Submarine Network Series- Part 3- Submarine Systems’ Performance Metrics

Submarine cable systems, vital lifelines of global connectivity, pose unique challenges compared to their terrestrial counterparts. As a result, measuring and determining service performance in submarine networks requires specialized approaches. Let's delve deeper into the distinctive metrics and considerations that govern the evaluation of submarine systems' performance.

Unique Challenges of Submarine Systems

Component Health Monitoring: In submarine networks, assessing service performance involves monitoring the health status of essential components such as Basic Units (BUs) and intermediate repeaters. This data is gathered by coherent transponders placed at cable ends, presenting a more complex monitoring scenario compared to terrestrial networks.


Total Output Power Constraint: Unlike terrestrial systems, submarine systems face Total Output Power (TOP) constraints, altering the calculation of Signal-to-Noise Ratio (SNR). TOP constraints lead to signal depletion and noise accumulation, necessitating adjustments in performance evaluation methodologies.


Evolution of Coherent Modems: Advancements in coherent modems, particularly on D+ submarine optical cables, redefine system capacity parameters. Consequently, the ITU updated commissioning processes to accommodate these advancements, ensuring accurate performance assessments.

Performance Metrics for Submarine Systems

Optical Signal-to-Noise Ratio (OSNR): OSNR quantifies the ratio of service signal power to noise power within a specified bandwidth. However, comparing OSNR between systems with different baud rates requires careful consideration to ensure accurate performance evaluation.


Signal-to-Noise Ratio ASE (SNRASE): SNRASE, akin to OSNR, accounts for noise within the signal bandwidth, enabling comparisons across systems with varying baud rates.


Effective SNR (ESNR): ESNR offers a line rate-independent measurement by considering both linear and non-linear noises. Unlike traditional metrics like Q factor, ESNR remains consistent across different line rates, providing valuable insights into future system capacities.


Generalized OSNR (GOSNR): GOSNR combines linear and non-linear noise contributions, offering a comprehensive assessment of system performance. Its baud-independent version, GSNR, provides a holistic view of performance, accounting for effects like guided acoustic-optic wave Brillouin scattering (GAWBS) and signal droopSubmarine cable systems are more challenging compared to terrestrial ones. As a con- sequence, they have major differences in the ways information about service performance can be determined and measured.

Some major differences are the following:

­ In submarine networks the service performance can be determined by information de- scribing the health status of basic network components (BUs, intermediate repeaters). This information is obtained by coherent transponders which are placed at the ends of a submarine cable. Their terrestrial counterparts are by far more easy to monitor. Terrestrial networks can process more data with regard to each unit’s contribution to the whole system’s performance.


­ Total output power (TOP) constraint is another key difference between terrestrial and submarine systems as it changes the way that total SNR is calculated. The TOP constraint in submarine amplifiers results in signal depletion whereas amplifier noise is accumulated because the total channel power (S+N) remains fixed with distance.

New coherent modems used on D+ submarine optical cables change the parameters that define total system capacity and so ITU updated the commissioning process (thus the final tests before going commercial) in a G.977.1 recommendation.

Optical signal-to-noise ratio (OSNR) refers to the ratio of service signal power to noise power for a valid bandwidth (0.1 nm, so ~12.5 GHz at 1550 nm). OSNR is used to quantify the linear noise from amplified spontaneous emission (ASE). However, if two systems are using different baud rates, the OSNR of the higher baud rate system will be reported as higher (although the two systems may have the same noise level). So, OSNR has to be defined without reference to channel spacing and symbol rate.

Signal-to-noise ratio ASE (SNRASE) is similar to OSNR except that the noise bandwidth is equivalent to the signal bandwidth. This leads to a measurement not dependent on symbol rate and which accounts for all noise detected on the receiver side. Therefore, by implementing this approach it is easier to compare the SNR metric for signals with different baud rates.

Similar to the baud independence of SNR, a metric that provides a line rate-independent measurement of performance would be useful for evaluating potential system capacities. Effective SNR (ESNR) measures linear and non-linear noises and reports their impact to the signal performance. As a result, ESNR will not be changed for the same noise levels, no matter the signal’s line rate. This is an improvement over Q factor, which varies for signals experiencing the same total noise on different line rates. In this way, ESNR can be used as a future teller (i.e., if measured at one line rate, it can then be used to predict performance of higher data transmission rates for the same cable system).

As performance of upcoming systems depends also on optical nonlinearity (SNRNL), it would be convenient to measure both linear and non-linear performance. Generalized OSNR (GOSNR) sums the non-linear and the linear noise of the wet plant optical systems. GOSNR’s updated baud-independent version is GSNR. Other effects are both guided acoustic–optic wave Brillouin scattering (GAWBS) and signal droop. GAWBS is an effect which leads to a penalty for a given wet plant design. This effect is caused by the interaction between light and the acoustic modes that occur in the optical fiber.

GSNR is evaluated directly through simulations or analytic models and indirectly through experiments. Figure below summarizes all existing and new metrics and indicates at which exact point each of them is measured.

