Calculating Power Budget For Fiber-Optic Cable For M Series, Mx Series, And T Series Routers; Calculating Power Margin For Fiber-Optic Cable For M Series, Mx Series, And T Series Routers - Juniper T640 Hardware Manual

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Calculating Power Budget for Fiber-Optic Cable for M Series, MX Series, and T Series Routers

Related
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Calculating Power Margin for Fiber-Optic Cable for M Series, MX Series, and T Series Routers

Copyright © 2010, Juniper Networks, Inc.
tolerate different input power levels. Transceivers that have a longer reach can transmit
enough power to saturate the receivers on PICs that have a shorter reach. Specifically,
LR transceivers can saturate IR PICs, and both IR and LR transceivers can saturate SR
PICs. Transceivers in the same reach class can also saturate one another.
To prevent saturation, you might need to attenuate power at the PIC receiver, particularly
if you know that it has a shorter reach than the transceiver that is sending the signal.
Determine the amount of attenuation needed by measuring the power level at each
receiver. Attenuate the power to bring it within the allowable range; for short lengths of
fiber, with fiber and connector loss close to zero, an attenuator of 5 to 10 dB should be
sufficient.
For specifications of minimum and maximum input level (receiver sensitivity and receiver
saturation) and minimum and maximum output level (average launch power) for the
SONET/SDH PICs supported on the T640 router, see the T640 Core Router PIC Guide.
T640 PIC Description on page 29
M Series, MX Series, and T Series Transceiver Overview
Supported Network Interface Standards by Transceiver
To ensure that fiber-optic connections have sufficient power for correct operation, you
must calculate the link's optical power budget, which is the maximum amount of optical
power it can transmit. When you calculate the optical 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 optical
power budget ( ), you assume minimum transmitter power (
P
B
sensitivity ( ):
P
R
P = P – P
B T R
The following hypothetical optical power budget equation uses values measured in
decibels (dB) and decibels referred to one milliwatt (dBm):
P = P – P
B T R
P = –15 dBm – (–28 dBm)
B
P = 13 dB
B
T640 PIC Description on page 29
M Series, MX Series, and T Series Transceiver Overview
Supported Network Interface Standards by Transceiver
After calculating a link's optical power budget you can calculate the optical power margin
( ), which represents the amount of power available after subtracting attenuation or
P
M
Appendix E: T640 Router Cable and Wire Guidelines and Specifications
) and minimum receiver
P
T
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