dielectric strengthening members (normally a glass rod epoxy), an aramid yarn, and 3 mm buffer tubing with an additional layer of Kevlar surrounding each fiber. The ripcord is a parallel cord of strong yarn that is situated under the jacket(s) of the cable for jacket removal. Distribution cables have an overall Kevlar wrapping, a ripcord, and a 900 micrometer buffer coating surrounding each fiber. These fiber units are commonly bundled with additional steel strength members, again with a helical twist to allow for stretching. A critical concern in outdoor cabling is to protect the fiber from damage by water. This is accomplished by use of solid barriers such as copper tubes, and water-repellent jelly or water-absorbing powder surrounding the fiber. Finally, the cable may be armored to protect it from environmental hazards, such as construction work or gnawing animals. Undersea cables are more heavily armored in their near-shore portions to protect them from boat anchors, fishing gear, and even sharks, which may be attracted to the electrical power that is carried to power amplifiers or repeaters in the cable. Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, dual use as power lines, installation in conduit, lashing to aerial telephone poles, submarine installation, and insertion in paved streets. Capacity and market In September 2012, NTT Japan demonstrated a single fiber cable that was able to transfer 1 petabit per second () over a distance of 50 kilometers. Modern fiber cables can contain up to a thousand fiber strands in a single cable although the highest strand-count single-mode fiber cable commonly manufactured is the 864-count, consisting of 36 ribbons each containing 24 strands of fiber. In some cases, only a small fraction of the fibers in a cable may actually be in use. Companies can lease or sell the unused fiber to other providers who are looking for service in or through an area. Depending on specific local regulations, companies may overbuild their networks for the specific purpose of having a large network of dark fiber for sale, reducing the overall need for trenching and municipal permitting. Alternatively, they may deliberately under-invest to prevent their rivals from profiting from their investment. Reliability and quality Optical fibers are very strong, but the strength is drastically reduced by unavoidable microscopic surface flaws inherent in the manufacturing process. The initial fiber strength, as