Until quite recently, most of new DWDM networks were optimized only for 10G noncoherent transmission. Today, we are facing new challenges demanded by the 100G coherent market. Should we add 100G to the existing network, or create a new coherent-only overlay?
The correct answer must take some key points into account. Before naming a few, let´s look at some important concepts.
10G Networks
Modulation:
The most common modulation scheme used for 10G networks is called On-Off Keying (OOK). OOK is the simplest form of amplitude-shift keying (ASK) modulation. It represents the information as the presence or absence of a carrier wave. The problem begins when we try to apply OOK at higher rates. Even for the most sophisticated electronics available, this kind of modulation does not seem to be practical once the baud rate at 100G for example would be too high.
Nonlinear Effects
DWDM networks usually deals with high power scenarios. Sometimes the power of a wavelength can alter slightly the refractive index of the fiber. This causes a phase change of the wavelength itself. The nonlinear effect that describes this phenomenon is known as SPM, or Self Phase Modulation. It´s not unusual when the phase of other wavelengths is affected. In this case the nonlinear effect is called XPM or Cross Phase Modulation.
100G Networks
Modulation:
The most common modulation scheme used for 100G networks is called Polarization Multiplexed-Quadrature Phase Shift Keying (PM-QPSK). The QPSK modulation scheme alters the phase rather than the amplitude. Taking 4 values of the phase, it is possible to achieve 2 bits per symbol. The 2 polarization schemes double the transmission rate. As a result, the baud rate is reduced 4 times when compared to a hypothetical 100G OOK signal.
Nonlinear Effects
Unlike the high-power density (large amount of power for a small spectral range) of 10G wavelengths, the coherent 100G wavelengths have a much smaller impact on the refractive index in the fiber (100G channels have smaller power density compared to 10G´s). This means a much smaller SPM/XPM impact for 100G-only networks. When a hybrid 10/100G network takes place, 10G wavelengths may cause XPM over 100G channels. This is even worse than XPM over 10G-only networks because coherent 100G are based on QPSK modulation which is based on phase values to encode 2 bits per symbol. Any phase change caused by XPM may be a problem.
Chromatic Dispersion
Chromatic dispersion occurs because different frequencies propagate through the fiber at different speeds. The different frequencies of the same channel also travel at different speeds. This causes distortion.
10G OOK networks deal with dispersion by deploying Dispersion Compensation Modules (DCM). These DCMs consist of kilometers of fiber with negative chromatic dispersion. A residual dispersion within the 10G receiver tolerance is achieved.
This seems a great solution to reach greater distances. However, low chromatic dispersion may be a problem for coherent transmission. When the channels travel together, the impact on the fiber´s refractive index is greater. This increases nonlinear effects such as XPM.
Options to improve the performance of 100G in 10G Networks
Gard Bands
One way to improve the performance of 100G in 10G networks is to isolate the 100G channels so the impact of 10G XPM is minimized. This isolation consists on a gard band for the channel. This means a spectral space in both sides of the 100G channel.
Gard bands, however, cause reduced spectral efficiency (the spectral space could be used for more channels). Besides, maintaining gard bands through the entire network may be a tough job.
Dispersion Compensation Schemes
Another way is to adjust the dispersion plan to increase the chromatic dispersion in order to reduce nonlinear effects. This is much easier for new networks, but it may require higher Capex budget for deployed networks (it may require DCMs changes…).
Should we add 100G to the existing network, or create a new coherent-only overlay?
The factors to consider are:
- Capex and Opex budgets
- If you consider a 100G-Coherent-Only network, how profitable this extra reach will be?
- How is the current wavelength utilization?
- How old is you DWDM Hardware?
For example:
It the traffic is ever-growing, the equipment is too old and you budget can support, the coherent-only network may be the best option.
However, if your network can support a great increase of traffic and reach requirements does not seem to be so profitable, a hybrid 10G/100G is likely the best option.
Carlos Rogério Roffé
