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Questions about Optical Budget Calculations and Actual SFP/SFP+ Maximum Ranges


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#1 KrisGT

KrisGT

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Posted 14 September 2016 - 02:51 PM

All the following questions are assuming the link has no extra connections and/or splices. Just a single cable between two optics.

 

So I am self taught on this topic. I was hired by my company and had to learn fast. One thing bugs me about my estimates on determining how far an optic can reach. I feel like I am missing an important variable somewhere.

 

So far I have been taking the Minimum Tx Power then subtracting the Rx Sensitivity and 0.75db for the connector.

 

So for a SFP-10G-ZR it would be: 0dbm - 24dbm = 24db - 0.75db = 23.25db

 

Then I subtract 3db for a safety margin = 20.25

 

After this I divide my new total by the loss per km. At 1550nm Cisco puts this at 0.25db/km

20.25db / 0.25db = 81km

 

Now this math adds up for the part above but it does not work for 1000BASE-LX/LH and 1000BASE-EX. Why can the EX reach up to 40km while the LX/LH only goes 10km?

 

These have the same wavelength of 1310nm, TX power and RX sensitivity. So what am I missing?

 

As far as attenuators are concerned, are they required for LX/LR optics? If you have a link going 500m will you damage the RX side??

 

I know for 40km optics and up you have to have one for shorter distances to not cause damage.



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