# MX104 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:

### How to Calculate 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 (P_{B}), you assume minimum transmitter
power (P_{T}) and minimum receiver sensitivity
(P_{R}):

P_{B} = P_{T} –
P_{R}

The following hypothetical power budget equation uses values measured in decibels (dB) and decibels referred to one milliwatt (dBm):

P_{B} = P_{T }–
P_{R}

P_{B} = –15 dBm – (–28
dBm)

P_{B} = 13 dB

### How to Calculate Power Margin for Fiber-Optic Cable

After calculating a link's power budget, you can calculate the
power margin (P_{M}), which represents the amount
of power available after subtracting attenuation or link loss (LL)
from the power budget (P_{B}). A worst-case estimate
of P_{M} assumes maximum LL:

P_{M} = P_{B} –
LL

P_{M} 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 |

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 power budget (P_{B}) 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 (P_{M}) is calculated as follows:

P_{M} = P_{B} –
LL

P_{M} = 13 dB – 2 km (1 dB/km) –
5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB

P_{M} = 13 dB – 2 dB – 2.5 dB
– 1 dB – 0.5 dB

P_{M} = 7 dB

The following sample calculation for an 8-km-long single-mode
link with a power budget (P_{B}) 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 (P_{M}) is calculated as follows:

P_{M} = P_{B} –
LL

P_{M} = 13 dB – 8 km (0.5 dB/km) –
7(0.5 dB)

P_{M} = 13 dB – 4 dB – 3.5 dB

P_{M} = 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.

## Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion

### 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.