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

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 and below"
 
Cable Plant Loss Budget

The cable plant "loss budget" is a function of the losses of the components in the cable plant - fiber, connectors and splices, plus any passive optical components like splitters in PONs.



Thus the loss budget of the cable plant is a major factor in the power budget of the fiber optic link and is what one calculates to compare against tested insertion loss (and even compares to OTDR loss measurements) to determine if the cable plant is properly installed.

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 also 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 Apple app store for your device for details.

Calculating Loss Budgets

Calculating a loss budget for a cable plant involves estimating all the component losses - fiber, splices and connectors - and summing them up.

Go here for more comprehensive discussion on how to calculate a loss budget.

 

Connector Loss

For each connector, we usually figure 0.3 dB loss for most adhesive/polish or fusion splice-on connectors. The loss spec for prepolished/mechanical splice connectors or multifiber connectors like MPOs will be higher (0.75 max per EIA/TIA 568)

When testing cable plants per OFSTP-14 (double ended), include connnectors on both ends of the cable when using the 1-cable reference For other options see the note below. When testing per FOTP-171 (single ended), include only one connector - the one attached to the launch cable.

 

Splice Loss

For each splice, figure 0.3 dB for multimode mechanical splices (0.3 max per EIA/TIA 568) and 0.15dB for singlemode fusion splices. 

 

Fiber Loss

For multimode fiber, the loss is about 3 dB per km for 850 nm sources, 1 dB per km for 1300 nm. (3.5 and 1.5 dB/km max per EIA/TIA 568) This roughly translates into a loss of 0.1 dB per 100 feet (30 m) for 850 nm, 0.1 dB per 300 feet(100 m) for 1300 nm.

For singlemode fiber, the loss is about 0.5 dB per km for 1310 nm sources, 0.4 dB per km for 1550 nm. (1.0 dB/km for premises/0.5 dB/km at either wavelength for outside plant max per EIA/TIA 568)This roughly translates into a loss of 0.1 dB per 600 (200m) feet for 1310 nm, 0.1 dB per 750 feet (250m) for 1300 nm.

 

So for the estimated loss of a cable plant, calculate the approximate loss as:

 

(0.5 dB X # connectors) + (0.2 dB X# splices) + (fiber attenuation X the total length of cable)

 

For more information see calculate a loss budget.

 

What about OTDR testing?

OTDRs are used for verifying individual events like splice loss on long links with inline splices or for troubleshooting. All standards require an insertion loss test for qualification of the link loss. In MM fibers, the OTDR will underestimate the loss considerably - as much as 3 dB in a 10 dB link - but the amount is unpredictable. In long distance SM links, the difference may be less, but there are other measurement uncertainties, like connector or splice loss, where the OTDR can show a gain.

What happens when you test with an OTDR with its limited distance resolution? Specifically, if you have singlemode fiber terminated with fusion spliced pigtials, you cannot see the both splice and the connector losses. Or what if you have a patch panel with connections using short patchcords?

For insertion loss testing, you simply sum up all the loss contributors and get a total for the cable run. In the case of an OTDR, you are analyzing each event.

So if you have a connection point where both fibers were terminated with spliced-on pigtails, you should analyze the event as the sum of 2 fusion splices and one connection, not each individually. A patchcord termination would be two connection losses, plus splices if the termination was by splicing on pigtails.

For more on OTDRs, see the FOA Online Reference Guide on Testing or  Lennie Lightwave's Guide.

Note On Including Connectors On The Ends And Test Methods

Many designers and technicians wonder when doing a loss budget whether the connectors on the end of the cable plant should be included in the loss budget. The answer is yes, they should be included for two reasons:

1) When the cable plant is connected up to communications equipment with patchcords, the connections to the patchcords will have loss.

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

Testing standards often include 3 different ways of setting the "0dB" reference for testing loss.

All three of these methods are approved in many standards, but it is important to realize they will give different loss readings due to the connections included when making the 0dB reference measurement. The math of these methods is discussed in detail here.

1 cable reference) If the "0dB" reference for the insertion loss test was done with only 1 reference test cable attached between the light source and power meter (which is the most common way,) the connectors on both ends of the cable will be included in the loss so the loss budget should include both connectors.

3 cable reference) 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.

2 cable reference)  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.

Will the network run on that link?

Here is a table showing the loss margin for most fiber optic LANs and links. If the loss of the cable plant is less than the maximum loss allowed for the link, it should run (but you really want a little bit of margin!)

Evaluating Cable Plant Test Data Compared To The Link Loss Budget
To prove the cable plant was installed properly requires test data, of course. During the design phase, loss budgets calculated for each cable run should provide an estimate of the expected loss of the fibers in each cable link to compare to actual test results.

Short fiber optic premises cabling networks are generally tested in three ways, connector inspection/cleaning with a microscope, insertion loss testing with a light source and power meter or optical loss test set, and polarity data, meaning that the routing of fibers is confirmed so that when connecting equipment the tech can identify fiber pairs for transmit and receive. Polarity testing generally can be done with a visual fault locator to confirm that fibers are connected per the documented cable diagrams.

Outside plant (OSP) testing is more complex. If the cable plant includes cables concatenated with splices, it's expected to add OTDR testing to the connector inspection, insertion loss and polarity testing. If the link has passive devices like FTTH splitters or WDMs, those need to be tested and documented also.

There is one thing that whoever is reviewing the data - and going back to the design phase, whoever writes the test specifications based on the loss budgets in the first place - needs to understand: none of these are absolute numbers. The loss budget which is created early in the design phase estimates the loss of the cable plant based on estimates of component loss and therefore is not an absolute number, but an estimate to be used to compare to test data.

Test data is created by instruments and related components that make measurements which have  measurement errors. There are always factors in making measurements that cause the instrument reading to be inaccurate - only an approximation of the real value - and the real value is unknowable because of measurement errors. (If you are curious, look up the Heisenberg uncertainty principle.)

Let's look at this symbolically:

Example:The loss budget for each fiber in a cable plant link is 8.0 dB but the measured loss with a light source and power meter is 8.2dB. Should that fiber be rejected? Well, no, because the uncertainty of the loss budget is probably ~+/-0.5dB, providing a range of 7.5 to 8.5dB loss. The uncertainty of the loss test is probably in the same range, so the actual loss is in the range of 7.7 to 8.7dB. Thus there is considerable overlap of the loss budget and the measurement results, so there is no reason to reject this fiber. However if one fiber is testing at over 9dB, there is reason to double check tests to determine if it is acceptable. All this requires considerable judgement.