Four types of transmission media are commonly employed for local area networks:
(a) Twisted-pair electrical cable which is limited to data rates of around 3 Mbps and has typical error rates around one in 100000 when not screened. Screened twisted-pair can carry signals upto 10 Mbps with error rates better than one in 100 million over distances upto about 1 km but it is much more expensive than unscreened twisted-pair.
(b) Baseband co-axial cable for data rates upto 50 Mbps and typical error rates of 1 in 10 million.
(c) Broadband co-axial cable for data rates (the sum of many FDM channels) over 300 Mbps and typical error rates of 1 in 1000 million.
(d) Fiber optic cable for total data rates of more than 150 Mbps and extremely low error rates. The number of devices which may be attached to a fiber optic bus is small and therefore the ring topology which uses fiber optic links between access points is most suitable.
