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I just got fiber, and I'm super happy!

But recently I read about simplex vs duplex, and that simplex can only send data one way.

And I noticed that there's actually only one fiber from my ISP, which means it's simplex?

If I'm correct, how does that work? Is it like changing from upload to download every millisecond? If yes, wouldn't that add more delay?

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What you very probably have are BX (also called BiDi) optics. They use one colour of light in one direction and another (non-interfering) colour in the other direction. This requires different transceivers on both ends of the link. The ISP usually uses BX-D (downstream) optics which transmit at 1490nm and receive at 1310nm, and the customer uses BX-U (upstream) optics which transmit at 1310nm and receive at 1490nm. Because the two different colours you have a full-duplex connection.

Sander Steffann
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  • Thank you. Can you explain why physical duplex exists then? Just wondering. –  Jul 25 '15 at 21:00
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    BX optics are more complex as they need a multiplexer to split the different colours. That also limits their range. It's also much easier to manage networks that can use the same optics everywhere. For BX you need different types of transceivers at each end. Here is some marketing material from MRV on BX optics: http://www.mrv.com/sites/default/files/white_papers/a4_pdfs/mrv-wp-sfp_transceivers2010.pdf. Ras has also given a nice presentation if you want to learn more about optics in general: https://www.nanog.org/meetings/nanog57/presentations/Monday/mon.tutorial.Steenbergen.Optical.39.pdf – Sander Steffann Jul 25 '15 at 21:24
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    CATV on fiber often uses 3 wavelengths - data in, data out and TV signal. Different wavelengths are differently impacted by fiber bending, among other things, so sometimes one of the three services will drop out while the other two work. – Ecnerwal Jul 28 '15 at 16:18
  • The residential deployments I have seen usually have two fibers: one fiber with BX optics for data and one separate fiber for TV. Haven't seen them using three bands on one fiber here. – Sander Steffann Jul 28 '15 at 16:24
  • To add to Sanders post, its quite likely you have GPON if your connection is asymmetric. Its a form of WDM and shares optics between dwellings, making it cheaper to roll out than direct p2p fibre. – Linef4ult Nov 09 '15 at 13:05