It is very likely that you have heard of submarine cables. They are used in stretches of sea to connect seasons terrestrial and thus transmit signals from telecommunications over long distances. For this, they are installed on the ocean floor.

These cables receive additional mechanical protection to be installed under water: they usually have a steel interior and special insulation. They can be metallic, coaxial or optical - the most used today.


It is common for submarine cables to be used in international telecommunications networks to interconnect countries and continents. In Brazil, the system is used to interconnect the entire national coast.

The first submarine cables were installed more than 160 years ago: it was in the 1850s, shortly after the invention of wireless telegraphy (created by Samuel Morse, in 1843). Initially, then, this network was used to telephony and then for data transmission.

First installations

The first telegraph line was established in 1844 between Baltimore and Washington, DC, in the United States. The funding came from the American Congress and the first official transmission was: “What hath God wrought!”(What a work God has done!).

There are disagreements as to the dates, but the first documented underwater telegraph cable was launched in 1851 on the Dover channel. Soon after, the idea of ​​creating a network that crossed the Atlantic and allowed the tech be used to interconnect different continents.

The first transnational system came through the hands of the Atlantic Telegraph Company, in 1857, to link North America and Europe. For this purpose, two ships, a British and an American, carried 2.500 nautical miles (4.630 km) of cable, from Ireland. Unfortunately, however, when about 750 km had already been covered, the cable broke.

In the following year, 1858, a new attempt was made, but after the launch of 250 km of cable, there was another break. Still in 1858, another test was carried out: this time, two ships left the middle of the Atlantic towards ports on opposite sides. The process was successful and the message “Glory to God in the highest, and on Earth, peace, good will to men” was sent.

As the activity was still in its infancy, the system was quite slow and its bandwidth allowed it to carry only two words per minute. In addition, a few weeks later, the cable failed due to the voltages used.

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Another eight years passed before, finally, reliable operations were established between Europe and North America - which occurred in 1866. After that, many other submarine metallic cables were installed, but were still used only for the transmission of telegraphic messages.

It took another 90 years before the invention of the submarine coaxial cable, in 1956. With it, communication between several individuals simultaneously was possible. Just over a decade later, in the 1970s, the optical cables - they have become the best option for subsea communication and are in use today.

Structure in Brazil

No Brazil, the first submarine cable was inaugurated in 1857. It was part of the first Brazilian telegraph line and connected Praia da Saúde, in Rio de Janeiro, to the city of Petrópolis. There were 15 km of submarine cable in a line whose total length was 50 km.

In 1874, the country's first fully submarine cable came; inaugurated by D. Pedro II, it connected Rio de Janeiro, Salvador, Recife and Belém. In the following year, the line to connect Recife, João Pessoa and Natal was created. Still in 1875, Irineu Evangelista de Souza, the Baron of Mauá, participated in the organization and financing of the installation of the first international submarine cable in the country; installed by the British Eastern Telegraph Company, it connected Brazil to Portugal.

Submarine cable structure in Brazil
Credit: Reproduction

In recent years, other submarine cables have been launched to connect Brazil to various parts of the world. Those presented in the figure above are the main ones.

In 2021, an undersea fiber optic cable was inaugurated to connect directly from Brazil to Europe – without having to go through the United States. The equipment is 6 kilometers long and connects Fortaleza, in Brazil, with Sines, in Portugal, and also has passages through French Guiana, Madeira Island, the Canary Islands and Cape Verde.

The era of optical cable

Current submarine cables are made of optical fiber and allow the transport of all types of digital information - that is, telephone, internet and other data. Generally, they are 69 mm in diameter and weigh about 10 kg per meter. For deep waters, however, thinner and lighter cables are used. All continents except the Antarctica, are connected by them.

The first such system was implemented in 1982 in the Canary Islands. It was only in 1988, however, that the era of submarine long-distance optical cable began: a network with mass transmission capacity was installed between the oceans Pacifico and Atlantic to connect the USA, France and England.

Also in 1988, came the first system designed to use the dense wavelength division multiplexing technique (Dense Wavelength Division Multiplexer - DWDM). As a result, the USA, Great Britain, Germany and the Netherlands were interconnected and the traffic capacity between North America and Europe was increased to 20 thousand voice circuits.

