Juniper MX10004 - Calculate the Fiber-Optic Cable Power Budget for an MX10004 Router

transmission. Passive media components such as cables, cable splices, and connectors cause
aenuaon. Although aenuaon is signicantly lower for opcal ber than for other media, it sll
occurs in both mulmode and single-mode transmissions. An ecient opcal data link must transmit
enough light to overcome aenuaon.
Dispersion is the spreading of the signal over me. The following two types of dispersion can aect
signal transmission through an opcal data link:
• Chromac dispersion, which is caused by the dierent speeds of light rays.
• Modal dispersion, which is caused by the dierent propagaon modes in the ber.
For mulmode transmission, modal dispersion usually limits the maximum bit rate and link length.
(Chromac dispersion or aentuaon does not usually limit the maximum bit rate or link length.) For
single-mode transmission, modal dispersion is not a factor. However, at higher bit rates and over longer
distances, chromac dispersion limits the maximum link length.
An ecient opcal data link must have enough light to exceed the minimum power that the receiver
requires to operate within its specicaons. In addion, the total dispersion must be within the limits
specied for the type of link in the Telcordia Technologies document GR-253-CORE (Secon 4.3) and
Internaonal Telecommunicaons Union (ITU) document G.957.
When chromac dispersion is at the maximum allowed, its eect can be considered as a power penalty
in the power budget. The opcal power budget must allow for the sum of component aenuaon,
power penales (including those from dispersion), and a safety margin for unexpected losses.
Calculate the Fiber-Opc Cable Power Budget for an MX10004 Router
Calculate the link's power budget when planning ber-opc cable layout and distances to ensure that
ber-opc connecons have sucient power for correct operaon. The power budget is the maximum
amount of power the link can transmit. When you calculate the power budget, you use a worst-case
analysis to provide a margin of error, even though the parts of a congured system don’t operate at the
worst-case levels. We cannot use the phrase "all parts . . . don't operate." You can write "none of the
parts operate," however.
To calculate the worst-case esmate for the ber-opc cable power budget (P
B
) for the link:
1. Determine values for the link's minimum transmier power (P
T
) and minimum receiver sensivity
(P
R
). For example, in the following example, we measure (P
T
) and (P
R
) in decibels per milliwa (dBm):
P
T
= –15 dBm
P
R
= –28 dBm
98