SUSTAINABLY ADDRESSING GLOBAL BANDWIDTH GROWTH

BY BRIAN LAVALLÉE

Industry analysts at TeleGeography forecast that global used international bandwidth is expected to grow at a compounded annual growth rate (CAGR) of 33% between 2022 and 2029, roughly doubling every 2.5 years. However, the carbon footprint of networks that address this growth simply cannot scale linearly from socioeconomic and environmental perspectives. This means a “bending of the curve” is required in that the carbon footprint grows at a lower rate than associated bandwidth grows. In other words, the watts/bit, space/bit, and cost/bit decrease as network capacity increases to address global bandwidth growth, which shows no signs of abating over the next decade. There’s no “Plan B” for submarine networks because there’s no technology today, or on the innovation horizon, that can reliably and cost-effectively scale to submarine cables based on fiber-optic transmission technology. This means that as an industry, we must continually evolve what we’ve been doing for decades until the next eureka technology moment. How can our industry reliably, cost-effectively, and sustainably scale to address bandwidth demand growth between continental landmasses? By adopting and adapting several ingenious technologies and network architectures available today and tomorrow. These technologies span the Open Systems Interconnection (OSI) model, from the Physical layer to the Application layer. We’ll discuss options available to submarine cable operators to maintain pace with global bandwidth growth and do so in a cost-effective and sustainable manner, which are often seen as contradictory design and business goals.

OPTICAL BYPASS, A GAMECHANGER

Over a decade ago, modems based on coherent optical transmission were introduced, initially to address long-haul terrestrial networks, say from New York to Los Angeles. However, after a field trial over an existing submarine wet plant, the proverbial lightbulb went off, as coherent modems provided better performance than incumbent modems using traditional Intensity-Modulation Direct-Detection (IMDD) On-Off Keying (OOK). This field trial was a monumental turning point in the submarine network industry and changed how submarine wet plants were designed, built, and maintained right up to today. Improve- ments in Submarine Line Terminal Equipment (SLTE) modems have steadily progressed to bring us ever closer to the dreaded Shannon Limit, but we can’t surpass this physical limit. We can get as close as technologically possible albeit with less equipment, via higher channel rates, for vastly reduced power and space requirements.

When the first 40Gb/s coherent optical modems were introduced, a new architecture change called “Optical Bypass” was also introduced and deployed in the traditional Cable Landing Station (CLS). Historically, early end-to-end optical networks had a clear demarcation between submarine and terrestrial networks, shown in Figure 1, due to political, economic, technological, and other reasons. This demarcation point served the industry well for decades but was rendered obsolete when replaced by an innovative and simpler end-to-end network architecture based on “Optical Bypass,” which Ciena pioneered over a decade ago.

The advent of coherent modems, Reconfigurable Optical Add/Drop Multiplexers (ROADMs), and intelligent optical power management meant traditional Optical-Electrical-Optical (OEO) stages in the CLS were no longer required. SLTE could now physically be relocated further inland directly into the Central Office (CO) of Communications Service Providers (CSPs) or data centers of Internet Content Providers (ICPs). Power Feed Equipment (PFE) usually remains in the CLS for a variety of reasons, such as safety due to very high voltages involved in powering the long chain of wet plant optical amplifiers, which are commonly (and incorrectly from a technical perspective) referred to as “repeaters” for historical reasons.

By leveraging coherent modems, ROADMs, intelligent optical power balancing, and data-driven intelligent control resulted in revolutionary SLTE providing a much simpler end-to-end network architecture, overland and undersea, as shown in Figure 2. This innovative approach was, and still is, significantly less expensive to build, own, and operate because considerably less equipment is required in the CLS—and in the terrestrial backhaul networks connecting the CLS to inland Point-of-Presence (PoP) on each end of the submarine cable network. For example, what used to take six modems per channel wavelength (two for the submarine cable and four for the two terrestrial backhaul networks) could be done with just two modems relocated in the inland PoP SLTE. This reduced complexity results in considerably less power, space, cost, and latency.

