FOA Guide

Power Budgets And Loss Budgets

The terms "power budget" and "loss budget" are often confused.

The power budget refers to the amount of fiber optic cable plant loss that a datalink (transmitter to receiver) can tolerate in order to operate properly. Sometimes the power budget has both a minimum and maximum value, which means it needs at least a minimum value of loss so that it does not overload the receiver and a maximum value of loss to ensure the receiver has sufficient signal to operate properly.

The loss budget is the amount of loss that a cable plant should have if it is installed properly. It is calculated by adding the estimated average losses of all the components used in the cable plant to get the estimated total end-to-end loss. The loss budget has two uses, 1) during the design stage it is used to ensure the cabling being designed will work with the links intended to be used over it and 2) after installation, the loss budget for the cabling is compared to the actual test results to ensure the cable plant is installed properly.

Some standards refer to the loss budget as the "attenuation allowance" but there seems to be very limited use of that term.

Obviously, the power budget and loss budget are related. A data link will only operate if the cable plant loss is within the power budget of the link.

Remember the calculated loss budget is an estimate that assumes the values of component losses and does not take into account the uncertainty of the measurement. Be aware of this because if measurements are close to the loss budget estimates, some judgement is needed to not fail good fibers and pass bad ones! This is discussed in depth in the page on "Installation Deliverables."

Power Budget

All datalinks are limited by the power budget of the link. The power budget is the difference between the output power of the transmitter and the input power requirements of the receiver, both of which are defined as power coupled into or out of optical fiber of a type specified by the link. The power budget is not just a straightforward determinant of the maximum loss in the cable plant that the link can tolerate. As shown below, cable plant loss is only a part of the power budget. Distortion impairments, for example from dispersion (modal and chromatic dispersion in MM fiber, chromatic and polarization mode dispersion in SM fiber), reduce the power budget. In multimode gigabit Ethernet networks, for example, transceivers have a dynamic range (transmitter output to receiver sensitivity) of about 5-6 dB before dispersion is factored in, leaving a power budget of about 2 dB.

Noise in transceivers, mainly in the receiver, affect the power budget also. The receiver has an operating range determined by the signal-to-noise ratio (S/N) in the receiver. The S/N ratio is generally quoted for analog links while the bit-error-rate (BER) is used for digital links. BER is practically an inverse function of S/N. Transceivers may also be affected by the distortion of the transmitted signal as it goes down the fiber, a big problem with multimode links at high speeds or very long OSP singlemode links.

When testing a fiber in a cable plant to determine if the cable plant will allow a specific link to operate over it, the test should be made from transceiver to transceiver, e.g. the cable plant with patchcords installed on either end that would be used to connect the transceivers to the cable plant. When doing a link loss budget (below) for the cabling to be used with a given link to determine if the link will operate over that link, the loss of the patchcords may also be included.

Testing The Power Budget  For A Link

How is the power budget determined? You test the link under operating conditions and insert loss while watching the data transmission quality. The test setup is like this:

Measuring fiber optic link power budget
Connect the transmitter and receiver with patchcords to a variable attenuator. Increase attenuation until you see the link has a high bit-error rate (BER  for digital links) or poor signal-to-noise ratio (SNR for analog links). By measuring the output of the transmitter patchcord (point #1) and the output of the receiver patchcord (point #2), you can determine the maximum loss of the link  and the maximum power the receiver can

From this test you can generate a graph that looks like this:
fiber optic BER
A receiver must have enough power to have a low BER (or high SNR, the inverse of BER) but not so much it overloads and signal distortion affects transmission. We show it as a function of receiver power here but knowing transmitter output, this curve can be translated to loss - you need low enough loss in the cable plant to have good transmission but with low loss the receiver may overload, so you add an attenuator at the receiver to get the loss up to an acceptable level.

