ler, is more expensive, and supports higher speeds than multi-mode fiber. Measur- ing approximately the same diameter of a strand of human hair, it is used mainly in carrier networks and in undersea cabling. The fact that single-mode fiber carries light pulses faster than multi-mode fiber can be explained by a geometric rule: A straight line is the shortest distance between two points. Light travels faster in a straight line than if it zigzags along a path, which is precisely what happens to light waves if they reflect, or "bounce," off the inner wall of the fiber strand as they travel. These zigzag paths also cause the signals to attenu- ate, lose power, and fade over shorter distances. The small core of single-mode fiber keeps the light signal from bouncing across the diameter of the fiber's core. Thus, the straighter light signal travels faster and has less attenuation than if it had a bouncier ride through the core. When the light pulses travel in narrower paths, fewer repeaters are needed to boost the signal. Single-mode fiber can be run for 80 miles without regeneration (boost- ing). In contrast, signals on copper cabling need to be repeated after approximately 1.5 miles. For this reason, telephone companies originally used fiber for outside plant cabling with cable runs longer than 1.24 miles. The main factor in the increased expense of single-mode fiber is the higher cost to manufacture more exact connectors for patch panels and other devices. The core is SO small that connections and splices require much more precise tolerances than with multi-mode fiber. If the connections on single-mode fiber do not match cores exactly, the light will not be transmitted from one fiber to another. It will leak or disperse out of the end of the fiber core at the splice. Multi-mode fiber has a wider core than single-mode fiber. The wider core means that signals can only travel a short distance before they require amplification. In addi- tion, fewer channels can be carried per fiber pair when it is multiplexed because the 