mid Buffer tube Strong member Ripcord Tight buffer Aramid yarn FIGURE 4.12 Scheme of the structure of a fiber cable: (a) tight-buffered cable and (b) loose tube cable. For current telecommunication applications, practically only single-mode fibers are used, also in the access area. Thus, all the telecommunication cables include single-mode fibers, even if multimode fiber cables and even mixed fiber cables are still used for other applica- tions or for compatibility with legacy systems generally in the access area. The first element in determining the characteristics of a fiber cable is the choice of mate- rials used to package the cable. Depending on the environment in which the cable has to be deployed, different require- ments are imposed to the design and different materials have to be chosen. For indoor cables, used for LANs or internal cabling of carriers central offices, fire safety is the number one factor. Moreover, especially in the LAN application, protection of the inner fibers against excessive bending and easy installation in the typical LAN conduit is another key requirement. In the case of outdoor cables, moisture resistance and temperature tolerance are the major factors when choosing materials. They also need to be ultraviolet resistant and capable of sustaining the stresses related to typical installation procedures without damage. Finally, aerial self-supporting cables must endure extreme temperature ranges from sunlight heat to freezing snow. They also must survive high wind loading. The structure of a fiber cable, schematically reported in Figure 4.12, is designed to allow these requirements to be fulfilled. The fibers are sustained by an internal strength member made from steel, Aramid yarn [43], or some other material with high resistance to traction and high elasticity. The role of the strength member is to give mechanical resistance to the cable and to allow cable trac- tion during installation. When a cable is pulled into a duct, the tension is applied to the strong member instead of the