Why Subsea Optical Cables are Important in a Hyperscale Network World
An estimated 436 submarine cables are spread over nearly 1.3 million kilometers (km) of the ocean floor (figure 1). Of greater significance is that 97% - 99% of the world’s data is transmitted over said cables. Mobile operators and hyperscalers responsible for those transmission lines must utilize specialized testing solutions to ensure cost-effective deployment and operation. Failure to do so could literally bring communications to a halt.
Figure 1: Submarine cables stretch more than 1.3 million kilometers across the ocean floor worldwide. Image courtesy of TeleGeography.
Maintaining transmissions through subsea cables has a profound impact on more than just how people gather and exchange information. It affects the global economy. The worldwide submarine cable system market is expected to rise from $14.40 billion in 2021 to $16.15 billion by the end of 2022. Growth will continue – reaching $22.7 billion by 2026, according to Research and Markets.
Two elements are driving the growth of submarine cable systems:
COVID-19 – The global pandemic altered how we live, work, and stay in touch. Those changes are expected to continue, at least in the near-term. Demand for video conferencing, streaming services, and other online technologies are now integral to our day-to-day lives, both personally and professionally.
Hyperscalers – Hyperscale data centers (5,000+ servers and more than 10,000 square feet) are an even bigger factor. Such facilities are used by global technology corporations to deliver key services worldwide. Figure 2 shows the growth projection of hyperscale data centers, which utilize a flexible architecture for a homogenous scale-out of greenfield applications, according to Synergy Research Group.
Figure 2: Hyperscale data centers are expected to have consistent growth for at least the next three years. (Courtesy of Synergy Research Group).
Proper Deployment and Optimal Operation
Keeping all this data flowing is no simple task. An average of 100+ submarine cables suffer a break every year. Often times, the cause is accidental but with the growing importance of data, those responsible for maintaining those cables must be more aware of nefarious activities or risk having networks fail.
On average, network operators commission cable ships annually to lay or repair underwater cables. Engineers on those ships must understand all installation requirements and know the specific parameters for installation. They rely on Coherent Optical Time Domain Reflectometer (C-OTDRs) and optical spectrum analyzer (OSAs) measurements to determine a method of ensuring the proper laying of the cable, as well as to monitor cable operation and accurately locate faults.
Why a C-OTDR is Necessary
A C-OTDR, such as the Anritsu MW90010A, is the instrument of choice when it comes to optical submarine network verification. It accurately locates faults to within 10 meters (m) through a technique in which backscatter light travels back to the C-OTDR using the optical feedback path. Additionally, two repeaters are connected by the optical fiber, which is typically between 40 km and 90 km long, for testing purposes.
At the receiving end of the C-OTDR, the backscattered optical signal is extremely small compared to the amplified spontaneous emission (ASE) optical signal emitted from the EDFA in a subsea system. A C-OTDR measures backscattered light by causing interference between the light source itself and the backscattered light on the receiving side. Backscattered light has high coherence, but ASE does not interfere with it, so only backscattered light can be extracted.
Contrast this to a conventional OTDR. A direct detection type OTDR can only measure the integrated power, so the backscattered light is buried in the ASE, resulting in nothing being seen.
Accurate Fault Location – at Deep Depths
Another reason C-OTDRs are preferred is that they have higher data point resolution compared to an OTDR. For example, the MW90010A has 1.2 million data points, so it can more accurately locate faults in a submarine network. Each 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.
A traditional OTDR has data point resolution based on the km range setting of the instrument. For example, an OTDR with 50,000 data points is affected by the range setting. This factor is critical related to submarine networks, as the distance in subsea cables is several orders of magnitude longer than terrestrial networks.
A C-OTDR with fewer data points is problematic, as well, especially when long links must be measured. For example, if a C-OTDR has 10,000 data points and the measurement range is 8,000 km, data point inaccuracy will be 800 m. A delay in locating the fault at the end of the fiber will be caused in this scenario, as shown in figure 3. The result is the network will be down and/or operate at a slower rate for a longer period, creating significant financial loss.
Figure 3: A C-OTDR data point resolution will impact measurement accuracy.
Other key benefits of a C-OTDR are:
- The required 10 m resolution is maintained regardless of km range setting so the C-OTDR is not the weak link during fault location
- Processing time of the C-OTDR is reduced while the trace is calculated
- Approximately 8 samples per second can be made with the C-OTDR
Importance of Measuring Signal Power
There can be 160 or more laser signals at various wavelengths that are multiplexed in submarine cables. Accurate power tests on these signals are required for subsea optical cables to transmit according to specifications. If the power is too low, the signal will not be received at the other end of the cable. Power exceeding the specification may result in transmission equipment damage.
To prevent either scenario, engineers employ an OSA to display the optical power of the signal under test. Optical-Signal-to-Noise Ratio (OSNR) can be made with the OSA to acquire highly accurate noise power measurements. When conducting the tests, engineers should use the On/Off measurement method. It allows OSNR analysis of polarized multiplex signals by turning off every channel, so the noise power of each can be measured individually in accordance with the IEC 61282-12 standard.
Other necessary tests to maintain performance specifications are made with the OSA, as well. These include channel wavelength, gain tilt (flatness of each channel power), and spectrum width.
You can learn more about optical submarine cables by watching the Why is Submarine Cable testing so important? webinar recently conducted by Anritsu.