Rise of Hyperscalers Places Greater Importance of Testing Subsea Optical Cables
As published in the September Issue of SubTel Forum Magazine
By Shu Zhuang
September 27, 2022
At last count, there are an estimated 436 submarine cables stretching more than 1.3 million kilometers around the globe, according to TeleGeography. Those cable are essential to how we all communicate and gather information, as they transmit between 97% – 99% of the world’s data. To ensure their proper deployment and operation – and to efficiently locate and repair any faults – advanced testing solutions and processes must be incorporated.
Maintaining transmissions through subsea cables has a profound impact on more than just how people live their lives. It has a major financial impact. The global submarine cable system market is expected to grow from $14.40 billion in 2021 to $16.15 billion by the end of this year. Growth will continue – reaching $22.7 billion by 2026, according to Research and Markets.
There are two main factors contributing to this growth:
COVID-19 – The global pandemic has changed the way in which people live. Remote working environments are expected to continue for the foreseeable future, creating more demand for video conferencing and other streaming technologies.
Hyperscalers – Perhaps a bigger reason for the growing deployment of subsea optical cables is the influx of hyperscale data centers. Such facilities are used by global technology corporations to deliver key services worldwide. A hyperscale data center is defined as one that has more than 5,000 servers, occupies 10,000 square feet, and has a flexible architecture for a homogenous scale-out of greenfield applications. Figure 2 provides the growth projection of hyperscale data centers, according to Synergy Research Group.
Hyperscale data centers are the reason, Google, Meta, Microsoft, and Amazon are prominent players in the submarine cable market. Each is making considerable investments into new subsea cables. In fact, the capacity deployed by private network operators like hyperscalers is outpacing traditional Internet backbone operators. By 2024, the group is expected to own more than 40 long-distance cables connecting every continent with the exception of Antarctica.
Understanding Submarine Cables
With such investment comes equally high expectations. Submarine cables integrate various elements (figure 3) that must be tested before the cables are dropped down to the seabed at depths of 3+ kilometers (km). Monitoring must also be done on the cables to ensure proper data transmission and for networks to meet key performance indicators (KPIs). Verification needs to be done on:
- Optical cable – There are typically several fiber pairs within an optical submarine cable. The optical cable leverages DWDM technology to maximize the capacity.
- Submarine Line Terminal Equipment (SLTE) – To meet the growing demand, SLTE is operating at higher speeds ranging from 200Gbit/s to 800Gbit/s.
- EDFA Section – This layer contains several components, including Erbium-Doped Fiber Amplifier (EDFAs). EDFAs are optical amplifiers that amplify the optical signal along the cable. More than 100 EDFAs may be in a single subsea cable optical link.
- Optical Feedback Path – A part of the EDFA section, the path is essential when conducting network tests.
Ensuring Installation and Operation
On average, more than 100 submarine cables suffer a break annually. Many are caused accidentally by fishing vessels as they pull their anchors. Given the growing importance of the traffic transmitted over subsea cables, however, there is growing concern that nefarious acts may be undertaken to damage cables, as well.
Network operators deploy cable ships to lay or repair underwater cables. On average, one of these deep-sea vessels is commissioned each year to meet the growth in deep sea cables. Engineers on those ships who are responsible for laying the subsea cables face a number of challenges. Examples include understanding all installation requirements and knowing the specific parameters for installation. Coherent Optical Time Domain Reflectometer (C-OTDR) and OTDR measurements form the main method of ensuring the proper laying of the cable, as well as to monitor cable operation and accurate locate faults when they occur.
Advantages of a C-OTDR
A C-OTDR is the optimum instrument to accurately measure and characterize the optical submarine network. It accurately locates faults to within 10 meters (m). It works on the same basic principles of an OTDR. Conventional OTDR technology, however, isn’t a viable option because an EDFA only amplifies in the forward direction and employs components that are unidirectional. Backscattered light – critical for measuring optical cable – is not able to return via its original path, as a result.
To offset this scenario, the majority of installed and planned systems include the aforementioned optical feedback path within the EDFA enclosure. Backscatter light travels back to the C-OTDR using this path, allowing a C-OTDR to monitor the submarine cable using OTDR principles. Two repeaters are connected by the optical fiber, which is typically between 40 km and 90 km long, for testing purposes, as well.
