Juniper NFX150, Hardware Manual

Page: 50 / 152

See Also Clearance Requirements for Airflow and Hardware Maintenance for an NFX150 Device
on page 45
•
•
Rack Requirements for NFX150 Devices on page 48
Network Cable and Transceiver Planning for NFX150
• Understanding NFX150 Devices Fiber-Optic Cable Signal Loss, Attenuation, and
Dispersion on page 50
•
Calculating the Fiber-Optic Cable Power Budget for an NFX150 Device on page 51
• Calculating the Fiber-Optic Cable Power Margin for an NFX150 Device on page 52
Understanding NFX150 Devices Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion
To determine the power budget and power margin needed for fiber-optic connections,
you need to understand how signal loss, attenuation, and dispersion affect transmission.
The NFX150 devices use various types of network cable, including multimode and
single-mode fiber-optic cables.
•
Signal Loss in Multimode and Single-Mode Fiber-Optic Cables on page 50
• Attenuation and Dispersion in Fiber-Optic Cable on page 50
Signal Loss in Multimode and Single-Mode Fiber-Optic Cables
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. However, LEDs are not coherent light sources. 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 (layers of lower
refractive index material in close contact with a core material of higher refractive index),
higher-order mode loss occurs. Together, these factors reduce the transmission distance
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 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 to multimode fiber, single-mode fiber has a 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 cables that are connected
to the NFX150 devices, see
“Pluggable Transceivers Supported on NFX150 Devices” on
page 54 . Exceeding the maximum transmission distances can result in significant signal
loss, which causes unreliable transmission.
Attenuation and Dispersion in Fiber-Optic Cable
An optical data link functions correctly provided that modulated light reaching the receiver
has enough power to be demodulated correctly. Attenuation is the reduction in strength
Copyright © 2018, Juniper Networks, Inc. 50
NFX150 Network Services Platform Hardware Guide