ou may encounter some Web sites as well as trade publications that refer to the target transmission distance. In addition to variances in transmission distance based on the diameter of the fiber in micrometers, the transmission distance also varies based upon modal bandwidth expressed in megahertz per kilometer. Chapter Five In a copper-medium environment bandwidth represents a measure of the information-carrying capacity of the media. In an optical-medium environment other factors affect bandwidth and the transmission capacity of the media. Those factors include the dispersion, or spreading, of light pulses as they travel down the core of a fiber. In a multimode fiber envi- ronment, the primary source of dispersion is modal dispersion. Modal dis- persion, as indicated earlier in this book, occurs when individual modes of light take different paths through a fiber, thus traversing the length of the fiber at different times. All multimode fibers have a direct correlation between modal spreading and their usable bandwidth; that is, as modal dis- persion increases, the usable bandwidth, which is referred to as modal band- width, decreases. The metric used to define modal bandwidth is MHz.km, which denotes the signaling rate over a specified distance. Thus, any increase in a fiber's modal bandwidth will result in a direct increase in the transmission distance obtainable over a fiber at a given data rate. We will observe this effect when we observe a table of IEEE 802.3z optical media limitations and a figure that plots transmission distance as a function of fiber bandwidth. In addition, after we complete our tour of Gigabit Ether- net specifications, we will tabulate the various flavors of Gigabit Ethernet. 1000BASE-LX A second optical-medium Gigabit Ethernet specification is 1000BASE-LX. Here the mnemonic LX denotes the use of a long-wave- length laser. This specification denotes the use of 1300-nm laser trans- ceivers operating on either single-mode or multimode fiber. When used with 62.5-um fiber, a transmission d