The coherent 100G transmission systems from network equipment vendors are showing an amazing robustness and tolerance for high PMD and CD. Some are reporting a PMD tolerance of up to 100ps DGD and above, while the CD tolerance can be in the range of up to 50.000ps/nm.

Therefore the ideal network for deploying such coherent systems would be a green field deployment on fibers without any compensation devices such as DCMs, as in this case they would just add attenuation without adding any real value.

But what is the reality of the 100G rollout situation?

In practise there are many brown field installations, meaning many of these networks are deployed already and have several 10G services running over the fibres already. The expectation from the market has been that you can simply add additional 100G coherent services by using free channels in the WDM grid. But the early adopters among the operators have had to learn it the hard way that 100G coherent and 10G non-coherent 10G NRZ can not be put side by side without any extra considerations.

The fact that the coherent systems typically have in average a 6dB lower sensitivity leads to the consequence that they are driven with about 6dB higher optical power. This causes Non Linear Effects (NLEs) such as 4 Wavelength Mixing  that will generate some cross talk, Self Phase Modulation (SPM) and Cross Phase Modulation (XPM).

The solution for this issue is simple, but also in a certain way unpleasant. By using so called Guard Bands the 100G and 10G NRZ services (actually also any other lower rate service) can coexist on one fiber. A Guard Band is simply a gap of 200-400GHz in the WDM grid  (4-8 Channels if using a common 50GHz spaced grid) with no service deployed separating the two technologies, meaning coherent 100G on one side of the Guard Band and 10G NRZ and other lower rate services on the other side.

Guard bands of 200GHz – 400GHz between coherent and non-coherent services prevent cross-channel effects

As one of the drivers for 100G is the increased spectral efficiency this is pretty unpleasant as it means that a certain part of the spectrum can simply not be used. So far one system vendor has claimed in press releases that their transmission equipment does not require any Guard Band, but it is not known if this has been proven in any service provider trials yet.

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