led LSZH cables. They produce little smoke and no toxic halogen compounds. But they are the most expensive jacket material. Embedded just below the cable jacket there is the ripcord: a thin but very strong thread. Its role is to split the cable easily without harming cable interiors. Fiber optic cables are available in a wide variety of physical constructions. They can be anything from simple simplex or duplex (zipcord) cables used for jumpers to 144-fiber cable for intercity transmission. Most of the fibers used in these cables come down to two basic configurations-900uum tight-buffered fibers or 250 um coated fibers (also called bare fibers). Actually, tight- buffered fibers cover a coated fiber (the coating is soft plastic) with a thick layer of harder plastic, making it easier to handle and providing physical protection. There are two basic types of fiber optic cable constructions: Tight-buffered cable and loose tube cable. Their structure is shown in Figure 4.12a and b. Multiple color-coded 900 um tight-buffered fibers can be packed tightly together in a compact cable structure, an approach widely used indoors; these cables are called tight- buffered cables. Tight-buffered cables are used to connect outside plant cables to TE, and also for linking various devices in a premises network. Multifiber, tight-buffered cables often are used for intrabuilding, risers, general build- ing, and plenum applications. On the other hand, multiple (up to 12) 250 um coated fibers can be put inside a color coded, flexible plastic tube, which usually is filled with a gel compound that prevents moisture from seeping through the hollow tube. Buffer tubes are stranded around a dielectric or steel central member. Aramid yarn is used as primary strength member. Then an outer PE jacket is extruded over the core. These cables are called loose tube cables. Loose tube structure isolates the fibers from the cable structure. This is a big advantage in handling thermal and other stresses encountered outdoors, which is why most loose tube 