I am writing this article after analyzing several networks that are currently considering the implementation of C Band, upgradeable to L Band Optical Line Systems (OLS). While being “future-ready” is an essential KPI for every network planner, it is equally important to keep a close watch on current network utilization, realistic traffic projections, and optimization of the existing spectrum.
Let us carefully evaluate each aspect of this OLS dilemma so that we can determine when and how it makes sense to implement C-to-L Band OLS.
 |
| View of the DWDM Spectrum with C and L Band Source: Researchgate |
1. C Band already offers significant capacity
Gone are the days when C Band meant 96–120 channels of 50GHz spacing, each carrying 100G optics. Today, C Band can support flexible spectrum configurations—such as 64 channels of 400G or even 32 channels of 800G wavelengths.
This represents enormous capacity with more efficient grooming options. For instance, with 32 channels of 800G per segment (approx. 500 km), we achieve 25.6 Tbps of bandwidth—all on a cost-effective C Band OLS.
2. The importance of accurate traffic projections
An OLS is fundamentally designed based on traffic projections. If a five-year forecast indicates that C Band spectrum will not be exhausted (even after applying grooming optimizations), then deploying an L Band-ready OLS is an unnecessary expense.
Such an approach increases the upfront cost of the solution, while L Band usage may not be required until after year five. By then, the additional cost of a C+L OLS becomes an overhead, impacting provisioning economics and overall profitability.
There have been instances where bandwidth providers chose C+L OLS upfront but later struggled to sell services due to higher entry costs—leading to reduced margins and lower market competitiveness.
3. Cost implications of C-to-L upgradeable OLS
While it may sound appealing to demand C+L upgradeable OLS, the CAPEX and OPEX implications must be understood:
Component manufacturing for C+L systems is not yet cost-effective.
Switch-gain amplifiers with dual directions are expensive compared to EDFA-based C Band OLS.
Low-degree ROADMs (for smaller sites) are often unavailable; most C+L ROADMs are 20–32 degrees, adding cost.
Some vendors require separate ROADMs for C and L Bands, effectively doubling costs. Others mitigate this using couplers or integrated designs, but pricing remains high.
Amplification remains a challenge. No single amplifier can handle both bands today. Instead, L Band amplifiers must be cascaded with C Band amplifiers—further inflating costs.
For operators whose traffic will remain stable for the next five years, deploying a C+L OLS is commercially unjustifiable, as competitors running pure C Band deployments will enjoy lower costs and better margins.
4. The evolving industry landscape
Just two years ago, C+L ROADMs and WSS were not commercially available. Today, they are. Similarly, research is ongoing for day-one C+L amplifiers.
If an operator invests prematurely in today’s C+L upgradeable OLS, they may miss out on more cost-effective and technically advanced solutions that will arrive in the near future.
Thus, the decision to deploy C+L OLS must be purpose-driven and strategically timed.
When should C+L OLS be considered?
Despite the above constraints, there are specific scenarios where C+L OLS may be justified today:
a) Confirmed near-term growth: Bandwidth is projected to exceed 12.8 Tbps within two years.
b) Captive demand: Customers with steady, predictable YoY growth (e.g., ~25%).
c) Hyperscalers as primary customers: These clients often demand full-spectrum allocations (multiples of 500GHz), which validates building a C+L network.
d) Spectrum slicing strategy: C Band is reserved for captive bandwidth, while L Band is leased for alien wavelengths—enabling day-one dual-band utility.
 |
| Concept of C Upgradeable to L band Source: Ribbon Communications |
Conclusion
An OLS is a long-term investment—this much is agreed. However, the cost must be balanced against realistic capacity growth and market demand.
Engineering-driven choices that overlook commercial realities can result in revenue shortfalls and reduced margins. Therefore, the decision to adopt C+L OLS must be based on accurate traffic projections, well-defined timelines, and market conditions, ensuring that the network delivers maximum yield for the investment.
So till then my friends,
Happy Network Planning.
Cheers,
Kalyan