Figure: Summation of existing and new cable performance metrics and indication of the exact location where each one of them is measured.

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Cited: Papapavlou, C., Paximadis, K., Uzunidis, D., & Tomkos, I. (2022). Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends. Telecom.

Future Perspectives and Conclusion

The performance of upcoming submarine systems hinges on the accurate measurement and evaluation of both linear and non-linear factors. As technology advances, metrics like ESNR and GSNR will play pivotal roles in predicting and optimizing system capacities.

In conclusion, submarine systems present unique challenges that demand specialized performance metrics and evaluation methodologies. By embracing advancements in coherent modems and refining performance metrics, we pave the way for more efficient, reliable, and high-capacity submarine networks, ensuring seamless global connectivity for generations to come.

Submarine Network Series: Part 2- Past Evolution of Submarine Transmission Systems

The history of submarine cables can be traced back to the late 1840s when a newly discovered natural polymer called gutta percha provided the basic means for insulating cables for undersea use. In 1850, a relatively short-distance cable was submerged between the UK and France, but unfortunately, it lasted only for a few messages and was replaced by a more durable successor a year later. The first cable connecting Europe to the US became operational on August 16, 1858, but the deployment of cables across the Pacific Ocean proved to be more challenging, and transpacific telegraph cables only became operational in 1902.

It took approximately fifty more years for newer advanced cables to support voice and data communications to be deployed. TAT-1 was the first transatlantic cable used for telephony, which became operational on September 25, 1956, connecting Scotland to Clare Ville, initially supporting transmission of just 36 telephone channels. The first coaxial cables capable of transmitting frequency-multiplexed voice signals became operational in the 1960s, which eventually led to optical technology in 1986 and 1988, using optical fiber links for high-speed/capacity submarine communications.

Some significant milestones in the submarine era history include the first submarine cable supporting phone data (from San Francisco to Oakland) in 1884, the first submarine high-voltage direct current cable connecting the island of Gotland to mainland Sweden in 1954, and the first deployment of repeaters in the 1940s, boosting the TAT-1, which was the first telephone cable crossing Atlantic. The first transpacific submarine coaxial telephone cable linking Japan, Hawaii, and the US mainland was operational in 1964. The first submerged international fiber-optic cable that connected Belgium to the United Kingdom was in 1986, and the first submerged transoceanic fiber-optic cable, named TAT-8, that connected the USA to the United Kingdom and France, was in 1988.




Figure 1 depicts the historical evolution of submarine transmission systems leading to optical technology. In Figure 2, a transatlantic cable capacity comparison is illustrated from TAT-1 in 1956 to TAT-14 in 2001. Figure 3 shows the worldwide submarine cable map, which comprises 487 cables, stretching over 1.3 million km, and 1245 landing stations that are currently in service or under construction. These networks were deployed by consortia of major telecom network operators.

However, during the current century, major cloud service providers such as Google, Facebook, and Microsoft have become active investors in submarine networks. From 2016 to 2018, Google invested approximately $47 billion dollars in capital expenditure (capex) to expand and upgrade Google Cloud infrastructure. Nowadays, Google counts 134 points of presence (PoPs) and 14 major subsea cable investments globally, interconnecting all six continents. These investments have played a significant role in enabling Google to provide reliable and high-speed cloud services to customers globally.

Submarine Network Series: Part 1- The Evolution and Future of Submarine Networks: A Focus on Space Division Multiplexing

The Submarine series of posts discusses the evolution of submarine networks alongside terrestrial ones over the past several decades. While there are similarities between these two network categories, such as the need to cover ultralong-haul distances and transport huge amounts of data, there are also important differences that have dictated their different evolutionary paths. The series focuses on space division multiplexing (SDM) as the ultimate solution to cover future capacity needs and overcome problems of both networks.

The introduction section provides a brief history of submarine cables, which were first submerged to transmit telegraphy data approximately one and a half centuries ago. Today, submarine cable systems have become a basic component of the whole global backbone network infrastructure and serve as the most crowded, yet isolated, of deep-water networks. The traffic generated by end-users has been boosted beyond the average 50% annual growth rate due to the COVID-19 pandemic, increasing the need for speed and more bandwidth.

We will review recent and future submarine technologies, focusing on all critical sectors: cable systems, amplifiers' technology, submarine network architectures, electrical power-feeding issues, economics, and security. The authors provide an overview of all recently announced SDM-based submarine cable systems, compare their performance (capacity-distance product), and analyze the reasons that led to the first SDM submarine deployment. They also report up-to-date experimental results of submarine transmission demonstrations and perform a qualitative categorization that relies on their features.

Based on all latest advances and their study findings, the authors try to predict the future of SDM submarine optical networks mainly in the fields of fiber types, fiber counts per cable, fiber-coating variants, modulation formats, as well as the type and layout structure of optical amplifiers. Results show that SDM can offer higher capacities (in order of Pb/s) compared to its counterparts, supported by novel network technologies: pump-farming amplification schemes, high counts up to 50 parallel fiber pairs, thinner fiber coating variants (200 µm), and optimum spectral efficiency (2-3 b/s/Hz).

