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MX204 Network Cable and Transceiver Planning
Calculating Power Budget and Power Margin for Fiber-Optic Cables
Use the information in this topic and the specifications for your optical interface to calculate the power budget and power margin for fiber-optic cables.
You can use the Hardware Compatibility Tool to find information about the pluggable transceivers supported on your Juniper Networks device.
To calculate the power budget and power margin, perform the following tasks:
Calculating Power Budget for Fiber-Optic Cable
To ensure that fiber-optic connections have sufficient power for correct operation, you need to calculate the link's power budget, which is the maximum amount of power it can transmit. When you calculate the power budget, you use a worst-case analysis to provide a margin of error, even though all the parts of an actual system do not operate at the worst-case levels. To calculate the worst-case estimate of power budget (PB), you assume minimum transmitter power (PT) and minimum receiver sensitivity (PR):
PB = PT – PR
The following hypothetical power budget equation uses values measured in decibels (dB) and decibels referred to one milliwatt (dBm):
PB = PT – PR
PB = –15 dBm – (–28 dBm)
PB = 13 dB
Calculating Power Margin for Fiber-Optic Cable
After calculating a link's power budget, you can calculate the power margin (PM), which represents the amount of power available after subtracting attenuation or link loss (LL) from the power budget (PB). A worst-case estimate of PM assumes maximum LL:
PM = PB – LL
PM greater than zero indicates that the power budget is sufficient to operate the receiver.
Factors that can cause link loss include higher-order mode losses, modal and chromatic dispersion, connectors, splices, and fiber attenuation. Table 1 lists an estimated amount of loss for the factors used in the following sample calculations. For information about the actual amount of signal loss caused by equipment and other factors, refer to vendor documentation.
Estimated Link-Loss Value
Higher-order mode losses
Modal and chromatic dispersion
Multimode—None, if product of bandwidth and distance is less than 500 MHz-km
The following sample calculation for a 2-km-long multimode link with a power budget (PB) of 13 dB uses the estimated values from Table 1 to calculate link loss (LL) as the sum of fiber attenuation (2 km @ 1 dB/km, or 2 dB) and loss for five connectors (0.5 dB per connector, or 2.5 dB) and two splices (0.5 dB per splice, or 1 dB) as well as higher-order mode losses (0.5 dB). The power margin (PM) is calculated as follows:
PM = PB – LL
PM = 13 dB – 2 km (1 dB/km) – 5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB
PM = 13 dB – 2 dB – 2.5 dB – 1 dB – 0.5 dB
PM = 7 dB
The following sample calculation for an 8-km-long single-mode link with a power budget (PB) of 13 dB uses the estimated values from Table 1 to calculate link loss (LL) as the sum of fiber attenuation (8 km @ 0.5 dB/km, or 4 dB) and loss for seven connectors (0.5 dB per connector, or 3.5 dB). The power margin (PM) is calculated as follows:
PM = PB – LL
PM = 13 dB – 8 km (0.5 dB/km) – 7(0.5 dB)
PM = 13 dB – 4 dB – 3.5 dB
PM = 5.5 dB
In both examples, the calculated power margin is greater than zero, indicating that the link has sufficient power for transmission and does not exceed the maximum receiver input power.
CB-RE and RCB Interface Cable and Wire Specifications for MX Series Routers
Table 2 lists the specifications for the cables that connect to management ports and the wires that connect to the alarm relay contacts.
In routers where the Routing Engine (RE) and Control Board (CB) are integrated into a single board, a CB-RE is known as Routing and Control Board (RCB). The RCB is a single FRU that provides RE and CB functionality.
Table 2: Cable and Wire Specifications for Routing Engine and RCB Management and Alarm Interfaces
Routing Engine console or auxiliary interface
RS-232 (EIA-232) serial cable
1.83-m length with RJ-45/DB-9 connectors
Routing Engine Ethernet interface
Category 5 cable or equivalent suitable for 100Base-T operation
One 4.57-m length with RJ-45/RJ-45 connectors
Alarm relay contacts
Wire with gauge between 28-AWG and 14-AWG (0.08 and 2.08 mm2)
Understanding Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion
This topic describes signal loss, attenuation, and dispersion in fiber-optic cable.
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable
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 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, higher-order mode loss results. Together these factors limit the transmission distance of multimode fiber compared with 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.
Exceeding the maximum transmission distances can result in significant signal loss, which causes unreliable transmission.
Attenuation and Dispersion in Fiber-Optic Cable
Correct functioning of an optical data 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 splices, and connectors. Although 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 over time. The following two types of dispersion can affect an optical data link:
Chromatic dispersion—Spreading of the signal over time resulting from the different speeds of light rays.
Modal dispersion—Spreading of the signal over 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.
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.
When chromatic dispersion is at the maximum allowed, its effect can be considered as a power penalty in the power budget. The optical power budget must allow for the sum of component attenuation, power penalties (including those from dispersion), and a safety margin for unexpected losses.