How’s this possible? Because modern coherent modems provide enough optical margin to propagate not only across long submarine cables at much higher channel rates, but also over terrestrial backhaul routes to the inland PoPs on each end of a submarine cable. In the CLS, ROADMs based on Wavelength Selective Switch (WSS) technology not only replaced legacy Synchronous Digital Hierarchy (SDH) Interconnection Equipment (SIE), which consumed enormous amounts of power, but also provided intelligent submarine optical power management. A summary of this more sustainable network architecture is provided below.

• Significant savings in power, space, and cost by eliminating a considerable amount of equipment.

• Simpler end-to-end network architecture to own and manage due to less overall equipment required.

• Reduced latency due to the elimination of multiple OEO conversion stages in the CLS on each end.

• Elimination of SIE in the CLS on each end of the cable, as the market shifts to wavelength services.

• Elimination of Terrestrial Line Terminal Equipment (TLTE) by replacing it with SLTE relocated from the CLS inland to the inland Central Office or data center on each end.

Submarine network operators can operate the submarine cable and terrestrial backhaul network segments on each end as a single, unified, and seamless all-optical photonic— no OEO stages—connection. Optical Bypass was rapidly adopted globally and helped stimulate the drive towards Open Cables where SLTE, and the wet plant they connect to on each end, come from different vendors. By leveraging open Application Programming Interfaces (APIs) and a next-generation domain controller, submarine cable operators could more easily manage multi-vendor Open Cables. This allows submarine cable operators to select from a broader vendor supply chain to build best-of-breed endto-end networks that not only have the best capacity, latency, cost, and reliability, but also the most sustainability (i.e., lowest power/ bit and space/bit).

Coherent Modems

The introduction of coherent modems over a decade ago was a pivotal moment for the network industry, overland and undersea. Coherent modems continue to evolve since introduced with throughput-optimized Forward Error Correction (FEC), Frequency Division Multiplexing (FDM), Probabilistic Constellation Shaping (PCS), rich in- strumentation and metrics, streaming telemetry, and other technologies. These enabled submarine cable operators to transport ever-increasing amounts of data across their undersea network assets. Each generation of modem increased the amount of data transmitted per channel for an aggregate higher overall submarine cable capacity—albeit using fewer modems. Increasing overall cable capacity with fewer modems yields improved economies of scale, to combat price erosion, and better overall sustainability.

Fewer modems hosted in fewer platforms provide significant savings in associated cost, power, and space. For example, Figure 3 compares 6.4Tb/s of capacity over three generation of Ciena’s WaveLogicTM modems yielding an 87.5% reduction in footprint and 80% savings in energy consumption. These savings translate into tangible reductions in upfront CAPEX and ongoing OPEX, especially as it relates to soaring energy costs in many parts of the world. Although not talked about in the past as much as it is now, sustainability has always been a design goal for each generation of coherent modem technology, and the result of this focus is quantifiably impressive with additional benefits upcoming with future generations.

Ciena recently announced our latest generation of WaveLogic 6 optical technology, which will be leveraged in Ciena’s GeoMesh Extreme submarine network solution. Compared to WaveLogic 5 Extreme, WaveLogic 6 Extreme will provide ~ 15% of spectral efficiency improvement, 50% reduction in power and space, 1Tb/s per channel over transpacific reaches of 12,000km, and be supported in existing host platforms to further reduce waste. This provides improved capacity to address global growth in a highly sustainably manner.

As we continually approach the Shannon Limit, business benefits will start to shift from massive increases in capacity on existing cables to reducing the cost, space, and energy per bit in a more environmentally and economically sustainable manner. In other words, providing the same submarine cable capacity, at the Shannon Limit, with less energy-consuming hardware will become the focus of SLTE modem innovation going forward. However, new wet plant innovations will allow us to side-step the Shannon Limit constraints of traditional submarine cable designs with a new generation of high-performance wet plants.

Wet Plants

Forecasted growth cannot be addressed without the need for ongoing SLTE modem innovation, wet plant innovation, and more cables deployed around worldwide, and this is exactly what’s happening now. Several new submarine cables have been announced along different submarine network routes that incorporate new wet plant technologies with additional wet plant technologies already being looked at for the future.

