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):
