An optical fiber with a core diameter of less than 10 microns. Used for high-speed transmission over long distances, it provides greater bandwidth than multimode, but its smaller core makes it more difficult to couple the light source. Increasingly, singlemode fiber is used for shorter distances. When singlemode fiber is used in shorter distances, such as a campus or metropolitan area network (MAN), step-index fiber is used. For longer distances and for transmitting multiple channels (WDM), dispersion-shifted fiber is used. Contrast with multimode fiber. See dispersion-shifted fiber, step-index fiber, dispersion and fiber optics glossary.