Wet plants evolved a few years ago with the introduction of uncompensated cables. Chromatic dispersion, once the enemy of IMDD-OOK optical transmission, which turned light on and off to represent digital ones and zeros, is a friend of coherent optical transmission. Uncompensated wet plants are easier to design, operate, and maintain with improved total cable capacity by allowing coherent SLTE modems to better perform. This yields more submarine cable capacity using existing undersea assets consuming the same amount of energy. However, uncompensated cables alone cannot address Shannon Limit constraints, so more wet plant innovation is required and is already being tested in labs worldwide.

Spatial Division Multiplexing (SDM), shown in Figure 4, increases the number of fiber pairs from a traditional 4 to 8 pairs to 16 to 24 pairs, and even higher in the future. This provides massive increases in total capacity compared to traditional cables deployed just a few years ago and is a promising wet plant technology.

Although SDM is a commercially available option to massively increase the information-carrying capacity of submarine cables, other more forward-looking wet plant innovations are being investigated to further increase wet plant capacities. One such innovation is Multi-Core Fiber (MCF) cables, shown in Figure 5, which add two or more optical cores to a single optical fiber, which theoretically doubles the capacity of the fiber. If the cores are far enough apart, say with 2 cores, MCF is “uncoupled,” while more cores closer together are “coupled.” The latter means optical transmission in adjacent cores will interact and interfere with each other thus requiring new techniques, such as Massive Input Massive Output (MIMO) used in wireless network transmission. A new generation of coherent modems that exploit coupled MCF submarine cables will subsequently require a significant amount of time, investment, and development.

There’s also the possibility of leveraging C+L Band technology in future submarine cables, which has been sidelined with the advent of C-band SDM cables, that essentially doubles the supported cable capacity. By leveraging the latest SLTE modems alongside SDM, MCF, and C+L Band wet plants, we should be able to achieve submarine cables with total capacities in the multiple petabits per second (Pb/s) range, where 1 petabit is 1,000,000,000,000,000 bits. From a sustainability perspective, using fewer massive capacity submarine cables to address global bandwidth demand growth results in a smaller overall carbon footprint. However, valid questions about diversity and availability quickly come to the forefront of discussions so a balance between the number of cables required to address global bandwidth growth, diversity, availability, and sustainability is required, as these business goals are tightly intertwined and, often, contradictory.

INTELLIGENT, ANALYTICS-DRIVEN AUTOMATION

Most SLTE deployed in the past few years is highly instrumented and supports streaming telemetry data via open APIs providing real-time insights into the performance of the submarine network. This measured performance data can be fed into an analytics engine leveraging machine learning and artificial intelligence to produce actionable insights. Once a change has been applied to the network, and its state changes, it will be reflected in real-time streaming telemetry to provide closed-loop automation. A network that can adapt allows submarine network operators to maximize the utilization of their existing network assets.

For example, let’s consider the typical 25-year design lifespan of a submarine cable, which means that it ages over time and remains within the forecasted number of fiber repairs due to human and natural faults; the deployed capacity will remain intact for 25 years. However, what if the submarine cable has just been deployed? Can we trade off end-of-life margin early in the cable lifespan and run it “hotter”? Analytics can provide insights into whether an operator can “upshift” channel rates for a limited time early in the lifespan of a submarine cable. Analytics can also take inventory of all submarine and terrestrial backhaul assets and provide insights into maximizing already deployed network assets. This avoids, or at least delays, deploying more energy-consuming equipment to address bandwidth growth, which leads to improved sustainability.

SUSTAINABILITY, A CRITICAL DESIGN GOAL

There are many technology and architecture options available to improve the sustainability of submarine networks, including terrestrial backhaul networks at each end. By constantly innovating SLTE connected to existing submarine cables, the watts/bit is continually reduced as capacity grows—meaning fewer submarine cables, and their associated CO2 footprint, are required. However, existing global submarine cable network infrastructure simply can’t address global bandwidth growth—meaning more cables will be required.

Because of continual technology innovation across our industry, we’ve been able to maintain pace with healthy growth in submarine network bandwidth demand while simultaneously reducing the associated carbon footprint per bit. A question often raised these days is, will addressing global bandwidth growth result in an ever-increasing overall carbon footprint, even with impressive sustainably gains achieved, due to so many more bits being transported overall? Time will tell how far we can push submarine network technology and architectures, but what I do know is that sustainability is at the forefront of design goals, as it should be. STF