Optical transceiver question

Wed Sep 7 23:31:07 UTC 2016

Frank Bulk wrote:
> We recently purchased some generic optics from a reputable reseller that
> were marketed to reach 60 km.

transceivers don't work like that.  They are sold with a specific
optical budget, normally rated in dB at a specific wavelength.  The km
equivalent is usually based on G.652 fibre at an attenuation of 0.25dB
per km at 1550nm, or 0.35dB loss at 1330 nm.

This means that your optical link needs to fit within the optical budget
of your transceivers.  If you put a pair of muxes in the middle, you
need to subtract the mux- and demux attenuation from your budget. Each
patch should be assumed to lose ~0.5dB, each splice should assume 0.1dB
and you need to add in a 2dB overhead for loss over time.

You need to feed all these figures into an optical budget calculator and
see what it comes out with. There are plenty online, some good, some not
so good.

If you're using a technology which is affect by dispersion, you may need
to subtract a couple more dB to compensate for this, or if the link is
too long or the technology requires it (10G dwdm or non coherent 100G),
you may need to implement optical dispersion compensation.

When dealing with optical links of this sort of size, the first and most
important thing to start out with is an accurate characterisation of
your optical link, end-to-end, at multiple wavelengths, but particularly
at 1550nm because that is the reference point for C band WDM.  Your
provider should provide this as part of the hand-off process.  Once you
have this data, you should be able to work out what you can support on
it.  If you don't have this data, you cannot ever know for sure what
your optical link can support.

For more information, check out Richard Steenbergen's presentations on
optical design at https://www.nanog.org/resources/tutorials:

Optical Networking 101 (Feb 4 2013)
Everything You Always Wanted to Know About Optical Networking (June 13,
2016)

These are both excellent introductions to optical design, and it's worth
spending the time watching the youtube videos associated with these talks.

Nick