You must realize that not all transmitters have the same power output nor do receivers have the same sensitivity, so you test several (often many) to get an idea of the variability of the devices. Depending on the point of view of the manufacturer, you generally error on the conservative side so that your likelihood of providing a customer with a pair of devices that do not work is low. It's easier that way.


Furthermore, if your link uses multimode fiber at high bit rates (or singlemode on long links at very high bit rates), there will be dispersion. Dispersion spreads out the pulses, causing a power penalty. That's why high speed Ethernet at 10G has a loss budget of 2dB while the power budget calculated from transmitter and receiver specifications is about 6dB.

Calculating Cable Plant Link Loss Budget

Loss budget analysis is the calculation of a fiber optic cabling system's estimated loss performance characteristics. This is sometimes confused with the communication system "power budget" which is a specification of the dynamic range of the electronics, the difference between the output power of the transmitter coupled into the fiber and the minimum received power required at the receiver for proper data transmission. The communications system power budget will set a limit for the loss of the cable plant.

The cable plant loss budget needs to consider transceiver wavelength, fiber type, and link length plus the losses incurred in splices, connections and other passive devices like FTTH or OLAN PON splitters. Attenuation and bandwidth/dispersion are the key parameters for the cable plant loss budget analysis.

FOA has a online Loss Budget Calculator web page that will calculate the loss budget for your cable plant. This is a good page to bookmark on your smartphone, tablet and/or laptop to have for making calculations in the field.

FOA Loss Budget App
FOA has a free app for iOS smartphones and tablets that will calculate loss budgets for the cable plant you are designing or testing. See the app store for your device for details.

Analyze Link Loss In The Design Stage

Prior to designing or installing a fiber optic cabling system, a loss budget analysis is recommended to make certain the system will work over the proposed link. That same loss budget will be used as to compare test results after installation of the cabling to ensure that the components were installed correctly. Both the passive and active components of the circuit have to be included in the loss budget 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 ensure 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 typical 0.2 km multimode link. The link may be analyzed and tested in two ways, with or without the patchcords that connect the equipment. With the patchcords, the cable plant has 5 connections (2 connectors at each end to connect to patchcords connecting to the transmitter and receiver), 3 connections at patch panels in the link) and one splice in the middle. Without the patchcords, the cable plant has 3 connections (2 connectors at each end for the transmitter and receiver), 1 connection at a patch panel 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.

Loss Budget

At the top is a fiber optic link with a transmitter connected to. a cable plant with a patchcord. The cable plant has 1 intermediate connection and 1 splice plus, of course, "connectors" on each end which become "connections" when the transmitter and receiver patchcords (or reference test cables) are connected. At the receiver end, a patchcord connects the cable plant to the receiver.

Note: A connector is the hardware attached to the end of a fiber which allows it ti be connected to another fiber or a transmitter or receiver. When two connectors are mated to join two fibers, usually requiring a mating adapter, it is called a connection. Connectors have no loss; only connections have loss.

Below the drawing of the fiber optic link above is a graph of the power in the link over the length of the link.  The vertical scale (Y) is optical power at the distance from the transmitter shown in the horizontal (X) scale. As optical signal from the transmitter travels down the fiber, the fiber attenuation and losses in connections and splice reduces the power as shown in the green graph of the power.

Note: That graph above looks like an OTDR trace. The OTDR sends a test pulse down the fiber and backscatter allows the OTDR to convert that into a snapshot of what happens to a pulse going down the fiber. The power in the test pulse is diminished by the attenuation of the fiber and the loss in connectors and splices. In our drawing, we don't see reflectance peaks but that additional loss is included in the loss of the connector.

On the left side of the graph, we show the power coupled from the transmitter into its patchcord, measured at point #1 (the end of the transmitter patchcord) and the attenuated signal at the end of the patchcord connected to the receiver shown at point #2. We also show the receiver sensitivity, the minimum power required for the transmitter and receiver to send error-free data.

