se the distance or spacing. Trials have been Chapter 2 Traditional Transmission Media successfully completed at distances of 2,500 miles (4,000 km) and 4,000 miles (6,400 km). Components of Fiber Optics The two factors that determine the performance characteristics of a given fiber implementation are the type of cable used and the type of light source used. The following sections look at the components of each. Fiber-Optic Cable Fiber-optic cable is available in many sizes. It can have as few as a couple pairs of fiber or it can have bundles that contain upward of 400 or 500 fiber pairs. Figure 2.8 shows the basic components of fiber-optic cable. Each of the fibers is protected with cladding, which ensures that the light energy remains within the fiber rather than bouncing out into the exterior. The cladding is surrounded by plastic shielding, which, among others things, ensures that you can't bend the fiber to the point at which it would break; the plastic shielding therefore limits how much stress you can put on a given fiber. That plastic shielding is then further reinforced with Kevlar reinforcing material-material that is five times stronger than steel-to prevent other intrusions. Outer jackets cover the Kevlar reinforcing material, and the number and type of outer jackets depend on the environment where the cable is meant to be deployed (e.g., buried underground, used in the ocean, strung through the air). There are two major categories of fiber: multimode and single mode (also known as monomode). Fiber size is a measure of the core diameter and cladding (outside) diameter. It is expressed in the format xx/zz, where XX is the core diame- Kevlar Reinforcing Material Glass Fiber and Cladding Plastic Shielding Outer Jackets Figure 2.8 Fiber-optic cable Fiber Optics ter and ZZ is the outside diameter of the cladding. For example, a 62.5/125-micron fiber has a core diameter of 62.5 microns and a cladding diameter of 125 microns. The core diameter of the fiber in multimode ranges from 50 microns to 62.5 microns, 