4 - Media

Abstract

This chapter discusses the physical properties of various media. Each medium has its unique advantages and disadvantages, and these are additive to the other features of the local area network chosen. Media suitable for local area networks divide into three categories: twisted pair wiring, coaxial cable, and optical fibers. Twisted pair consists of two copper conductors, each covered with insulation, usually polyvinyl chloride (PVC). The conductors used are typically 22 or 24 gauge. The two wires are twisted about each other so that each is equally exposed to interfering signals picked up from the environment. This feature is particularly important when differential transmission is used, because in differential transmission, information is conveyed by the difference in voltage between the two wires. Coaxial cable consists of a copper conductor surrounded by insulation, which is in turn surrounded by a tube-shaped conductor of solid copper, solid aluminum, or metal braid. Optical fibers combine the compactness of twisted pair with bandwidth which exceeds that of broadband coaxial cable. Their most appealing feature is that they experience little electrical interference, even from lightning strikes. In addition, they operate well even when the ground potential among various buildings varies widely.