The Fiber Optic Association - Tech Topics
Cable
Plant Link Loss Budget
Analysis
Loss budget analysis is the calculation and verification of a fiber optic system's operating characteristics. This encompasses items such as routing, electronics, wavelengths, fiber type, and circuit length. Attenuation and bandwidth are the key parameters for budget loss analysis.
Analyze Link Loss In The Design Stage
Prior to designing or installing a fiber optic system, a loss budget analysis is reccommended to make certain the system will work over the proposed link. Both the passive and active components of the circuit have to be included in the budget loss calculation. Passive loss is made up of fiber loss, connector loss, and splice loss. Don't forget any couplers or splitters in the link. Active components are system gain, wavelength, transmitter power, receiver sensitivity, and dynamic range. Prior to system turn up, test the circuit with a source and FO power meter to ensure that it is within the loss budget.
The idea of a loss budget is to insure the network equipment will work over the installed fiber optic link. It is normal to be conservative over the specifications! Don't use the best possible specs for fiber attenuation or connector loss - give yourself some margin!
The best way to illustrate calculating a loss budget is to show how it's done for a 2 km multimode link with 5 connections (2 connectors at each end and 3 connections at patch panels in the link) and one splice in the middle. See the drawings below of the link layout and the instantaneous power in the link at any point along it's length, scaled exactly to the link drawing above it.
Cable
Plant Passive Component
Loss
Step
1. Fiber loss at the operating
wavelength
| Cable Length | 2.0 | 2.0 | ||
| Fiber Type | Multimode | Singlemode | ||
| Wavelength (nm) | 850 | 1300 | 1300 | 1550 |
| Fiber Atten. dB/km | 3 [3.5] | 1 [1.5] | 0.4 [1/0.5] | 0.3 [1/0.5] |
| Total Fiber Loss | 6.0 [7.0] | 2.0 [3.0] |
(All specs in brackets are maximum values per EIA/TIA 568 standard. For singlemode fiber, a higher loss is allowed for premises applications. )
Step 2. Connector Loss
Multimode
connectors will have
losses of 0.2-0.5 dB typically. Singlemode connectors, which are
factory made and fusion spliced on will have losses of 0.1-0.2
dB. Field terminated singlemode connectors may have losses as
high as 0.5-1.0 dB. Let's calculate it at both typical and worst
case values.
| Connector Loss | 0.3 dB (typical adhesive/polish conn) | 0.75 dB (TIA-568 max acceptable) |
| Total # of Connectors | 5 | 5 |
| Total Connector Loss | 1.5 dB | 3.75 dB |
(All connectors are allowed 0.75 max per EIA/TIA 568 standard)
Step 3. Splice Loss
Multimode
splices are usually
made with mechanical splices, although some fusion splicing is
used. The larger core and multiple layers make fusion splicing
abut the same loss as mechanical splicing, but fusion is more
reliable in adverse environments. Figure 0.1-0.5 dB for multimode
splices, 0.3 being a good average for an experienced installer.
Fusion splicing of singlemode fiber will typically have less than
0.05 dB (that's right, less than a tenth of a dB!)
| Typical Splice Loss | 0.3 dB |
| Total # splices | 1 |
| Total Splice Loss | 0.3 dB |
(All splices are allowed 0.3 max per EIA/TIA 568 standard)
Step 4. Total Passive System Attenuation
Add the fiber loss, connector and splice losses to get the link loss.
| Best Case | TIA 568 Max | |||
| 850 nm | 1300 nm | 850 nm | 1300 nm | |
| Total Fiber Loss (dB) | 7.0 | 2.0 | 7.0 | 3.0 |
| Total Connector Loss (dB) | 1.5 | 1.5 | 3.75 | 3.75 |
| Total Splice Loss (dB) | 0.3 | 0.3 | 0.3 | 0.3 |
| Other (dB) | 0 | 0 | 0 | 0 |
| Total Link Loss (dB) | 8.8 | 3.8 | 11.05 | 7.05 |
Remember these should be the criteria for testing. Allow +/- 0.2 -0.5 dB for measurement uncertainty and that becomes your pass/fail criterion.
Equipment
Link Loss Budget
Calculation: Link loss
budget for network hardware depends on the dynamic range, the
difference between the sensitivity of the receiver and the output
of the source into the fiber. You need some margin for system
degradation over time or environment, so subtract that margin
(as much as 3dB) to get the loss budget for the link.
Step
5. Data From Manufacturer's
Specification for Active Components (Typical 100 Mb/s link)
| Operating Wavelength (nm) | 1300 |
| Fiber Type | MM |
| Receiver Sens. (dBm@ required BER) | -31 |
| Average Transmitter Output (dBm) | -16 |
| Dynamic Range (dB) | 15 |
| Recommended Excess Margin (dB) | 3 |
Step
6. Loss Margin Calculation
| Dynamic Range (dB) (above) | 15 | 15 |
| Cable Plant Link Loss (dB) | 3.8 (Typ) | 7.05 (TIA) |
| Link Loss Margin (dB) | 11.2 | 7.95 |
As a general rule, the Link Loss Margin should be greater than approximately 3 dB to allow for link degradation over time. LEDs in the transmitter may age and lose power, connectors or splices may degrade or connectors may get dirty if opened for rerouting or testing. If cables are accidentally cut, excess margin will be needed to accommodate splices for restoration.
NOTE: Many techs forget when doing a loss budget that the connectors on the end of the cable plant must be included in the loss budget. When the cable plant is tested, the reference cables will mate with those connectors and their loss will be included in the measurements.
Related Topics:
Table of the cable plant length and loss margins for most LANs and Links
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