A C-OTDR also has the added ability to transmit on an adjustable narrow wavelength, so the instrument is used in a live network alongside real traffic within the DWDM network. To conduct measurements, the C-OTDR transmits two pulses, both of which are typically placed as far away from live traffic as possible to minimize interference. A probe pulse is sent to a DWDM channel, while a dummy pulse occupies a second channel commonly adjacent to the probe pulse. A dummy pulse is necessary because of the automatic gain control system of the EDFA.
In a live system, the input to an EDFA is at a constant power level across multiple channels. Testing with a C-OTDR is often completed on a system with no traffic (aka “unlit”), too. When testing on an unlit system, the EDFA gain control cannot maintain a stable output due to the pulsing power nature of the C-OTDR. To compensate, the C-OTDR outputs pulses on two channels to ensures a constant input level to the EDFA. The test pulse is generated for a short period, while the load pulse is on for the remainder of designated time. The ratio between the two is determined by testing the pulse width selected on the C-OTDR.
On the receiver side of the C-OTDR, there are several enhancements over a standard OTDR:
- Improved filtering – Input of the C-OTDR is filtered to remove the active DWDM channels, as well as extra noise.
- Coherent detection – Coherent detection re-injects the original transmitted wavelength, allowing the resultant to show only data at exactly that wavelength. This method removes all other noise for improved signal-to-noise, so data from well below the normal noise floor can be reconstructed.
Coherent detection is necessary because a submarine network is comprised of many optical amplifiers that increase the power level to the DWDM wavelengths. It also increases the Amplified Spontaneous Noise (ASE) level. As each amplifier raises the ASE level, the coherent detection method enables the C-OTDR to detect signals that would normally be “hidden” within or below the noise.
More Accurate Fault Location at Any Depth
A C-OTDR is essential for submarine cable because it enables accurate fault location on any length of a submarine network. The data point resolution of many traditional OTDRs is typically based on the km range setting of the instrument. For example, an OTDR with 50,000 data points is affected by the range setting. It is a critical issue with submarine networks, as the distance in subsea cables is several orders of magnitude longer than terrestrial networks.
C-OTDRs are designed with 1.2 million data points and automatically reduce the number of points, depending on the distance range setting. The latter feature has several advantages:
- The necessary 10m resolution is maintained irrelative of the km range setting to ensure the C-OTDR is not the weakest point when locating a fault
- Processing time of the C-OTDR is reduced while the trace is calculated
- The C-OTDR is capable of conducting approximately 8 samples per second
Each 1.2 million data point sample is averaged over time before the trace is displayed on the C-OTDR screen. Faster processing is achieved because fewer data points are averaged when a range setting of less than 12,000 km is selected.
Using a C-OTDR with fewer data points can be detrimental when longer links must be measured. For example, if the C-OTDR has 10,000 data points maximum and the range is set to 8,000 km, an 800m data point inaccuracy will be created. As seen in figure 4, the inaccuracy can cause extended delays in locating the end fiber fault. The result is the network will be down and/or operate at a substandard level for a longer period. Given the value and investment in the networks, the financial implications can be astronomical in such a scenario.
Importance of Measuring Signal Power
Multiple laser signals – up to 160 or more – of different wavelength are multiplexed in submarine cables. Accurate testing of the power of these signals is necessary to ensure operation of subsea optical cables. If the power is too low, the signal will not be received at the other end. If it’s too high, the signal may break transmission equipment.
An optical spectrum analyzer (OSA) is an instrument that displays the optical power of the signal under test. The OSA conducts Optical-Signal-to-Noise Ratio (OSNR) for accurate noise power measurements. The On/Off measurement method is most effective on submarine optical cables. It allows OSNR analysis of polarized multiplex signals by turning off each channel, so the noise power of each can be measured individually in accordance with IEC61282-12.
During installation of submarine cables, the OSA is used for additional measurements, as well. Among the other tests the analyzer performs are channel wavelength, gain tilt (flatness of each channel power), and spectrum width.
The importance of submarine cables in global networks is becoming greater with the growth of hyperscale data centers. To ensure proper deployment of new cables and their ongoing operation, a new generation of C-OTDRs and OSAs have been developed. They allow for extremely accurate distance measurements and full characterization of optical events in submarine optical cables. The coherent technology and submarine cable optical feedback path of C-OTDRs ensure thousands of km of fiber can be characterized quickly and efficiently, helping to ensure the expensive task of fault restoration is completed as quickly and efficiently as possible.
About the Author
Shu Zhuang is Senior Product Marketing Manager at Anritsu Company. She has more than 20 years of experience in product marketing, pre-sales, global network design, system design engineering and system verification roles. She holds an MBA in Electrical and Computer Engineering from Stevens Institute of Technology.
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