**By Brian Lavallee, Senior Director of Solutions Marketing at Ciena**

For over a decade, the submarine cable industry has benefitted from the advent of coherent-based modems that have allowed operators to upgrade their submerged network assets to previously inconceivable, total information-carrying capacities. Channel speeds initially jumped from 10Gb/s to 40Gb/s, then to 100Gb/s, and now even higher, to 200Gb/s and above. Submarine cables deployed many years ago are now supporting an order of magnitude increase in total information-carrying capacity. The exact total capacity of an upgraded cable will ultimately depend upon its unique nature of performance, but all increases are astounding. Such cable upgrades breathe new life into legacy subsea wet plants – some over a decade old – allowing cable operators to maintain pace with over 40 percent CAGR in demand.

The introduction of coherent Submarine Line Terminating Equipment (SLTE) was a sea change milestone for the industry. Operators had to get used to massive increases in capacity that their subsea assets could support. However, there’s a looming limit to how many bits can be crammed into an optical fiber – referred to as Shannon’s Limit – and is a red-hot topic of discussion these days, and for good reason.

**So, what is Shannon’s Limit?**

A brilliant renaissance man named Claude Shannon, referred to as the “Father of Information Theory”, wrote a groundbreaking paper in 1948 that effectively created the discipline of information theory as we know it today…a full seven decades ago! His paper showed that the maximum theoretical capacity of any communications channel (fiber optic-based submarine cable in our case) is limited by bandwidth and noise. Bandwidth is a range of optical frequency (usable wet plant spectrum) in which to transmit a signal. Noise is anything that disturbs this signal, such as linear and nonlinear effects associated with submarine fiber optic-based communications.

Shannon showed how to calculate the maximum theoretical data rate over a communications medium (ex. optical fiber) in the presence of noise, without incurring transmission errors and referred to it as the Channel Capacity. In the submarine network industry, we refer to it as Shannon’s Limit, and it is essentially the same thing. So why does all this matter? Because we’re approaching Shannon’s Limit when it comes to the maximum theoretical capacity of both existing and new submarine cables. This means the days of massive increases in per channel capacities may be coming to an end via diminishing returns. There is hope on the horizon, so all is not lost.

**A simple equation, with profound consequences**

The mathematical equation defining Shannon’s Limit is shown below, and although mathematically simple, contains complex implications in the real world where theory and engineering rubber meets the road. The equation has particularly profound (and costly) consequences when new submarine cable plans have not been taken into account. Have you taken Shannon’s Limit into account?

*Figure 1: Mathematical equation for Shannon’s Limit*

Let’s dive deeper into this relatively simple equation to better understand its profound effect on our industry. C is the maximum capacity carried over a communication medium (submarine optical fiber), given in bits per second (b/s). B is the bandwidth (usable submarine optical fiber spectrum), given in Hertz. S/N, the Signal-to-Noise ratio is a ratio and thus has no units.

The implication of this equation is clear; if you want to increase the capacity (C) of a submarine optical fiber, you increase the amount of usable spectrum (B), increase the signal level, and/or reduce the noise level. There are real-world limitations (ex. nonlinear impairments, FEC overhead, performance margins, component aging, dispersion, etc.) to account for, which will further decrease the achievable capacity.

**Approaching Shannon’s Limit**

To get as close to Shannon’s Limit as we can, there’s a wealth of technological wizardry incorporated into the latest coherent-based SLTE. It’s the ingenious integration of software, hardware, and mathematical advancements that yielded revolutionary modems changed our industry over the past decade. However, we’ll eventually reach a limit as to how many bits we can stuff into a fiber.

Fortunately, there are techniques, besides modem-centric advances, that address Shannon’s Limit directly and/or indirectly. For example, we can significantly increase the bandwidth (B) in Shannon’s equation by leveraging L-band to double the usable spectrum, deploy wider band repeaters with more usable spectrum, deploy more fiber pairs per submarine cable to multiply the overall available spectrum, and integrate newer multi-core fibers into future wet plants. We can also increase the capacity by increasing the signal-to-noise ratio by leveraging Raman amplification, developing and deploying lower loss fibers, and using a higher total output power within subsea optical fibers.

Of course, there are several business and technology tradeoffs associated with all these options, but they represent prospects to challenge Shannon’s Limit, directly and indirectly. It should be noted that these prospects are applicable to newer uncompensated submarine wet plants before they’re deployed on the bottom of the seabed. This means that wet plant technology decisions decided before it’s laid upon the seabed will ultimately dictate your Shannon’s Limit. A low initial cost wet plant will hamper its upgradability and associated revenues. This is why each and every submarine cable has and will continue to have, a unique personality in terms of performance.

**Optimizing submarine cable capacity**

There are indirect opportunities outside the optical transmission domain to be leveraged. Operators can adopt statistically multiplexed packet switching, which allows operators to migrate away from traditional Time Division Multiplexing (TDM). TDM consumes bandwidth whether it’s being used or not. Packet switching optimizes bandwidth utilization by using the lulls between end-user data communication to transmit other end-users data. Packet switching allows far more user traffic to be carried over the same physical submarine network because it exploits the statistical nature of data traffic, which represents the vast majority of submarine cable traffic both today and well into the future. As well, packet-based switching is equally applicable to both old, new, and even yet to be deployed submarine cables.

It should also be noted that the amount of existing bandwidth already deployed is not always being used in an optimized manner. For instance, legacy protection bandwidth dedicates bandwidth in case of a fault, so under normal conditions, the protection bandwidth could be carrying revenue-generating data. Traffic also often doesn’t follow the most optimized path to its destination, which leads to the inefficient consumption bandwidth along its path between endpoints. By virtualizing underlying physical network assets, overland and undersea, SDN control-based intelligence can better optimize the consumption of available bandwidth on an adaptive basis. This implicitly delays having to deal with Shannon’s Limit, since the better use of overall bandwidth effectively means that less bandwidth is required.

Submarine cables are designed for guaranteed full-filled End-of-Life (EOL) performance, meaning early in a typical 25-year lifespan, more traffic can be transported by “borrowing” some EOL margin to increase channel rates via intelligent SLTE. As bandwidth-on-demand consumption models continue to be adopted across all parts of the global network, borrowing EOL margins for temporary increases in overall submarine cable information-carrying capacity just makes sense. It’s called Liquid Spectrum and will be an optical game changer, overland and undersea.