The difference between the transmitter output and the receiver sensitivity is the power budget. Expressed in dB, the power budget is the amount of loss the link can tolerate and still work properly -
to send error-free data. The difference between the transmitter output (point #1) and the receiver power at its input (point #2) is the actual loss of the cable plant experienced by the fiber optic data link.

The difference between the power coupled into the cable plant and the power at the receiver is the loss of the cable plant. That's what we estimate when we calculate a loss budget.
It's also what is called "insertion loss" tested with a test source and power meter.

Note: This concept gets many questions - but two are most common. Why do you include the loss of the connectors on the ends if they are connected to a transmitter and receiver. And what about testing a permanently installed cable plant from patch-panel (or wall outlet) to another patch panel, not including the final patchcords used to connect equipment.

Why do you include the connectors on each end? Depending on the design of the transceivers (and especially if they have pigtailed lasers or detectors), practically every factor in connector loss affects coupling to a transmitter or receiver as well. Whether these connections are included in the loss budget should depend on whether the margin for the link to be use on the cable plant was specified to include these connectors. As far as we know almost all system specifications are considering connection losses at both ends. Unless you know the system was not specified for loss including the end connectors, include them in calculations of the loss budget.

Testing is another issue. When the cable plant is tested, the reference cables will mate with those end connectors and their loss will be included in the measurements but the results depends on the method used to set the "0dB" reference.

If the "0dB" reference for the insertion loss test was done with only one reference test cable attached between the light source and power meter which is the most common way, the connectors on the end of the cable will be included in the loss so the loss budget should include both connectors.
Most tests are specified and done with the one cable reference when the test equipment is compatible with the connectors.

If the "0dB" reference for the insertion loss test was done with three cables, the launch reference cable, a receive reference cable and a third reference cable between them, a method used for many plug and jack (male/female) connectors such as MPOs, the loss budget should not incude the connectors on the end. When making the "0dB" reference with three cables, two connections are included in setting the reference so the measured value will be reduced by the value of those two connections. If the loss budget is calculated without the connectors on the ends, the value will more closely approximate the test results with a 3-cable reference. The three cable reference is generally done with plug/jack or male/female connectors like the MPO or when doing a "channel" test specified in some standards that includes the permanently installed cable plant with patchcords attached but excludes the connectors on each end that attach to transceivers.

While the two-cable reference method is rarely used, it includes only one connector. Thus you could use the same approach when calculating loss budgets for this test method.

Whatever test method is presumed, it must be documented when the loss budget is calculated.

Example: Cable Plant Passive Component Loss - Calculating a Loss Budget
For this analysis, we'll use our 0.2 km cable plant above without the patchcords so it has 3 connections and one splice.

Step 1. Fiber loss at the operating wavelength over 200m (0.2 km)

Cable Length (km)
0.2 0.2

Fiber Type Multimode
Wavelength (nm) 850 1300 1310 1550
Fiber Atten. dB/km 3 [3.5] 1 [1.5] 0.4 [1/0.5] 0.3 [1/0.5]
Total Fiber Loss 0.60 [0.7] 0.20 [0.3]

(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. Connection Loss

Multimode connectors will have losses of 0.2-0.5 dB typically (see note about "connector" vs. "connection" loss). Singlemode connectors, which are factory made and fusion spliced on will have losses of 0.1-0.2 dB. Field terminated singlemode connectors (not recommended) may have losses as high as 0.5-1.0 dB and unacceptable reflectance.

Let's calculate it at both typical and worst case values.

Remember that we include all the components in the complete link, including the connectors on each end.

Connector Loss 0.3 dB (typical adhesive/polish conn)  0.75 dB (TIA-568 max acceptable)
Total # of Connectors 3  3
Total Connector Loss 0.9 dB  2.25 dB

Note: When people say connector loss, they really mean "connection" loss - the loss of a mated pair of connectors, expressed in "dB." Thus, testing connectors requires mating them to reference connectors which must be high quality connectors themselves to not adversely affect the measured loss when mated to an unknown connector. This is an important point often not fully explained.  In order to measure the loss of the connectors you must mate them to a similar, known good, connector. When a connector being tested is mated to several different connectors, it may have different losses, because those losses are dependent on the reference connector it is mated to.

