Signal Loss in Multimode and Single-Mode Fiber-Optic Cable for the M120 Router
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.
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.
The router uses optical lasers for SONET/SDH PIC single-mode interfaces. These optics
comply with IR-1 of Bellcore GR-253-CORE Issue 2, December 1995 and ANSI TI.105.06.
For information about the maximum transmission distances and wavelength ranges
supported by multimode and single-mode (ATM and SONET/SDH) PIC interfaces, see
the
M120 PIC Guide
. Exceeding the maximum transmission distances can result in
significant signal loss, which causes unreliable transmission.
Related
Documentation
Fiber-Optic and Network Cable Specifications for the M120 Router on page 241
•
•
Attenuation and Dispersion in Fiber-Optic Cable on M Series, MX Series, and T Series
Routers on page 243
•
Calculating Power Budget for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 244
•
Calculating Power Margin for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 244
Attenuation in M120 SONET/SDH PICs
SONET/SDH transceivers in the different reach classes—such as short reach (SR),
intermediate reach (IR), or long reach (LR)—generate different output power levels and
tolerate different input power levels. Transceivers that have a longer reach can transmit
enough power to saturate the receivers on PICs that have a shorter reach. Specifically,
LR transceivers can saturate IR PICs, and both IR and LR transceivers can saturate SR
PICs. Transceivers in the same reach class can also saturate one another.
To prevent saturation, you might need to attenuate power at the PIC receiver, particularly
if you know that it has a shorter reach than the transceiver that is sending the signal.
Determine the amount of attenuation needed by measuring the power level at each
receiver. Attenuate the power to bring it within the allowable range; for short lengths of
fiber, with fiber and connector loss close to zero, an attenuator of 5 to 10 dB should be
sufficient.
Copyright © 2010, Juniper Networks, Inc.
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M120 Multiservice Edge Router Hardware Guide
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