T1

In the 1950s, the only method for connecting phone lines was with a pair of copper wires. For each phone line entering a building, there had to be a pair of copper wires. Wire congestion was a huge problem in central offices (COs) and under streets in metropolitan areas at the time. Imagine the CO of a large city, where tens of thousands of phone lines terminated, each requiring a pair of wires. These COs also needed to communicate with each other, which required even more wiring.

In 1961, Bell Labs in New Jersey invented the T1 as a means for digitally trunking multiple voice channels together between locations. The T1 delivered a 12:1 factor of relief from the congestion, as it could replace 24 two-wire phone lines with 4 wires. Back then, this was a major shift in thinking. Remember that at the time, digital technology was practically nonexistent. The first T1 went into service in 1962, linking Illinois Bell’s Dearborn office in Chicago with Skokie, Illinois. Today, you would be hard-pressed to find a company that doesn’t deploy multiple T1s.