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Calculating Fiber Loss and Distance Estimates

Calculating Fiber Optic Loss with Distance

 

There are a number of ways to tackle the problem of determining the power requirements for a particular fiber optic link. The easiest and most accurate way is to perform an Optical Time Domain Relectometer (OTDR) trace of the actual link.

This will give you the actual loss values for all events (connectors, splices and fiber loss) in the link. In the absence of an actual OTDR trace, there are two alternatives that can be used to estimate the power requierements of the link. 

 

  1. Estimate the total link loss across an existing fiber optic link in the fiber length and loss variables are known
  2. Estimate the maximum fiber distance if optical budget and loss variable are known.

 

Loss variables are connectors, splices and attenuation per kilometer of the fiber. If actual values for all of the loss variables are not known, an estimation 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 following table includes commonly accepted loss values in these calculations:

 

Fiber

Type

Wavelength

Fiber

attenuation

/ km*

Fiber

attenuation

/ km #

Connector

Loss

Splice

Loss

Multimode

50/150µm

850 nm

1300 nm

3.5 dB

1.5 dB

2.5 dB

0.8 dB

0.75 dB

0.75 dB

0.1 dB

Multimode

52.5/125µm

850 nm

1300 nm

3.5 dB

1.5 dB

3.0 dB

0.7 dB

0.75 dB

0.75 dB

0.1 dB

Single

Mode

9µm

1310 nm 0.4 dB 0.35 dB

0.75 dB

0.1 dB

Single

Mode

9µm

1550 nm 0.3 dB 0.22 dB 0.75 dB 0.1 dB

* These values are per TIA/EIA and other industry especifications and are the values used by Transition Networks in all link loss calculations.

# These values are one example of the performance that can be obtained with a new fiber installation.

 

The IEEE also recommends maximum cable distances as defined in the table below: 

 

Standard

Data-Rate

(Mbps)

Cable Type

IEE

Standard-Distance

10BASE-FL 10

850nm-Multimode

50/125µm 62.5/125µm

2km.
100BASE-FX 100

1300nm-Multimode

50/125µm 62.5/125µm

2km.
100BASE-SX 100

850nm-Multimode

50/125µm 62.5/125µm

300m.
1000BASE-SX 1000

850nm

Multimode 50/125µm  
850nm

Multimode 62.5/125µm

550m.
220m.
1000BASE-LX 1000

1300nm

Multimode 50/125µm

62.5/125µm 1310nm

Single Mode 9/125µm

5km.
1000BASE-LH 1000

1550nm

Single Mode 9/125µm

70km.

 

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 splices and connectors, are known. This calculation is simply the sum of all worst-case loss variables in the link:

 

Link Loss = [fiber length (km) × fiber attenuation per km] + 
                   [splice loss × # of splices]
                   [connector loss × # of connectors] + [safety margin]

 

For example: Assume a 40 km. single mode link at 1310nm with 2 connector pairs and 5 splices.

Link loss = [40km × 0.4dB/km] + [0.1dB × 5] + [0.75dB × 2] + [3.0dB] = 21.0dB

 

In this example, an estimated 21.0dB of 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 is established to identify any potential performance issues. (Please check our OTDR Test Measurement Devices category).

 

Estimate Fiber Distance

This calculation will stimate the maximum distance of a particular fiber optic link given the optical budget and the 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 × # of splices]
                       - [connector loss × # 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 × 5] - [0.75dB × 2] - [3.0dB]} / [0.4dB/km] = 52.5km.

 

In this example, an estimated 52.5 km. distance is possible before dissipating the optical power to a value below the Rx sensitivity. As always, it is very important to measure and verify the actual link loss values once the link is established to identify potential performance issues. Actual maximum distances will be very depending on: 

  • Actual optical fiber attenuation per km
  • 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

 

Source: FOA (The Fiber Optic Association, Inc.)

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