With this and other techniques, it was possible to expand the bands and reduce the costs of equipment, cables and installation services. As a result, hiring prices fell. In addition, protection mechanisms were created for both submarine cables and optical systems - which brought new paradigms of reliability and availability: the capacity already reaches the terabits and the wavelength reaches 60-90 lambdas.

At the end of the XNUMXth century and the beginning of the XNUMXst century, there was an effective increase in the supply of bandwidth thanks to the new submarine cable systems launched in the oceans Pacific, Atlantic, Southeast Asia and South America. On the American continent, there are three large capacity underwater optical networks: SAM1, South American Crossing and 360 Network.

Evolution has taken the time to transmission of signals, which was previously measured in minutes, drops to milliseconds with the use of optical fiber. The largest underwater fiber optic cable in the world is SEA-ME-WE 3: it is 38 thousand km long and interconnects 32 countries in Southeast Asia, the Middle East and Europe.

How does the submarine cable work?

In addition to the high transmission capacity of submarine cables, they reach great distances: they can reach 9 thousand km without the need for regeneration of signal. In systems with optical fiber of third generation (of 1300nm) the spacing between the repeaters can be of up to 60 km whereas in the systems of fourth generation (1550nm), they can reach up to 100 km.

The devices used in subsea systems are designed to withstand water pressure up to 8 m in depth (that is, 800 atmospheres). They are highly reliable structures, whose useful life normally reaches 25 years.

It is common for submarine cable networks to be built in a ring. Thus, they can surround continents, countries or islands in order to offer connectivity over the entire extension of the locality and guarantee redundancy to protect and self-restore network traffic in the event of failure.

As the ring is bidirectional, data travel in any direction and can depart or reach any point. When a failure is detected, the system automatically redirects traffic - that is, the repair is instantaneous.

Scheme with installation with earth station and submarine cables
Credit: Reproduction

Earth station

It is at the Earth Station that the equipment that regenerates the optical signal and demultiplexes the signals (which separates them into channels and later distributes them to end users) remains. On the continent, the submarine cable arrives and leaves earth stations.

In general, these stations are located at points that are far from service-consuming centers. Then, to allow efficient distribution, points of presence (POPs) are installed. Both the earth station and the POPs use energy and security with 100% redundancy.

To manage traffic, monitor signals, identify problems and maintain the system (24 hours a day, 7 days a week), there are system management centers (Network Operation Centers - NOC). They are usually built on an earth station or on a POP.

Dense wavelength division multiplexing

Current subsea systems can transmit several independent optical signals, each with its own wavelength. In order for them to be combined into a single fiber, dense wavelength division multiplexing is used. The wavelengths currently used in subsea cables have transmission speeds 2,5Gbps and 10Gbps.

The equipment that does this process is located at earth stations. Typically, they have the ability to allow for a gradual increase (from one to several wavelengths) when there is a need to increase capacity.

Digital synchronous hierarchy (Synchronous Digital Hierarchy - SDH)

SDH is the equipment that performs multiplexing and protects optical networks. As the systems are standardized according to international standards, they can be easily interconnected with other submarine, terrestrial and satellite networks.

Optical amplifiers

It is natural that the signal subsea cables suffer losses due to the distance traveled. To alleviate this occurrence, optical amplifiers are used. These devices are connected to the cable at predefined intervals and return the optical pulses to their original amplitude - this is not about signal regeneration, which is done at the earth station.

This equipment is designed to carry fiber capacity for thousands of kilometers between earth stations. To work, the amplifiers need 4 thousand V of energy, which is supplied remotely by earth stations.

How is a project for the installation of cables?

The deployment of an underwater cable is a complex project. Three aspects are fundamental in this process:

  • the supplier of the underwater optical cable and other equipment;
  • the company specialized in launching the cable into the ocean;
  • a operator telecommunications

The cable supplier is responsible for making the optical cables and the transmission equipment. The telecommunications operator, in turn, is the one who orders the subsea optical network system.

The company that launches the cable makes a detailed study of the characteristics of the ocean bed (depth zones, topographic and geological profile, and physical and chemical characteristics). With this information, it traces the safest route to the system. Then, she may be responsible for maintaining the network.

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