Underwater optical cables

Since the 1990s, data transport on underwater optical cables has accounted for almost all intercontinental traffic. Due to their performance in terms of throughput and quality of service, these infrastructures have largely replaced satellites and wireless networks, which are now used to serve isolated areas. By February 2019, there were some 380 cables in service, representing about 1.2 million kilometres of pipes, and some 60 others under design or construction.

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The development of underwater optical cables

Optical technologies of submarine cables consist in sending information in the form of light pulses along a fibre, which gives access to higher data rates than analogue technologies.

The first transatlantic optical cable (TAT 8) will be put into service in 1988 between the United States, France and Great Britain. Between 1988 (TAT 8) and 2002 (APPOLO), the capacity of similar submarine cables will be multiplied by a factor of 5,000 (and by a factor of 40,000 over a wider reference period, from 1988 to 2009).

During the 20th century, the capabilities of optical cables, coupled with their rapid evolution, will lead to the premature abandonment of all analogue cables. Transmission satellites, until now used in addition to analogue cables, will also be outdated, before being reserved for certain sectors (television, telephone services in sparsely populated areas, etc.).

Intercontinental wireless networks will also gradually be marginalized. Thanks to the use of optical technologies, submarine cables quickly accounted for 99% of intercontinental data exchanges.

Underwater optical cables have exponential transport capacities

An underwater optical cable consists of a protective sheath (1 and 2 in the diagram below), a metal reinforcement (3, 4, 5 and 6), an insulating sheath (7) and pairs of optical fibres (8).

Cutting of an underwater cable optical fiber and description of the data path

Tactis - Fibre optique cables sous-marin

  • Each pair of optical fibers is activated by multiplexers. A multiplexer is a device that cuts and encodes each incoming data as light rays, injected into the fiber at distinct wavelengths (up to 160 wavelengths or "colors" in the early 2000s. The optical fiber pair then transmits these wavelengths to a demultiplexer. This device recovers the signal and translates it into data that can be used by the ground segment.The wavelengths of light are transmitted through a window, i.e. via a frequency interval (also known as a "bandwidth"). Any bandwidth has several characteristics:
    • Signal attenuation: measures the loss of signal for each km travelled.
    • Bandwidth: measured in nm, it is proportional to the capacity available on a transmission system, measured in bit/s.
    • The rate: represents the capacities transmitted on each color (2.5 to 400 Gbit/s).

    By 2014, most long-distance networks use 1,550 nm wide windows, which limits loss while ensuring significant transmission capacity.

    The capacity of a cable is used to estimate the amount of data it can transmit. It is traditionally measured in flow rates:

    • E1 : 2Mbit/s
    • DS3: 45 Mbit/s
    • STM1: 155 Mbit/s
    • STM4: 622 Mbit/s
    • STM16: 2,500 Mbit/s
    • STM64: 10,000 Mbit/s


    The efficiency of submarine cable transmissions

    The efficiency of the transmissions depends on the number of colours passing through each optical fibre. This number varies according to the type of multiplexer used: SDH (Synchronous Digital Hierarchy) multiplexers allow time multiplexing of waves; WDM (Wavelenght Division Multiplexing) multiplexers allow wave frequencies to be multiplexed. In practice, the complementary use of these technologies significantly improves the capabilities of optical fibres:

    • The classic WDM-SDH multiplex 4 to 16 wavelengths, for 2.5 to 10 Gbit/s per color. For example, the SEA-ME-WE3 submarine optical cable, which connects Europe, the Middle East, South Asia and Australia, uses 8 wavelengths.
    • CWDM (Coarse WDM) is a technology similar to WDM, allowing control of operating costs.
    • DWDM (Dense WDM) and UDWDM (Ultra-Dense WDM) allow multiplexing of a large number of wavelengths, for 10 to 40 Gbit/s per color. For example, the Tangerine cable, between France and England, uses 96 wavelengths.
    • High Capacity WDM multiplexes up to 64 wavelengths, at a maximum capacity of 400 Gbit/s per color. E.g.: the experimental Paris-Lyon link, which mobilizes 44 wavelengths for an activable capacity of 17.6 Tbit/s.

    It is theoretically possible to transport large capacities, measured in Tbit/s, on a pair of optical fibres. However, large intercontinental submarine cables have available capacities ranging from a few tens to a few thousand Gbit/s. This difference between practice and theory finds several technical and/or economic explanations:

    • The technical obsolescence of some submarine cables (the costs of modernization are significant, whereas each infrastructure has a lifespan of only 25 to 30 years).
    • The lack of sufficiently attractive markets or local competition (managers are not encouraged to use the full potential of their infrastructure and/or the new technologies at their disposal).
    • The attenuation of the signal over very long distances.


