Fibre Standard ( ISO Category ) |
TIA 492AAAA ( OM1 ) |
TIA 492AAAB ( OM2 ) |
TIA 492AAAC ( OM3 ) |
TIA 492AAAD ( OM4 ) |
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common Ethernet applications [ table 2 ].
The light grey section above highlights the requirements for 1000BASE-SX Ethernet ( 1Gb / s short wavelength ). The maximum channel attenuation and maximum lengths are listed for each type of supported fibre . In this case , for OM1 62.5 / 125 fibre the maximum allowed loss is 2.6dB and the maximum supported distance is 275m .
When using OM2 50 / 125 fibre the loss and distance are increased to 3.6dB and 550m respectively .
These charts make easy work of determining loss budgets as long as the application that will be running over the fibre is known . Fortunately , some fibre certifiers incorporate a database with these values so the user needs to simply choose the appropriate standard and the pass / fail limits are set automatically .
Calculating loss budgets for networking equipment
Lastly , when the networking equipment that will be running over the fibre is known , the requirements of that gear can be used to determine the loss budget .
Each piece of optical networking equipment , even SFP / GBIC modules will have specifications for output power and receiver sensitivity . These
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specifications tell the user exactly how much loss the equipment can sustain and still operate properly .
Transmitter output specifies how ‘ bright ’ the light is that ’ s emitted from the transmitter port . The specification will be in dBm ( not dB ). A typical device may have an output power of -20dBm . Don ’ t let the negative confuse you , it doesn ’ t mean negative power . 0dBm is a reference to 1mW ( miliwatt ) of power , and a negative dBm value simply means less than 1mW .
Power scale : 0 dBm = 1 mW
-50 dBm = . 00001 mW
Note : dBm is an amount of power . dB is a change in power from one level to another .
A receiver usually has two specifications , Receiver Sensitivity and Dynamic Range .
Receiver Sensitivity is the weakest ( darkest ) signal the receiver can detect and the Dynamic Range is how much brighter than the Sensitivity specification the light can be without blinding the receiver . For example , a receiver with a Sensitivity of -30dBm and a Dynamic Range of 20dB can detect light within a range of -10 to -30dBm .
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Table 2 : The TIA-568.3-D ( 2016 ) standard |
If the transmitter injects light into the fibre at -20dBm , and the weakest light the receiver can detect is -30dBm , the difference between the two is 10dB . Meaning that the system can have at most 10dB of loss before the signal is too weak for the receiver to detect .
What if the receiver was paired with a transmitter that output -5dBm of power ? The signal would be too strong and overpower the receiver . With optical systems , care must be taken to not overdrive receivers because it will be just as detrimental as having too little signal .
For more information on fibre optic testing and tester , visit www . idealnetworks . net
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30 | February 2017 |