By Jay Gill
For almost two decades, the L-band has been a sleeping giant of optical fiber capacity in wavelength-division multiplexing (WDM) technology, offering the potential to double capacity in a single stroke. Most deployed WDM systems operate in the C-band, while the adjacent L-band (see Figure 1) has remained largely unused despite its attractive transmission characteristics and the availability of L-band technologies.
Infinera’s recent announcement that Windstream has deployed the Infinera C+L solution signals that the time for L-band has finally come.
For skeptical observers, who have seen over-optimistic L-band predictions for years and may have questioned whether we would ever see widespread deployment of C+L optical systems (using both C- and L-band transmission on the same fiber pair), we can now answer “Yes!” with more certainty than ever before.
But that’s not the last question. Read on for a few FAQs about C+L and learn how Infinera is helping operators future-proof their networks with our C+L solution.
Q: Why hasn’t L-band taken off before?
A: There’s an obvious logic to C+L: capacity demand keeps growing inexorably, and using the L-band can double fiber capacity to help meet that demand. But those statements have been true for years, yet C+L has languished. The simple reason is that engineers kept finding ways to squeeze more capacity out of the C-band. Coherent transmission enabled the 10-fold jump from 10 gigabits per second (10G) to 100G per wavelength. The use of dual polarization doubled capacity, while 16 quadrature amplitude modulation (16QAM) doubled it yet again. With such huge capacity gains achieved by applying advanced coherent processing in increasingly powerful digital signal processor (DSP) chips, L-band demand kept receding into the future.
Q: Why now?
A: The pace of innovation in coherent signal processing is not slowing down. Over the next couple of years, new DSPs using the latest 7 nanometer (nm) silicon will come to market, delivering more sophisticated technology, including probabilistic constellation shaping (PCS). As amazing as these new optical engines will be, they won’t even come close to matching the fiber capacity gains of the past because of a little problem called the Shannon Limit, the fundamental limit on fiber capacity. At best, PCS and other new tools will squeeze another 10 to 20 percent out of the C-band, edging ever closer to the Shannon Limit. Given that sobering reality, we’re going to need to look elsewhere for the next massive increase in capacity.
Q: Why C+L-band? Why not something else?
A: L-band is not the only solution. Some network operators have deployed a lot of fiber and still have spare fiber pairs to light up. In areas where it’s cost-effective to deploy more fiber, operators are using advanced new fiber types that offer incrementally better capacity. Spatial-division multiplexing (SDM) promises to effectively increase the fiber count in new cables, and hence capacity, assuming the remaining technical and cost challenges can be worked out.
SDM is looking like the favored approach in subsea systems, but the economics are driven by the ability to electrically power the subsea amplifier chains. Stay tuned for more information on Infinera’s subsea advantages in this respect.
But in terrestrial networks, L-band has one crucial advantage: the spectrum is – quite literally – already “in the ground,” i.e. most existing deployed fiber is capable of supporting L-band transmission alongside the existing C-band. All we need to do is add L-band-capable equipment to use it.
Q: What’s required to make C+L work?
A: Using the L-band is simple in principle – just add L-band transponders and line system components to the existing C-band gear, as shown in Figure 2 – but there are a few important challenges in practice. What operators need is a solution that allows them to set a foundation for L-band deployments with minimal up-front cost and add L-band capacity only when it is actually needed due to C-band exhaustion, while also avoiding operational complexity and impact to existing services and maximizing the performance and agility of the overall C+L system.
Infinera C+L Solution
That’s exactly what the Infinera C+L solution is designed to achieve, as described below.
Built-in expandability with low first-in cost
Infinera’s FlexILS L-band-ready line system components are intelligently designed to enable easy expansion from C to C+L without service impact, and with minimal extra cost compared to a C-band-only system.
Extending ICE4 benefits to the L-band
Infinera’s fourth-generation Infinite Capacity Engine (ICE4)-based L-band meshponders are built with the same vertically integrated optical engine technology that goes into ICE4 C-band meshponders, including photonic integrated circuits (PICs) tailored for the L-band. That means all of ICE4’s benefits get extended from the C-band to the L-band, including:
- Optical leadership, delivering industry-leading capacity-reach from metro to subsea, demonstrated most recently on the MAREA subsea cable
- Superior total cost of ownership (TCO) and agility enabled by the sliceable super-channel architecture and Instant Bandwidth, as discussed in our recent three-part blog series, including a testimonial from Telia Carrier
No service impact – “double the capacity without a hit”
In addition to the L-band-expandable line system, Infinera has developed advanced, innovative control software that tackles the challenges of dynamic power balancing and performance optimization across the C- and L-bands and allows the addition of L-band capacity without complex planning restrictions and without service impact. Less sophisticated C+L systems require the balanced addition of capacity across the C- and L-bands, which means it isn’t possible to fill the whole C-band first and only then start adding L-band capacity. Compared to those systems, Infinera C+L allows deferred capital investment, with faster return on investment, and lowers operational complexity and cost for lower TCO.
Figure 3 summarizes these Infinera C+L solution elements and benefits.
In summary, the demand for L-band is finally here, and Infinera is ready to meet that demand with a complete C+L solution designed to lead the industry.