Understanding J-EX8200 Switch Fiber-Optic Cable Signal Loss, Attenuation, andDispersionTo determine the power budget and power margin needed for fiber-optic connections,you need to understand how signal loss, attenuation, and dispersion affect transmission.J-EX8200 Ethernet Switches use various types of network cable, including multimodeand single-mode fiber-optic cable.• Signal Loss in Multimode and Single-Mode Fiber-Optic Cable on page 78• Attenuation and Dispersion in Fiber-Optic Cable on page 78Signal Loss in Multimode and Single-Mode Fiber-Optic CableMultimode fiber is large enough in diameter to allow rays of light to reflect internally(bounce off the walls of the fiber). Interfaces with multimode optics typically use LEDsas light sources. However, LEDs are not coherent light sources. They spray varyingwavelengths of light into the multimode fiber, which reflects the light at different angles.Light rays travel in jagged lines through a multimode fiber, causing signal dispersion.When light traveling in the fiber core radiates into the fiber cladding (layers of lowerrefractive index material in close contact with a core material of higher refractive index),higher-order mode loss (HOL) occurs. Together, these factors reduce the transmissiondistance of multimode fiber compared to that of single-mode fiber.Single-mode fiber is so small in diameter that rays of light reflect internally through onelayer only. Interfaces with single-mode optics use lasers as light sources. Lasers generatea single wavelength of light, which travels in a straight line through the single-mode fiber.Compared to multimode fiber, single-mode fiber has a higher bandwidth and can carrysignals for longer distances. It is consequently more expensive.For information about the maximum transmission distance and supported wavelengthrange for the types of single-mode and multimode fiber-optic cables that are connectedto line cards on the J-EX8200 switches, see “Optical Interface Support in J-EX8200Switches” on page 51. Exceeding the maximum transmission distances can result insignificant signal loss, which causes unreliable transmission.Attenuation and Dispersion in Fiber-Optic CableAn optical data link functions correctly provided that modulated light reaching the receiverhas enough power to be demodulated correctly. Attenuation is the reduction in strengthof the light signal during transmission. Passive media components such as cables, cablesplices, and connectors cause attenuation. Although attenuation is significantly lowerfor optical fiber than for other media, it still occurs in both multimode and single-modetransmission. An efficient optical data link must transmit enough light to overcomeattenuation.Dispersion is the spreading of the signal over time. The following two types of dispersioncan affect signal transmission through an optical data link:• Chromatic dispersion, which is the spreading of the signal over time caused by thedifferent speeds of light rays.78Dell PowerConnect J-Series J-EX8216 Ethernet Switch Hardware Guide
