Twisting provides cancellation of magnetically induced fields and currents on a pair of conductors. Magnetic fields arise around other heavy current-carrying conductors and around large electric motors. Various grades of copper cables are available, with Grade 5 being the best and most expensive. Grade 5 copper, appropriate for use in 100-Mbps applications, has more twists per inch than lower grades. More twists per inch means more linear feet of copper wire used to make up a cable run, and more copper means more money.
Shielding provides a means to reflect or absorb electric fields that are present around cables.
Shielding comes in a variety of forms from copper braiding or copper meshes to aluminized mylar tape wrapped around each conductor and again around the twisted pair. Fiber optics are being used more often as user applications demand higher and higher bandwidths. More commonly, it means the capacity or amount of data that can be sent through a given circuit. A bandwidth of 100 Mbps is standard using fiber optic cables. When first introduced, fiber was considered only for special applications because it was expensive and difficult to work with.