(All connectors are allowed 0.75 max per EIA/TIA 568 standard, generally much too high except for array - MPO - connectors.)

 Remember that we include all the components in the complete link, including the connectors on each end. In our example above, the link includes patchcords on each end to connect to the electronics. We need to assess the quality of these connectors, so we include them in the link loss budget and if we test the link end to end, including the patchcords, these connectors will be included in the test results when connected to launch and receive reference cables. On some links, only the permanently installed link, not including the patchcords, will be tested. Again, we still need to include the connectors on the end as they will be included when we test insertion loss with reference test cables on each end.

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.

Typical  TIA 568 Max
   850 nm  1300 nm  850 nm 1300 nm
Total Fiber Loss (dB) 0.6 0.2  0.7 0.3
Total Connector Loss (dB) 0.9 0.9  2.25  2.25
Total Splice Loss (dB) 0.3 0.3  0.3  0.3
Other (dB) 0 0  0 0
Total Link Loss (dB) 1.8
 3.25 2.85

Note the big difference between the typical values and the TIA worst case values. Which should be used for evaluating the cable plant? If you use typical field installed connectors of the adhesive/polish type or SOCs - fusion splice on connectors, the lower/typical values are probably a good choice. If you use MPO or prepolished splice connectors with mechanical splices, the TIA values may be closer. 

In either case it is important to realize that these are estimates, just estimates, and some judgement is required.

Remember these should be the criteria for testing. Allow +/- 0.2 -0.5 dB for measurement uncertainty and that becomes your pass/fail criterion.

We can use the FOA Loss Budget Calculator web page to make the calculations. We just enter the data into the proper fields. Scroll down and click "Reset" to clear the data fields.

Try calculating the loss budget for a 25km OSP singlemode link that has 8 splices and connectors just at each end. Use the typical losses (scroll down to see the full list) in the calculator below.


FOA's online Loss Budget Calculator web page  will calculate the loss budget for your cable plant. This is a good page to bookmark on your smartphone, tablet and/or laptop to have for making calculations in the field.

Try some other cable plants for practice - try 13km singlemode at 1310nm, 4 splices and connectors only on the ends. Use the typical component loss data below the calculator or use your own estimates.

Equipment Link Power Budget Calculation: Link loss budget for network hardware depends on the dynamic range of the electronics, the difference between the sensitivity of the receiver and the output of the transmitter 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) 850
Fiber Type MM
Receiver Sens. (dBm@ required BER) -21
Average Transmitter Output (dBm) -13
Dynamic Range (dB) 8
Recommended Excess Margin (dB) 3


Step 6. Power Margin Calculation

Dynamic Range (dB) (above) 8  8
Cable Plant Link Loss (dB) 1.8 (Typ)  3.25 (TIA)
Link Loss Margin (dB) 6.2  4.75

Note that a link like this may have dispersion penalties, common for MM links at 1G or above.
As a general rule, the Link Loss Margin should be greater than approximately 3 dB to allow for link degradation over time. Sources in the transmitter may age and lose power, connectors or splices may degrade or connectors with multiple matings or may get dirty if opened for rerouting or testing. If cables are accidentally cut, excess margin will be needed to accommodate splices for restoration. The 3dB rule, of course, is irrelevant if the power budget is ~2dB like some of the 10G multimode links. Then the need for the best quality installation is critical!

Related Topics:

Guidelines On What Loss To Expect When Testing Fiber Optic Cables For Insertion Loss With A Meter and Source or OLTS

Table of the cable plant length and loss margins for most LANs and Links

More detailed information can be found on the FOA Online Reference Guide.

Return To The FOA Home Page

Return To FOA Tech Topics

(C) 2004-18 The Fiber Optic Association, Inc.