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Want a Cost-effective Solution for Scaling Fiber Capacity? Super C Could Be the Answer

headshot of Paul Momtahan

November 30, 2023
By Paul Momtahan
Director, Solution Marketing

Demand for bandwidth continues to grow strongly. However, the cost of laying or leasing and then lighting new fibers is very high and can be subject to lengthy delays. For some operators, new fiber may not even be an option. For many network operators, fiber is therefore one of the most valuable assets, and there is an ongoing requirement to maximize fiber capacity. But what is the best option for doing this?

Transponder Spectral Efficiency Gains Are Becoming Incremental

In the past we could rely on improvements in transponder technology for major increases in fiber capacity. For example, going from 10 Gb/s direct-detect wavelengths to 100 Gb/s coherent wavelengths increased fiber capacity by a factor of 10. It did this by shrinking the amount of unused spectrum between wavelengths and increasing the number of bits per symbol. However, with flexible-grid optical line systems and high baud rates, there is little more to be gained in terms of the reducing the unused spectrum between wavelengths, and gains in bits per symbol are being limited by the Shannon limit. This law/theorem puts a limit on the amount of information that can be communicated over a channel with a given bandwidth and amount of noise, and it provides an upper bound on the maximum spectral efficiency that can be achieved. High-performance embedded optical engines, such as Infinera’s 800 Gb/s-per-wavelength ICE6, are typically between 1 and 2 dB from the Shannon limit. The maximum possible scope for improved spectral efficiency is thought to be in the 30% to 40% range, with the next generations of high-performance transponders, leveraging 5-nm or 3-nm CMOS DSPs, typically targeting spectral efficiency improvements in the 10% to 20% range.

Scaling Fiber Capacity Requires More Spectrum

The only other way of scaling fiber capacity is with more spectrum. Outside of Japan, DWDM networks have traditionally used C-band fiber spectrum. The C-band was preferred over the L-band for its lower noise and more power-efficient EDFA amplification, and at the time for its lower chromatic dispersion. The Japanese market was the exception to this as G.653 dispersion-shifted fiber was widely deployed, which made the L-band favorable for DWDM. Over time, the amount of C-band spectrum that vendors have supported in DWDM systems evolved from 3.2 THz to 4.8 THz, the extended C-band. So, what is the best way of increasing the amount of spectrum beyond 4.8 THz?

Extending into the L-band Can Be Expensive

One approach to scaling network capacity that has gained traction over the last few years has been expanding into the L-band, doubling the amount of spectrum from 4.8 THz to 9.6 THz with C+L. However, C+L comes with a number of economic and technical challenges. C+L networks typically require doubling the number of components such as amplifiers and wavelength-selective switches (WSSs), which increases the cost, though Infinera’s FlexILS does provide the option to initially deploy a C-band-only system, then add the L-band hardware while in service when the extra capacity is required. Technical challenges of C+L relate to stimulated Raman scattering (SRS), which causes both the C-band and the L-band to tilt, with power going from the C-band to the L-band, as shown in Figure 1. SRS creates challenges in terms of C+L recovery speed, provisioning speed and complexity, and the topologies that can be supported. And while Infinera’s FlexILS line system overcomes these challenges with tools including sophisticated link control software and ASE idler hardware, this hardware also adds incremental cost to C+L solutions.

SRS creates challenges for C+LFigure 1: SRS creates challenges for C+L

Super C Provides a Cost-effective Alternative

If you don’t need the full 9.6 THz spectrum of C+L, which can provide in excess of 80 Tb/s over shorter distances, a more cost-effective alternative is Super C. Infinera’s Super C increases the amount of C-band spectrum by 27% to 6.1 THz, thus enabling 50+ Tb/s over shorter distances – and it does this while avoiding the costs and challenges related to C+L. Super C requires enhanced wavelength-selective switch and EDFA and Raman amplifier technologies that are capable of supporting a 6.1 THz C-band. However, Super C provides its extra 27% spectrum without the need to double the number of amplifiers and WSSs and without the need to add ASE idler hardware to combat the previously described SRS challenges.

Super C vs. C+LFigure 2: Super C vs. C+L

Extended C vs. Super C: Performance and Cost Comparison

One common question that comes up in discussions related to Super C is how does the performance and cost compare to “regular” extended C-band line systems? One common misconception relates to performance, with the assumption that the higher noise and ripple of Super C amplifiers will reduce performance and result in minimal total capacity gains with a significantly higher total cost. This misconception is based on assuming C-band amplifier designs for Super C amplifiers. Infinera’s Super C amplifiers leverage additional pumps and gain flattening filters to deliver better performance than best-in-class extended C-band amplifiers. In Infinera simulations leveraging 300 long-haul routes modeled on real networks, moving from extended C to Super C on average enabled 29.5% more capacity (20.7 Tb/s à 26.8 Tb/s) for only 27.6% more transponders, with the average wavelength speed actually increasing from 713.2 Gb/s to 725.6 Gb/s.

But what about cost? With some skilled engineering, Super C solutions can be quite cost-effective and only minimally more expensive than extended C solutions. But that is very good value considering the almost 30% improvement in fiber capacity, especially when compared to the much higher cost of alternatives such as C+L or lighting new fiber.

Future-proof with Super L

But what if the 6.1 THz of Super C  is not enough? Infinera’s GX optical line system also provides the option to further increase the total amount of spectrum with an additional 6.1 THz in the Super L band, giving a total of 12.2 THz, compared to the 9.6 THz of regular C+L. Super C plus Super L can enable total fiber capacity in excess of 100 Tb/s over shorter distances.

Summary

As bandwidth demand continues to grow strongly while transponder spectral efficiency gains become increasingly incremental, network operators need to explore additional strategies to scale fiber capacity, expanding the spectrum beyond the 4.8 THz of the extended C-band. And while C+L provides one option to double fiber spectrum, it has significant cost implications. Infinera’s Super C solution, which includes the GX’s Super C optical line system and ICE7-enabled Xponders, provides an alternative solution that can deliver 27% more spectrum and up to 30% more capacity with only a small cost premium over traditional extended C line systems. Furthermore, it comes with the option to add the Super L band in the future, taking total fiber capacity to over 100 Tb/s on shorter routes.