Then we will conclude that tradeoffs between capacity and implementation complexity and cost will have to be carefully considered for future deployments of submarine cable systems. SDM can provide a solution to cover future capacity needs and overcome problems of both submarine and terrestrial networks.

Google Cloud Platform - Submarine Network

Advanced Optical Series: Part 5 - A Deep Dive into Subsea Cabling: The Backbone of Global Computer Networking

Introduction:

Subsea cabling is an essential component of the global computer network. It is responsible for transmitting vast amounts of data between continents and connecting people and businesses worldwide. In this blog post, we will take a deep dive into the world of subsea cabling, exploring its history, technology, benefits, and challenges.

History of Subsea Cabling: 

The first subsea cable was laid across the English Channel in 1851. It was a copper cable covered in gutta-percha, a natural rubber, to protect it from seawater. Since then, subsea cabling has evolved dramatically, from simple copper cables to fiber optic cables capable of transmitting terabits of data per second.

Technology of Subsea Cabling: 

Subsea cables are made of several layers, each designed to protect the cable and ensure data is transmitted with minimal loss or disruption. The outer layer is typically made of polyethylene or other plastics to provide insulation and protect the cable from damage. The next layer is a steel armor designed to protect the cable from external pressure and damage from anchors and fishing nets. Inside the armor, the cable consists of several optical fibers, each capable of transmitting data at high speeds.

Benefits of Subsea Cabling: 

Subsea cabling offers several benefits for global computer networking. It is more reliable than satellite communication, as it is not affected by atmospheric conditions or solar flares. Subsea cabling also provides faster data transfer speeds and lower latency, making it ideal for real-time applications like video conferencing and online gaming.

Challenges of Subsea Cabling: 

Despite its benefits, subsea cabling also faces several challenges. The installation and maintenance of subsea cables are complex and expensive, requiring specialized ships and equipment. The cables are also susceptible to damage from natural disasters like earthquakes and storms, as well as human activities like fishing and anchoring.

Future of Subsea Cabling: 

The demand for subsea cabling is only expected to grow as more people and businesses connect to the internet. To meet this demand, researchers are exploring new technologies like self-healing cables, which can detect and repair damage automatically. There is also a growing interest in using subsea cables for renewable energy transmission, allowing offshore wind farms to transmit energy to the mainland.

Conclusion: 

Subsea cabling is the backbone of global computer networking, connecting people and businesses worldwide. It has come a long way since the first cable was laid across the English Channel, and it continues to evolve and improve. Despite its challenges, subsea cabling offers numerous benefits and is essential for the future of global communication and connectivity.

Features of SDM-Based Submarine Cable Systems

The evolution of SDM in submarine networks started with Suboptic’16 (powered by Alcatel-Lucent Submarine Networks (ASN) Ltd., founded in 1983) which was the first 16-FPs submarine cable system [8]. Until recently, in all networks, the initial goal was twofold: to increase the total cable capacity (up to 70% with regard to traditional cable) and to decrease the required cost and power per transmitted bit.

The innovative features that characterize the first generation of SDM submarine networks are [8] as follows.

­ A relatively high count of FPs (in the same cable) in order to increase the transported capacity.

­The deployment of lower effective area fibers in order to optimize cost through the use of a smaller number of regenerators.


­ The implementation of the novel “pump farming” repeaters’ technology. Pump farming means that a set of pump lasers isused to amplify a set of FPs. Reliability, redundancy, and better power management are the main advantages. In particular, reliability can be a cost-reduction factor as submarine cables’ failures and repairs (bringing downtime in provided services) are very costly.

­ SDM aims to achieve higher capacities by using the same amount of used power through a more efficient power management. The key concept is to reduce the optical power provided to each FP as a way to decrease the nonlinearities as implementing high count of FPs in the same cable.

However, in the event we want to cover small-distance undersea links (e.g., unrepeated- festoon networks) or if we want to increase the capacity of an existing traditional submarine cable system (consisting of a limited number of FPs), multiband transmission is a more effective solution compared with SDM, which is mostly preferable for long-haul distance links. More specifically, in multiband transmission there is no need to change the existing wet plant infrastructure during the upgrade process and this can result in both an increase (by double) of the capacity/FPs and in cost savings.


Table 2 presents the pros and cons of using SDM over a single band as opposed to multiple bands. The options presented are doubling the number of fibers at C band only and using the same number of FPs over the C + L bands. Note however that the C + L transmission is less efficient because C + L has to be separated and recombined (mux/de- mux) in the repeater for each span. This extra multiplexing/de-multiplexing leads to an extra loss per span of about a few dBs, which is contrary to the “optimizing efficiency” basic SDM concept.







Figure 6 shows the different types of submarine cables and the various types of fibers, respectively. The selection of the optimal cable type depends on the depth at which each cable is sinked. For example, double-armored (DA) submarine cable is used at the shore end, terminated at the beach manhole of the cable landing site, and interconnected with a much lighter land cable (LWA) moving toward the cable landing station.