Maximum Fiber Cable Distance Tutorial

Maximum Fiber Cable Distance Tutorial

July 5, 2007

The first thing to understand about calculating your maximum fiber cable transmision distance is that it is easy, but it also depends on a variety of factors described below.

To calculate the maximum transmission distance possible, before any losses are factored in, we start out using the IEEE's table of recommended distances for networking Ethernet, the most common transmission standard in use today. This table assumes a continuous cable length without any devices, splices, connector matings or other loss factors that effect signal transmission. In order to use the table you may need to refer to the specifications of your transmitting and receiving ports and those of the fiber optic cable used in your network and match them up with the first three columns in the table. You will then be able to determine your maximum potential transmission distance.

Maximum Cable Distance For Ethernet:*

Ethernet Standards (IEEE)	Data Rate	Cable Fiber Type	Maximum Distance (IEEE)
Ethernet (10Base-FL)	10 Mbps	50µm or 62.5µm Multimode @ 850nm	2km
Fast Ethernet (100Base-FX)	100 Mbps	50µm or 62.5µm Multimode @ 1300nm	2km
Fast Ethernet (100Base-SX)	100 Mbps	50µm or 62.5µm Multimode @ 850nm	300m
Gigabit Ethernet (1000Base-SX)	1000 Mbps	50µm Multimode @ 850nm	550m
Gigabit Ethernet (1000Base-SX)	1000 Mbps	62.5µm Multimode @ 850nm	220m
Gigabit Ethernet (1000Base-LX)	1000 Mbps	50µm or 62.5µm Multimode @ 1300nm	550m
Gigabit Ethernet (1000Base-LX)	1000 Mbps	9µm Singlemode @1310nm	5km
Gigabit Ethernet (1000Base-LH)	1000 Mbps	9µm Singlemode @1550nm	70km

* 1µm = 1 micron, 1nm = 1 nanometer wavelength

After we know the type of ethernet standard we are using above we can then calculate or estimate the amount of loss that occurs in different fiber optic cable types (62.5/125µm, 50/125µm or 9/125µm) at various transmission wavelengths (Multimode at 850nm or 1300nm, or Singlemode at 1310nm or 1550nm). Generally, higher wavelengths can support longer distances. However, to calculate the amount of cable loss more accurately you can use the TIA/EIA Cable Loss Estimates table below or you can refer to the specifications of your particular cable's fiber optic core.

EIA/TIA Cable Loss Estimates:**

Fiber Type	Loss Amount per 1 km
Multimode 62.5/125µm fiber @ 850nm	3.5dB / km
Multimode 62.5/125µm fiber @ 1300nm	1.5dB / km
Multimode 50/125µm fiber @ 850nm	3.5dB / km
Multimode 50/125µm fiber @ 1300nm	1.5dB / km
Singlemode 9/125µm fiber @ 1310nm	0.4dB / km
Singlemode 9/125µm fiber @ 1550nm	0.3dB / km

 

 

** Loss amounts are given in decibels (dB) per kilometer (km). Larger numbers mean higher loss amounts. The table assumes a continuous cable run without any devices, splices, connector matings or other factors involved in signal loss. Also note that this table assumes that the core/cladding diameter of your cable is the same throughout your transmission line. Mixing cable types is the subject of a different tutorial.

Calculating Total Link Loss

To calculate total link loss in a given transmission line we add the total cable loss (distance x loss amount from table above) to connector loss (at 0.75dB per connector), splice loss (at 0.1dB per splice) and a margin of error of 3dB as demonstrated below.

Example 1: Calculate the total link loss when cable distance and the number of connections and splices are known.

Assuming we have a cable distance of 23km over a singlemode 1310nm transmission line which has 3 connectors and 2 splices, our calculation looks like this:

(23km x 0.4dB) + (3 x 0.75db) + (2 x 0.1dB) +  3dB
      9.2dB          +     2.25dB     +     0.2dB     +  3dB
                        = 14.65dB

The total of 14.65dB represents the amount of power required to transmit across this transmission line.

Calculating Maximum Cable Distance

We can also calculate the maximum cable distance possible when the number of connectors, number of splices and the optical loss budget (calculated below) of your transmitting and receiving devices/ports are known. Again, we use 0.75dB loss per connector, 0.1dB loss per splice and add a 3dB loss as a margin of error.

Example 2: Calculate the maximum cable distance when the allowable loss budget of your devices and the number of connections and splices are known.

First we calculate our optical loss budget by subtracting our receiving port's minimum sensitivity (in dBs) from our transmitter's power output (in dBs) as follows:
From the product specifications of our devices we learn that our transmitting device has a -13.3dB power output and our receiving device has a -25.4dB minimum sensitivity, then our optical loss budget is calculated as follows: (Remember that subtracting a negative number is the same as adding the number.)

-13.3dB - -25.4dB = -13dB + 25.4dB = 12.1dB (This is our example's optical loss budget as derived from the specifications of our products.)

Assuming we are transmitting over a 62.5/125µm multimode fiber transmission line at 850nm and we have 2 connectors and 4 splices, our calculation looks like this:

12.1dB - (2 x 0.75db) - (4 x 0.1dB) -  3dB
12.1dB -     1.5dB       -     0.4dB      - 3dB
                       = 7.2

We then take this total and divide it by 3.5dB (taken from our TIA/EIA Loss Estimates table above), so

7.2dB / 3.5dB = 2.057 kilometers (the maximum total cable distance in our example)

Below are our two formulas summarized:

Product Specifications

The specifications of your transmitting, repeating, and receiving devices should be found in the product documentation that came with the device. Alternatively, these documents can often be found by "googling" the manufacturer and part number of your product(s). It is also important to note that maximum transmission distances are sometimes pre-calculated and provided in the specifications of your product literature, but these calculations often assume a continuous cable run without connector matings, splices or other loss factors such as temperature, aging, etc.

OTDRs

Optical time domain reflectometers (OTDRs) are the most accurate way to measure power requirements in a given transmission line. OTDRs can measure loss throughout your line as well as measure loss across each event: cable runs, connector matings, splices, etc.

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