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TEST & MEASUREMENT
Multimode Loss Budget 850nm 1300nm
Cable Length 90m 0.09km 0.09km
3.0dB/km 1.5dB/km
0.03dB 0.01dB
2 2
Cable Attenuation Coefficient
Cable Loss (length x attenuation)
# Connector pairs
Connector Insertion Loss (IL)
Reference-to-Standard grade 0.5dB 0.50dB
Connector Loss (# connectors x IL) 1.0dB 1.00dB
# Splices 0 0
Splice IL 0.30dB 0.30dB
0dB 0dB
1.03dB 1.01dB
Splice Loss (# splices x IL)
Total Link Budget
Multimode Loss Budget 850nm 1300nm
Cable Length 1500m 1.50km 1.50km
Cable Attenuation Coefficient 3.0dB/km 1.5dB/km
Cable Loss (length x attenuation) 4.50dB 2.25dB
# Connector pairs 2 2
Reference-to-Standard grade 0.5dB 0.50dB
Connector Loss (# connectors x IL) 1.0dB 1.00dB
# Splices 2 2
Splice IL 0.30dB 0.30dB
Splice Loss (# splices x IL) 0.60dB 0.60dB
Connector Insertion Loss (IL)
Total Link Budget
6.10dB
3.85dB
Using application-specific limits
It’s easiest to determine a loss budget when the application
that the fibre will support is known because you can look
up the requirements for each application. On the next
page is an excerpt from the TIA-568.0-D standard that lists
requirements for a few common Ethernet applications.
These charts make easy work of determining loss budgets
as long as the application that will be running over the fibre
is known. Plus, some fibre certifiers incorporate a database
with these values, so you can simply choose the appropriate
standard, and the pass/fail limits are set automatically.
Calculating loss budgets for networking equipment
When the networking equipment that will be running over
the fibre is known, the requirements of that can be used to
determine the loss budget. Each piece of optical networking
equipment, even SFP/GBIC modules will have specifications
for output power and receiver sensitivity. These tell the user
exactly how much loss the equipment can sustain and still
operate properly.
Transmitter output specifies how ‘bright’ the light is that
is emitted from the transmitter port. The specification will
be in dBm (not dB). A typical device may have an output
power of -20dBm. Don’t let the negative confuse you, it
doesn’t mean negative power. 0dBm is a reference to 1mW
(milliwatt) of power, and a negative dBm value simply
means less than 1mW.
Power scale:
0 dBm = 1 mW
-50 dBm = .00001 mW
Note: dBm is an amount of power. dB is a change in power
from one level to another.
A receiver usually has two specifications: receiver sensitivity
and dynamic range.
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