331、The coaxial cable is the common transmission medium.It has better shielding than twisted pairs, so it can span longer distances at higher speeds.Two kinds of coaxial cable are widely used.One kind, 5 0— ohm cable, is commonly used for digital transmission.The other kind, 7 5— ohm cable, is commonly used for analog transmission.This distinction is based on historical, rather than technical , factors ( e.g.early dipole antennas had an impedance of 3 0 0ohm, and it was easy to build 4:1 impedance matching transformers).A coaxial cable consists of a stiff copper wire as the c o r e , s u r r o u n d e d by an insulating material.The insulator is encased by a cylindrical conductor, often as a closely woven metal mesh.The outer conductor is covered in a protective plastic sheath.The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent noise immunity.The bandwidth possible depends on the cable length.For 1 — km cables a data rate of 1 to 2 Gbps is feasible.Longer cables can also be used, but only at lower data rates or with periodic amplifiers.Coaxial cables used to be widely used within the telephone system but have now largely been replaced by fiber optics on long— haul routes.In the United States alone, 1000 km of fiber is installed every day ( counting a 100— km bundle with 10 strands of fiber as 1000 km).Sprint is alread y 100 percent fiber, and the other major carriers are rapidly approaching that.Coax is still widely used for cable television and some local area networks, however(2)、Coaxial cable h a s _______shielding than twisted pairs, so it can span longer distances at higher speeds.