Multimode fiber cables are wider in diameter due to light modes and are effective over medium distances. They are generally used within LANs because they can be paired with LEDs and lower-cost light sources. Which statement best describes multimode fiber?

• A. Within LANs over medium distances due to lower-cost light sources
• B. Because they are cheaper to manufacture, they are used for long-haul WANs
• C. They are used for outdoor telecom backbones
• D. They operate only with lasers

Answer: (A)

Multimode fiber has a wider core that lets several light paths (modes) travel at once. Those multiple paths don’t arrive at the same time, which causes modal dispersion and limits how far the signal can travel without distortion. Because LANs involve relatively short distances, this dispersion is manageable, making multimode fiber practical for such environments. The larger core also makes it easy and inexpensive to couple light into the fiber using inexpensive sources like LEDs and VCSELs, which keeps costs down for local networks. That combination—short-to-medium distances and low-cost light sources—is what makes multimode fiber the go-to choice for LANs. In contrast, long-haul WANs and outdoor backbones rely on single-mode fiber and laser sources to support much greater distances with less dispersion. Lasers aren’t a requirement for multimode, since LEDs and VCSELs are the typical sources.