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How to Calculate the Fiber-Optic Cable Power Margin for QFX Series Switches

Before you calculate the power margin, calculate the power budget. See How to Calculate the Fiber-Optic Cable Power Budget for QFX Series Switches.

Calculate the fiber-optic data link's power margin when planning fiber-optic cable layout and distances to ensure that fiber-optic connections have sufficient signal power to overcome system losses and still satisfy the minimum input requirements of the receiver for the required performance level. The power margin (PM ) is the amount of power available after attenuation or link loss (LL) is subtracted from the power budget (PB).

When you calculate the power margin, 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 worst-case levels. A power margin (PM ) greater than zero indicates that the power budget is sufficient to operate the receiver and that it does not exceed the maximum receiver input power. This means the link will work. A power margin (PM) that is zero or negative indicates insufficient power to operate the receiver. See the specification for your receiver to find the maximum receiver input power.

To calculate the worst-case estimate for the power margin (PM) for the link:

  1. Determine the maximum value for link loss (LL) by adding estimated values for applicable link-loss factors; for example, use the sample values for various factors as provided in Table 1 (here, the link is 2 km long and multimode, and the power margin (PM) is 13 dBm).
    Table 1: Estimated Values for Factors Causing Link Loss

    Link-Loss Factor

    Estimated Link Loss Value

    Sample Link Loss Calculation Values

    Higher-order mode losses

    Multimode—0.5 dBm

    0.5 dBm

    Single-mode—None

    0 dBm

    Modal and chromatic dispersion

    Multimode—None, if the sum of bandwidth and distance is less than 500 MHz/km

    0 dBm

    Single-mode—None

    0 dBm

    Connector

    0.5 dBm

    This example assumes five connectors. Loss for five connectors: 5 (0.5 dBm) = 2.5 dBm.

    Splice

    0.5 dBm

    This example assumes two splices. Loss for two splices: 2 (0.5 dBm) = 1 dBm.

    Fiber attenuation

    Multimode—1 dBm/km

    This example assumes the link is 2 km long. Fiber attenuation for 2 km: 2 km (1 dBm/km) = 2 dBm.

    Single-mode—0.5 dBm/km

    This example assumes the link is 2 km long. Fiber attenuation for 2 km: 2 km (0.5 dBm/km) = 1 dBm.

    Clock Recovery Module (CRM)

    1 dBm

    1 dBm
    Note:

    For information about the actual amount of signal loss caused by equipment and other factors, see your vendor documentation for that equipment.

  2. Calculate the (PM) by subtracting (LL) from (PB):

    PB– LL = PM

    13 dBm – 0.5  dBm [HOL] – 5 x (0.5  dBm) – 2 (0.5 dBm) – 2 km (1.0 dBm/km) – 1 dB [CRM] = PM

    13 dB m – 0.5 dBm – 2.5  dBm – 1 dBm – 2 dBm – 1 dBm = PM

    PM = 6 dBm

    The calculated power margin is greater than zero, indicating that the link has sufficient power for transmission. Also, the power margin value does not exceed the maximum receiver input power.

    Refer to the specifications for your receiver to find the maximum receiver input power.