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How to Calculate Optical Fiber Loss and Distance Estimates

Posted By: technopediasite
Fiber loss and distance Estimates
Loss calculation

There are a number of ways to tackle the problem of determining the power requirement for a particular fiber optical link. The easiest & most accurate way is to perform an Optical Time Domain Reflectomere (OTDR) trace of the actual link. This will give you the actual loss values for the all events (Connectors, Splice and fiber loss) in the link.

In the absence of an actual OTDR traces, there are two alternatives that can be used to estimate the power requirements of the link:

1. Estimate the total link loss across an existing fiber optic link if the fiber length and loss variables are known.

2. Estimate the maximum fiber distance if the optical budget and loss variables are know.
Loss variables are connectors, splice and attenuation per kilometer of the fiber. If actual value for all of the loss variables are not known, an estimation for for each is needed to complete the calculations. In this case one would want to take a worst case approach to assure that there is adequate power available for the link. The blow table includes commonly accepted loss value used in this calculations:
 Accepted Optical fiber loss value
These values are per TIA/EIA and other industry specifications and are the values used by Transition Networks in all link loss calculation.
These values are the one example of the performance that can be obtained with anew fiber installation.

The IEEE also recommends maximum cable distances as defined in the table blow: 
Cable distances values
Recommends maximum cable Distance

Estimate total Link Loss
This calculation will estimate the total link loss through a particular fiber optic link where the fiber length, as well as the number of splice and connectors, are known. This calculation is simply the sum of all worst-case loss variables in the link:

Link Loss = [Fiber length (KM) x Fiber attenuation per KM] + [Splice loss x number of splices] + [ connector loss x number of connectors] + [safety margin]

For example : Assume a 40 KM single mode link at 1310nm with two connector pairs and 5 splices.
Link Loss=    [40KM x 0.4dB/km] +  [0.1dBx5] + [0.75dB x 2] + [3.0dB] = 21.0dB

In this example, an estimated 21.0dB power would be required to transmit across this link. Of course, it is very important to measure and verify the actual link loss values once the link established to identify any potential performance issues.

Estimate Fiber Distance
This calculation will estimate the maximum distance of a particular fiber optic link given the optical budget and number of connectors and splices contained in the link:

Fiber Length = [Optical budget] - [Link loss]
                                [Fiber Loss/KM]

Fiber Length = { [ ( min. TX PWR ) - (RX sensitivity)]  -  [ Splice loss x no. of splices] -  [ Connector loss x no. of connectors] -  [safety margin]} ➗ [ Fiber lost/KM]

For example : Assume a fast ethernet single mode link at 1310nm with 2 connector pairs and 5 splices.

Fiber Length=  [ ( -8.0dB) - ( -34.0dB)] - [0.1dB x5] - [0.75dB x 2] - [3.0dB
                                                       0.4dB/KM

Fiber Length=  [26.0dB] -  [0.5dB] - [1.5dB] - [3.0dB]
                                             [0.4dB/KM]

Fiber Length= 52.5 KM

In this example, an estimated 52.5 KM is possible before dissipating the optical power to a value of Rx sensitivity. As always, it is important to measure and verify the actual link loss value once the link is established to identify any potential performance issues. Actual maximum distances will very depending on:
Actual optical fiber attenuation per kilometer.
Optical fiber design and age.
Quality of connectors and actual loss per pair.
Quality of splices and actual loss per splice
Quantity of splices and connectors in the link.

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