· Copper Wire. This is the oldest f

· Copper Wire. This is the oldest form of electronic transmission medium. Its use dates back to the development of telegraph in the 1800s and earliest telephone systems. Early installations used open wires, but these were superseded by twisted pairs, which consist of a pair of insulated and twisted wires (see Figure 2.16). Twisted pairs are superior because of reduced crosstalk.2 They are very effective for relatively short distances (a few hundred feet), but can be used forup to a few kilometers. A twisted pair has a bandwidth to distance ratio of about1 MHz per kilometer. The performance of the twisted pair can be substantiallyimproved by adding a metallic shield around the wires. Shielded wires are muchmore resistant to thermal noise and crosstalk effects. Twisted pairs used for longdistance connections (e.g., telephone lines) are usually organized as a muchlarger cable containing numerous twisted pairs.