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MX480 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.

Tip:

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:

Calculate Power Budget for Fiber-Optic Cables

To ensure that fiber-optic connections have sufficient power for correct operation, you need to calculate the link's power budget (PB), 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 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

How to Calculate Power Margin for Fiber-Optic Cables

After calculating a link's PB, you can calculate the power margin (PM), which represents the amount of power available after subtracting attenuation or link loss (LL) from the 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.

Table 1: Estimated Values for Factors Causing Link Loss

Link-Loss Factor

Estimated Link-Loss Value

Higher-order mode losses

Single mode—None

Multimode—0.5 dB

Modal and chromatic dispersion

Single mode—None

Multimode—None, if product of bandwidth and distance is less than 500 MHz-km

Faulty connector

0.5 dB

Splice

0.5 dB

Fiber attenuation

Single mode—0.5 dB/km

Multimode—1 dB/km

The following sample calculation for a 2-km-long multimode link with a PB of 13 dB uses the estimated values from Table 1. This example calculates 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 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 PB of 13 dB uses the estimated values from Table 1. This example calculates 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 pPM 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 the examples, the calculated PM is greater than zero, indicating that the link has sufficient power for transmission and does not exceed the maximum receiver input power.

Understanding 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 MX10008 router uses various types of network cables, including multimode and single-mode fiber-optic cables.

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 reflect 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.

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 of the light signal during transmission. Passive media components such as cables, cable splices, and connectors cause attenuation. 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 transmit enough light to overcome attenuation.

Dispersion is the spreading of the signal over time. The following two types of dispersion can affect signal transmission through an optical data link:

  • Chromatic dispersion, which is the spreading of the signal over time caused by the different speeds of light rays.

  • Modal dispersion, which is the spreading of the signal over time caused by 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 limits the 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 within the limits specified for the type of link in the 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.

Routing Engine 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.

Note:

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

Port

Cable Specification

Maximum Length

Router Receptacle

Routing Engine console or auxiliary interface

RS-232 (EIA-232) serial cable

1.83 m

RJ-45 socket

Routing Engine Ethernet interface

Category 5 cable or equivalent suitable for 100Base-T operation

100 m

RJ-45 autosensing

Alarm relay contacts

Wire with gauge between 28-AWG and 14-AWG (0.08 and 2.08 mm2)

None

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Note:

We no longer include the RJ-45 console cable with the DB-9 adapter as part of the device package. If the console cable and adapter are not included in your device package, or if you need a different type of adapter, you can order the following separately:

  • RJ-45 to DB-9 adapter (JNP-CBL-RJ45-DB9)

  • RJ-45 to USB-A adapter (JNP-CBL-RJ45-USBA)

  • RJ-45 to USB-C adapter (JNP-CBL-RJ45-USBC)

If you want to use RJ-45 to USB-A or RJ-45 to USB-C adapter you must have X64 (64-Bit) Virtual COM port (VCP) driver installed on your PC. See, https://ftdichip.com/drivers/vcp-drivers/ to download the driver.