# Calculating the EX 8200 series Switch Fiber-Optic Cable Power Margin

Calculate the 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 (P_{M} ) is the amount of power available
after attenuation or link loss (LL) has been subtracted from
the power budget (P_{B} ).

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 (P_{M} ) 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 (P_{M} ) 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.

Before you begin to calculate the power margin:

- Calculate the power budget. See Calculating the EX 8200 series Switch Fiber-Optic Cable Power Budget.

To calculate the worst-case estimate for the
power margin (P_{M} ) for the link:

- 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 (P
_{B}) is 13 dBm):Table 1: Estimated Values for Factors Causing Link Loss

**Note:**For information about the actual amount of signal loss caused by equipment and other factors, see your vendor documentation for that equipment. - Calculate the (P
_{M}) by subtracting (LL) from (P_{B}):P

_{B}– LL = P_{M}13 dBm – 0.5 dBm [HOL] – 5 (0.5 dBm) – 2 (0.5 dBm) – 2 km (1.0 dBm/km) – 1 dB [CRM] = P

_{M}13 dBm – 0.5 dBm – 2.5 dBm – 1 dBm – 2 dBm – 1 dBm = P

_{M}P

_{M}= 6 dBmThe 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 specification for your receiver to find the maximum receiver input power.