T4000 Router Hardware Guide
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.
Related
Documentation
• Calculating Power Budget and Power Margin for Fiber-Optic Cables on page 348
Calculating Power Budget and Power Margin for Fiber-Optic Cables
1.
Calculating Power Budget for Fiber-Optic Cable on page 348
2.
Calculating Power Margin for Fiber-Optic Cable on page 348
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 (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
Calculating 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
A 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 (HOL), modal and chromatic dispersion, connectors, splices, and fiber attenuation.
Table 52 on page 349 lists an estimated amount of loss for the factors used in the following sample calculations.
348 Copyright © 2013, Juniper Networks, Inc.
