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 submarine 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.

Submarine cables vary in size depending on the number of fibers in the central core, the marine environment (sand or rock) and 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 submarine 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 network architecture, planned usage, deployment depth and environment, etc.

History of Submarine Cables

In 1850, the first undersea telegraph cable was installed between England and France. Since modern insulating materials did not yet exist, substances like India rubber and Gutta-percha were used to protect the copper wires, often unsuccessfully. Most early submarine cable networks were owned by the British, and the available science of that era severely limited bandwidth.

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.

Wet Plant
The wet plant is a term used to describe all the submarine cable network elements that reside underwater. The boundaries for this wet plant are the two beach manholes where the cable either enters or exits the water. Discrete parts of the wet plant include the cable, equalizers, branching units, and submarine repeaters.

Dry Plant 
Conversely, the dry plant, as it is known, consists of the submarine cable network segment on land between the beach manhole and the cable landing station. This includes power feeding equipment (PFE), submarine line terminals and land cable segments. 

Landing Station
The submarine cable system is connected to the terrestrial network at the submarine cable landing station, with the backhaul system providing connection to the dry plant. The landing station is where the head end and data center are found. These landing sites are usually located in areas with low levels of marine traffic, mild currents, and gently sloping sea floors so that the wet plant cables can be more easily buried/landed for linking to the dry plant.

Umbilical Cables
As the name suggests, umbilical cables provide a lifeline between the sea floor and a land-based position. This type of cable may be used to supply power to an undersea cable network from a ground position or floating position along the cable run.

Lengths/Distances
Over one million kilometers (km) of submarine cable currently transverse the world’s oceans. A cable run in the 100 km range is considered relatively short, since some transoceanic cables can be up to 20,000 km in length.

Repeaters/Amplifiers
The range of optical signals is limited due to attenuation. One way to overcome this is through the use of repeaters at discrete intervals over the cable run. For submarine cable networks, the repeaters are powered in series by the power feeding equipment (PFE) of the dry plant. To maintain the correct optical power levels and ensure adequate Optical Signal-to-Noise Ratio (OSNR) for data signals an electrically powered optical amplifiers can be used to repeat those signals. Most of the amplifiers utilized in submarine links are Erbium Doped Fiber Amplifiers (EDFA) which have inherently low noise and high (> 15 dBm) output power.

Design choices made for submarine cable 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.

Why Submarine Cable Networks Are Important

With so much worldwide data now traveling through submarine cable networks, their importance to global communication, commerce, and security cannot be overstated. This can be traced to the earliest history of these networks when British domination of the submarine cable industry engendered military, commercial, and political advantages.

Suppose your customer-facing website is slow because of poor network performance. Potential customers lose patience with the site and decide not to buy your products or services. Or, perhaps you’re looking to move into a different market. If your network can’t handle the demands, such a shift would place on it, your efforts will be in vain.

Submarine cable ownership today by individual governments has given way to consortiums of multiple telecom carriers who build, monitor, and maintain these huge networks. In recent years, many Internet Content Providers (ICP) and data center owner/operators have also entered the fray of submarine cable ownership to ensure their own interests in this valued commodity.

The range of uses for submarine cable knows no boundaries. With international internet and phone communication relying so heavily on these networks, almost any business, government, or individual who owns a computer is a daily user of submarine cable. To meet this growing demand, submarine internet cable bandwidth continues to increase. For example, the 6600 km MAREA cable from Virginia to Spain is capable of carrying 160 terabits per second (Tbps) of data.

Submarine Cable Networks Testing – What to Measure and Certify

The complexity of a submarine cable network 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 submarine 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 a 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.

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 life cycle of a submarine cable network, it is critical to use the right test and monitoring tools.

             Wet plant OTDR                                       Dry plant OTDR

Construction
During construction, end face inspection can detect dirt particles or other physical defects whose impact can either diminish or halt network performance (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 submarine cable networks in order to base line optical dispersion and assess if any dispersion compensation is required.

Activation
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.

                          ONMSi

       Service performance monitoring

Monitoring & Maintenance
Monitoring and maintenance are the keys to keeping the vital submarine cable networks performing optimally. To assess the fibers 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 submarine cable 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.