 fibers. Aramid yarn (Kevlar is its commercial name that is a product trademark) is a yellow color, fiber-looking material. It is strong and is used to bundle and protect the loose tubes or fibers in the cable. Every fiber is individually protected with a polymer tube. These polymer tubes are also colored SO that the fibers can be recognized at the two extremes of the cable. This is an important cable characteristic, since the circular symmetry make it impossible to recog- nize the single fiber from its position. The set of fibers is protected as a whole by different concentric tubes, each of which has a different role in assuring either cable resistance to mechanical, chemical or thermal aggressions or other cable characteristics, for example, resistance to burning or nonpoison- ous smoke if flaming. Technology for Telecommunications: Optical Fibers, Amplifiers, and Passive Devices A gel compound fills buffer tubes and cable interiors, making the cable impervi- ous to water. It needs to be completely cleaned off when the cable end is stripped for termination. The outer tube is called jacket and has a particular importance since it is the part of the cable that is exposed to the outside environment. For this reason, the jacket material is quite important; in a typical fiber cable the jacket is composed of the following materials: Polyethylene (PE) is the standard jacket material for outdoor fiber optic cables. PE has excellent moisture and weather-resistance properties. It has very stable dielec- tric properties over a wide temperature range. It is also abrasion resistant. Polyvinyl chloride (PVC) is the most common material for indoor cables; however, it can also be used for outdoor cables. It is flexible and fire-retardant. PVC is more expensive than PE and that has an impact for very long cables. Polyvinyl difluoride (PVDF) is used for plenum cables because it has better fire- retardant properties than PE and produces little smoke. Low smoke zero halogen (LSZH) plastics are used for a special kind of cable cal