    Operators: better security of lightpaths

    An underwater optical cable alone rarely ensures the routing of traffic from an overseas territory to the exchange nodes of the global Internet.

    It is therefore necessary for operators to interconnect their traffic with other submarine cables; these interconnections form lightpaths to delivery and exchange nodes (GIXs).

    These interconnections and the creation of these lightpaths are obtained either through direct purchase or by exchanging capacity from one submarine cable to another, and make it possible to define redundant and secure traffic routing routes. If only one submarine cable lands on a territory (as in the case of Mayotte or French Guiana in 2014), this redundancy cannot be implemented on the entire lightpath.

    The deployment of an underwater optical cable requires significant resources

    The installation, maintenance and operation of submarine optical cables require a significant mobilization of technical, human and financial resources.

    The installation of an underwater optical cable involves two complex operations:

    • The preparation of the installation of the cable, which requires:
      - Several thousand kilometres of cables,
      - A hydrographic campaign on all the planned routes,
      - A study of the bottoms in this same area,
      - The choice of the final routes.
    • Deploying the cable requires a particularly long and careful work of ensouillage or anchoring.


    The economic model of underwater optical cables

    The financing, establishment and operation of submarine optical cables are complex and risky operations:

    • An underwater optical cable represents a significant investment, as deployment costs are higher than terrestrial long-distance optical networks, particularly because of the technical nature of the operations (seabed studies, ensouillage operations, extraterritoriality management for the establishment of landing stations, etc.).
    • The maintenance of underwater optical cables requires significant technical, material and human resources. The main constraint is the permanent on-call duty of specialised cable ships, able to respond to the first damage and repair the infrastructure.
    • From a commercial point of view, the submarine cable manager is exposed to the risk of competition from its own customers (particularly in the case where they have acquired dark fibres or high-capacity circuits).


    Three modes of providing the capabilities of an underwater optical cable

    The marketing of a submarine cable can be carried out according to different alternative sharing methods:

    • The provision of dark fibre: this type of sharing is technically only possible on cables without repeaters. The buyer must then ensure the activation of the fibres for his own needs.
    • The provision of bandwidth or wavelength circuits, for data rates ranging from 2 Mbit/s to 10 Gbit/s. These circuits correspond to the traditional ways of sharing capacity between members of submarine cable consortia.
    • The provision of IP transit circuits: in a marginal, limited and punctual way, local operators can rent connectivity services, integrating the addressing of IP packets.


    Tactical expertise in underwater optical cables

    Tactis has more than 20 years of recognised expertise in this field, both with telecommunications operators, submarine cable consortia, government authorities, regulators, investors and major financial backers. Tactis first worked on projects in Europe, the Indian Ocean, then in the Mediterranean, in the Caribbean, North and South America and in recent years on projects specific to the African continent. In total, our teams have been involved in more than twenty projects in recent years.

    For submarine cable projects, Tactis conducts market studies, studies the technical and economic feasibility, assists in defining the technical specifications of a project, seeking financing, setting up the project, drafting technical specifications and negotiating with manufacturers, monitoring deployment and commissioning, audit/expertise following operational incidents (particularly interruptions) and monitoring return to service.

    Submarine cables (list of acronyms):

  • SAFE (South Africa Far East).
  • LION 1 (Lower Indian Ocean Network
  • LION 2 (Lower Indian Ocean Network 2).
  • Americas II.
  • GCN (Global Caribbean Network).
  • MCN (Middle Caribbean Network).
  • SSCS (Saba Statia Cable System).
  • SMPR-1 (Saint-Marteen Puerto-Rico Network 1).
  • ECFS (East Caribbean Fiber System)


Tactis - Directeur associé - Benjamin Fradelle- Aménagement numérique des territoires

Benjamin Fradelle

Associate Director
Since 2002, Benjamin Fradelle has been developing expertise in digital spatial planning, both in terms of defining the strategy of local authorities and in the technical and economic approaches associated with public initiative networks.

Contact Benjamin Fradelle

A question about underwater cables? Need to remove doubts in view of a future project? Send an email to Benjamin Fradelle, Tactis Associate Director.

Tactis - Directeur associé - Benjamin Fradelle- Aménagement numérique des territoires

Benjamin Fradelle

Associate Director Tactis
Since 2002, Benjamin Fradelle has been developing expertise in digital spatial planning both in terms of defining the strategy of local authorities and in the technical and economic approaches associated with public initiative networks.

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