Optical transport networks and services are in the midst of a significant transformation. After early deployments of 100-Gbps coherent line interfaces in the long-haul market, the technology is gradually transitioning to metro environments. This evolution requires entirely different design requirements in terms of performance, cost, power consumption, and form factor.
Conversely, equipment manufacturers continue to announce interface rates beyond 100 Gbps – such as 200 Gbps and 400 Gbps, or even 1 Tbps. Yet such large steps in capacity can result in substantially curtailed transmission reach, crippling the potential of higher bit rate interfaces and making them compete with typical lower-cost metro options. There have been various concerns about next-generation transport networks and if they’ll actually be able to support terabit per second interface rates and/or employ modulation formats beyond 16QAM. Network operators also want to know if the economics would ever make sense and if there are any implications for real-world multi-domain networks. Although some of the recent advances in ultra-high capacity interfaces have found an application in the rapidly evolving data center interconnect (DCI) market, there’s still a huge gap between interfaces that are able to serve long-distance optical transport applications and ones limited to metropolitan area networks. This disparity has created a need for greater flexibility.
While emerging technologies (e.g., software-defined network and network functions virtualization) are introducing flexibility in the upper layers of the transport stack, flexibility at the physical layer is proving more challenging – as evidenced by the inability of 100G QPSK, and also the extension to 200G 16QAM, to effectively scale across multiple network domains. In these circumstances, the question then arises – which set of Layer 0 technologies will enable an agile and future-proof transport network? What would be the impact of such agility on different market segments and on carrier economics?
International Tier 1 carrier Orange and Coriant recently collaborated in a live field trial to answer some of these questions. The following is an in-depth look at the results from this field trial, as well as the practical implications for real-world use cases and carrier economics.
Are networks ready for the future?
Since the advent of modern communication infrastructure, connectivity has moved beyond physical location-based contiguity to human interconnections, with a more recent explosion to an ever-growing network of machine-to-machine interactions (Figure 1). As of today, more than 25 billion devices are linked to each other via the Internet, and this number is expected to double in the next 5 years.
These developments have challenged the very core of the conventional transport network design and development cycle. Not only have the new realities imposed higher bandwidth demands, but they’ve also driven the need for dynamic service provisioning along with lower footprint and reduced transport costs. Data center operators and Internet content providers (ICPs) are at the forefront of these market trends. However, traditional carriers are facing the very same challenges, although within an entirely different construct owing to legacy infrastructure and previously deployed network domains and services.
Equipment vendors, of course, are trying to catch up with these rapid developments. However, it is clear that an evolutionary product portfolio will fall short when it comes to effectively meeting the plethora of use cases, given the sheer pace of change and the dynamic needs of different market segments. Recently deployed high-capacity flexible systems (which have added 200G 16QAM line rates), for example, have proven to be an incremental upgrade from commercial 100G coherent interfaces; they address bandwidth requirements for shorter reach and higher-capacity connectivity, but don’t scale well across multiple network domains due to the substantial reach and capacity discrepancy between 100G QPSK and 200G 16QAM. (For more on this, see "Flexi-rate optical interfaces go mainstream.")
What is needed is a streamlined interface strategy, one orchestrated to address dynamic, high-capacity traffic behavior, while focusing on a consolidated architecture targeting diverse network segments ranging from metro-based DCI to ultra-long-haul core networks. The right side of Figure 1 illustrates the concept of a next-generation "multi-flex" line interface designed to meet such needs that was the subject of the Orange field trial. The interface is designed to support line-rates up to 1 Tbps while quadrupling the spectral efficiency of current commercial 100G technology. Furthermore, support of a variety of modulation formats (QPSK, 8QAM, 16QAM, 32QAM, and 64QAM), paired with flexible rate capabilities and flexible grid transmission, aims to cost-effectively address the needs of multiple network domains. Software programmability optimizes cost per bit and power efficiency based on bandwidth consumption, traffic characteristics, and available system margins.
Field trial details
The field trial was conducted over a period of three weeks. The trial transmissions leveraged field-deployed fiber carrying live traffic between Lyon and Marseille (Figure 2A). The transport infrastructure consisted of G.652 standard single-mode fiber, spanning a total distance of 762 km, supported by hybrid amplification.
The first part of the trial focused on terabit transport in the C-Band. The trial demonstrated capacities of 24 Tbps, 32 Tbps, and 38.4 Tbps using 16QAM, 32QAM, and 64QAM, respectively (Figure 2B). The interface was based on a quad-carrier superchannel structure.
Most of these outcomes represented world-record achievements, either in terms of highest transport C-Band capacity and/or the distance over which such high speeds were transmitted in a live field trial. Specifically, transmission of 64QAM – with a spectral efficiency of 8 bps/Hz (38.4Tbps) – over 762 km marked a hallmark feat, considering the difficulties typical in bridging transparent, regeneration-free reach of 16QAM. Contributing technologies for this result included multi-lambda superchannel structures, fully programmable modulation options, and in-house digital signal processing (DSP) and forward-error correction (FEC) techniques.
Figure 2C illustrates the multi-flex operation of the interface, emulating a seamless legacy network upgrade scenario with flexible rate and flexible grid technologies. The spectral efficiencies ranged from 2.67 to 8.0 bps/Hz, while modulation formats encompassed QPSK to 64QAM, with variable system margins for the regional network under consideration. For commercial network deployments, the available system margin may be used to enable diverse network applications; conversely, margin may also be used towards progressive network upgrades, permitting a pay-as-you-grow approach.
The trial demonstrated that a multi-flex interface not only can maximize capacity on a link-by-link basis, but also can enable adaptive end-of-life margin planning and lowest cost per bit for applications under consideration. Most importantly, it will enable a consolidated architecture – for example, using similar FEC and DSP – catering both high capacity short-reach and performance-sensitive long-reach submarine network applications. It is clear that, for the foreseeable future of the optical transport business, the choice of architecture – considering return on investments in a given market space and constituent technologies – will be the secret sauce behind the new deployments.
Market trends, carrier economics, and portfolio consolidation
While the soaring bandwidth demands of internetworked smart devices have almost forced a revolution on optical transport technology design and development, network economics continue to diverge (Figure 3, left). With new network transformations expected to further create cost pressures, there is a clear requirement for convergence, not only in transport layers, but also in business models. A step-by-step approach would be to identify key industry trends, both technological and economic, and follow that up with strategically defined, streamlined portfolios.
For instance, the three major trends reshaping the global telecom landscape are social demographics, network economics, and energy constraints, which can be addressed by a flexible technology layer, network commoditization, and disaggregation. These trends not only represent technological evolution, but also represent transformation of the optical networking business, which affects both traditional carriers and growing over-the-top content providers.
Moving forward, it will be impossible for a market player to survive without streamlining its interface technology across diverse network domains, and catering to both niche and lower-end business with universal and scalable architectures. Figure 3 (right) establishes a few primary interface design requirements for different segments of the optical transport market. It is clear that while different markets necessitate different requirements, there exists a common denominator among the choices. A smart architecture can leverage this cohesion.
Dr. Danish Rafique works as a Senior Lead Eng. HW – Product and Technology, at Coriant.