Juniper CBL-M40-PWR-AU ユーザーズマニュアル
Fiber-Optic and Network Cable Guidelines
M40 Internet Router Hardware Guide
50
Multimode and Single-Mode Fiber
Multimode 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 LEDs as light
sources. LEDs are not coherent sources, however. They spray varying wavelengths 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, higher-order mode loss (HOL) results. Together these
factors limit the transmission distance of multimode fiber compared to single-mode fiber.
off the walls of the fiber). Interfaces with multimode optics typically use LEDs as light
sources. LEDs are not coherent sources, however. They spray varying wavelengths 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, higher-order mode loss (HOL) results. Together these
factors limit the transmission distance of 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. Lasers generate a
single wavelength of light, which travels in a straight line through the single-mode fiber.
Compared with multimode fiber, single-mode fiber has higher bandwidth and can carry
signals for longer distances. It is consequently more expensive.
layer only. Interfaces with single-mode optics use lasers as light sources. Lasers generate a
single wavelength of light, which travels in a straight line through the single-mode fiber.
Compared with multimode fiber, single-mode fiber has higher bandwidth and can carry
signals for longer distances. It is consequently more expensive.
For information about the maximum transmission distance and supported wavelength range
for the types of single-mode and multimode fiber-optic cable used by PICs on the M40 router,
see the M20 and M40 Internet Routers PIC Guide. Exceeding the maximum transmission
distances can result in significant signal loss, which causes unreliable transmission.
for the types of single-mode and multimode fiber-optic cable used by PICs on the M40 router,
see the M20 and M40 Internet Routers PIC Guide. Exceeding the maximum transmission
distances can result in significant signal loss, which causes unreliable transmission.
The router uses optical lasers for SONET/SDH PIC single-mode interfaces. These optics
comply with IR-1 of Telcordia Technologies document GR-253-CORE Issue 2, December 1995
and ANSI TI.105.06.
comply with IR-1 of Telcordia Technologies document GR-253-CORE Issue 2, December 1995
and ANSI TI.105.06.
Attenuation and Dispersion
A functional optical data link depends on modulated light reaching the receiver with enough
power to be correctly demodulated. 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 splices, and connectors. While attenuation is significantly lower for optical fiber than
for other media, it still occurs in both multimode and single-mode transmission. An efficient
optical data link must have enough light available to overcome attenuation.
power to be correctly demodulated. 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 splices, and connectors. While attenuation is significantly lower for optical fiber than
for other media, it still occurs in both multimode and single-mode transmission. An efficient
optical data link must have enough light available to overcome attenuation.
Dispersion is the spreading of the signal in time. The following two types of dispersion can
affect an optical data link:
affect an optical data link:
Chromatic dispersion—The spreading of the signal in time resulting from the different
speeds of light rays.
Modal dispersion—The spreading of the signal in time resulting from the different
propagation modes in the fiber.
For multimode transmission, modal dispersion, rather than chromatic dispersion or
attenuation, usually limits the maximum bit rate and link length. For single-mode
transmission, modal dispersion is not a factor. However, at higher bit rates and over longer
distances, chromatic dispersion rather than modal dispersion limits maximum link length.
attenuation, usually limits the maximum bit rate and link length. For single-mode
transmission, modal dispersion is not a factor. However, at higher bit rates and over longer
distances, chromatic dispersion rather than modal dispersion limits maximum link length.
An efficient optical data link must have enough light to exceed the minimum power that the
receiver requires to operate within its specifications. In addition, the total dispersion must be
less than the limits specified for the type of link in Telcordia Technologies document
GR-253-CORE (Section 4.3) and International Telecommunications Union (ITU) document
G.957.
receiver requires to operate within its specifications. In addition, the total dispersion must be
less than the limits specified for the type of link in Telcordia Technologies document
GR-253-CORE (Section 4.3) and International Telecommunications Union (ITU) document
G.957.