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I know what is dispersion, but what’s skew anyway?

In previous articles we defined what dispersion is and how it affects an optical link, also showed differences between modal dispersion and chromatic dispersion.


Now do you know, that for really high speed links, there is a also a skew as consideration factor?


In general, one might think about skew as dispersion as well, as after all it reflects in travel in time as well – just as dispersion does.


However, what skew is – is a difference in travel time measured in nanoseconds (ns) traveling on different fibers. Aha!


Have you seen how 40G QSFP module looks like? Here is how:





Figure 1: MPO connector of QSFP fiber module.




That’s interesting, right? 12 fibers here!



Is this difference in travel times becomes really important when dealing with multimode fiber transmitting on 40Gbit/s or 100Gbit/s  speeds.


In these 2 speed transmission electrical signal get demultiplexed into 4 or 10 different signals.


Like you can see in below figure, to get 100Gbit/s link sent over fiber, you need 10 different streams of 10Gbit/s signals. This is where skew must be limited as much as possible, otherwise these streams can’t be accurately multiplexed back in receiver side.





Figure 2: Principle of 100Gbit/s link multiplexing and de-multiplexing. 



For such datarate maximum skew value is <79 ns.


Considering, that such speeds at the current moment of writing usually are required in datacenters, where distances are short, that’s still OK.








Lomoveishiy – Finland

I needed those to connect my PC on the third floor to have internet access in that room, and ISP installed their modem on the first floor only. After dropping fiber patch cables, plugged in all cables into these media converters at both sides, and link came up instantly. Was much easier than I thought!

Raymond – USA

Great experience – units worked straight out of the box – just needed plug in cables and we were done. I also like the possibility to enable jumbo frames, while we do not have a need for this feature at the current moment it’s great to have this option.

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