This was the second transmission medium to be introduced (often called coax), around the mid-1920s. In the center of a coaxial cable is a copper wire that acts as a conductor, where the information travels. The copper wire in coax is thicker than that in twisted-pair, and it is also unaffected by surrounding wires that contribute to electromagnetic interference, so it can provide higher transmission rates than the twisted-pair. The center conductor is surrounded by plastic insulation, which helps filter out extraneous interference. This insulation is covered by a return path, which is usually braided-copper shielding or aluminum foil type covering. Outer jackets form a protective covering for coax; the number and type of outer jackets depend on the intended use of the cable (e.g., whether the cable is supposed to be strung in the air or underground, whether rodent protection is required). The two most popular types of coaxial cabling are used with Ethernet networks.
