Preparing for Router Installation
Fiber Optic and Network Cable Specifications
The router supports PICs that use various kinds of networ kc able,in cluding
multimode and single-mode fiber-optic cable. For informationabout the type
ofcable usedbyeachPIC,seetheM160 I nternet Router PIC Guide.
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable
Multimode fiber is largeenough in diameterto allow rays of light to reflect internally
(bounce off the walls of the fiber). Interfaces with multimode optics typically
use LEDs as light sources. LEDs are not coherentsources, however. They spray
varying wavelengths of light into the multimode fiber, which reflects the light at
different angles. Light rays travelin jagged lines through a multimode fiber, causing
signal dispersion. When light traveling in the fiber core radiates into the fiber
cladding, higher-order mode loss (HOL) results. Together these factors limit the
transmission distanceof multimode fiber compared to single-mode fiber.
Single-mode fiber is so small in diameter that rays of light can reflect internally
through one layer only. Interfaces with single-mode optics use lasers as
light sources. Las ers generate a single wavelength of light, which travels in
astraightlinethroughthesingle-modefiber. Comparedwithmultimode
fiber, single-mode fiber has higher bandwidth and can c arry signals for
longer distances. It is consequently more expensi ve.
For information about themaximum transmission distance and supported
wavelength rangefor the types of single-mode and multimode fiber-optic cable
used by PICs on the M160 router, see the M160 Internet Router PIC Guide.
Exceeding the maximum transmission di stances can result in significant
signal loss, which causes unreliable transmission.
The router uses optical lasers forSONET/SDH PIC single-mode interfaces.
TheseopticscomplywithIR-1ofBellcoreGR-253-COREIssue2,
December 1995 and ANSI TI.105.06.
Attenuation and Dispersion in Fiber-Optic Cable
Correct functioning of an opticaldata link depends on modulated light reaching
the receiver with enough power to be demodulated correctly. Attenuation is the
reduction in power of the light signal as it is transmitted. Attenuation is caused
by passive media components, such as cables, cable spli ces, and connectors.
While attenuation is significantly lower for optical fiber t han for other media, it
still occurs in both multimode and single-modetransmission. An efficient optical
data link must have enoughlight available to overcome attenuation.
Network Cable Specifications and Guidelines 71