Submarine Cable Networks
A submarine cable is a communications cable laid on or below the floor of the ocean or other body of water. A submarine cable network is an integrated system of cables collectively providing data transfer from one land-based location to another. This network includes the cable itself as well as other hardware components and infrastructure that make this large-scale communication possible. Despite the proliferation of satellite communications, over 95% of international data transfer is still performed via submarine cable networks.
These cables vary in size depending on the number of fibers in the central core, the marine environment (sand or rock), and the depth of water. They are specially constructed to protect the physical layer fibers carrying data from moisture and damage. The optical fibers in the inner layer of a cable are held inside a small plastic/nylon tube filled with a protective/lubricating gel, this tube is typically surrounded by multiple galvanized steel strands and encased in copper or aluminum tubing. This tubing is then further protected with a layer of polycarbonate, another aluminum water barrier, stranded steel wire for weight and mechanical strength, and then outside layers of mylar and polyethylene for additional waterproofing. There are many variations of cable construction which depend on the network's architecture, planned usage, deployment depth and environment, etc.
Submarine Cable Test Tools
Building, activating, and maintaining submarine cable networks presents unique challenges associated with accessibility, long distances, and constant exposure to the elements. Throughout the network life cycle, it is critical to use the right test and monitoring tools. See all submarine cable test tools.
During construction, end face inspection can detect dirt particles or other physical defects whose impact can either diminish or halt the performance of the network (and produce bad test results). Perhaps the most essential tool for physical layer fiber verification is the OTDR. Specialized types can be used for long haul wet plant OTDR as well as versatile bidirectional dry plant OTDR.
Another parameter which must be measured and controlled during cable deployment for submarine network construction is the amount of strain being placed on the actual fibers, this must be lower than 0.34% for submarine fiber grade (IEC 60794-3-20), higher values will reduce the life span of the fibers. Strain measurement can be performed using portable Brillouin OTDR. Optical dispersion tools are also pivotal to the characterization of networks in order to baseline optical dispersion and assess if any dispersion compensation is required.
Service activation requires tools for optical power measurement and spectrum analysis along with data network equipment testing. In order to quantify optical power, a ruggedized optical power meter is an essential tool. High quality Optical Spectrum Analyzers (OSA) for performing Optical Signal-to-Noise Ratio (OSNR) measurement and Network testers for latency, throughput, jitter, and frame loss/bit error rate analysis are also a must during this phase to validate and certify network performance.
Service performance monitoring
Monitoring & Maintenance
Monitoring and maintenance are the keys to keeping the vital cable networks performing optimally. To assess the fiber's physical health, an effective optical network monitoring system such as ONMSi can provide real-time fault, degradation, and security issue monitoring.
Fiber monitoring of links is an essential ongoing function. ONMSi can be used to detect fiber degradation and a wide array of fiber monitoring techniques may be utilized to meet the unique challenges associated with a fault. For example, to detect fluctuations in strain, temperature and other parameters inherent to submarine cables, fiber optic sensing tools play a critical role. In addition, service performance monitoring is key to pro-actively identifying data issues (such as latency, throughput, jitter, and frame loss/bit error rate) and is usually required in order to prove SLA compliance for customers.
Submarine Network Architecture
The overall architecture of a submarine cable system is probably best defined by the individual elements it is composed of from end to end. Each of these components plays an essential role in maintaining the integrity of data through thousands of miles of unpredictable ocean.
Design choices made for these networks typically relate to whether repeaters will be used, and if so, how many. Another design consideration is whether or not the cable runs will branch in multiple directions or simply travel point to point. Unrepeated cable systems have the inherent drawback of limited range, with 400km being the approximate limit. On the other hand, these unrepeated systems can be cheaper to deploy and maintain due to their lower complexity as there are no repeaters required and hence no powering.
Submarine Cable Network Testing – What to Measure and Certify
The complexity of these networks makes it extremely important to test and certify all segments of the wet plant, dry plant, and landing station including the interfaces between these elements. For the wet plant, OTDR testing is vital to measure attenuation across the entire length of the cable.
For repeated systems, this includes measurement through the in-line amplifiers. For non-repeated systems, long-range OTDR may be required. Other testing required to certify the wet plant includes chromatic dispersion (CD) and polarization mode dispersion (PMD).
The dry plant must also be tested and certified, including verification of error-free data transmission to the wet plant. The integrity of SLTE patching and the manhole splice should be verified. Optical Spectrum Analysis (OSA) through the dry plant connection is another critical test point. The data networking elements of the landing station must also be vetted, along with the fiber backhaul. As with a purely land-based network, latency, throughput, jitter, and frame loss are potential test metrics to assess performance. The time associated with protection switch service disruptions should also be evaluated.