bly send out an engineer with a T-Berd to perform invasive testing. TDM stands for time-division multiplexing. A T1 link is a TDM link because its 24 channels are divided into time slots. A T1 link is a serial link, SO one bit is sent at a time. The channels are cycled through at a high rate of speed, with each channel being dedicated to a slice of time. Telecom Glossary CHAPTER 20 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 and under streets in metropol- itan areas at the time. Imagine the central office 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 four wires. Back then, this was a major shift in thinking. Remember that at the time, digi- tal 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. In this chapter, I will go into detail about the design, function, and troubleshooting of T1s. While I usually try to simplify complex engineering topics, I feel that it's important to understand the principles of T1 operation. We live in a connected world, and much of that world is connected with T1s. Knowing how they work can save you countless hours of troubleshooting time when they break. Understanding T1 Duplex A full-duplex link can send and receive data at the same time. As anyone who's ever had a fight with his or her significant other over the phone can attest, both partie