About fiber-optic submarine cable networks

The first international undersea telegraph cable was installed in Europe in 1850 to connect England and France, whereas the first trans-Atlantic telephone cable was installed in 1956, and it had the capacity to transmit 36 analog phone channels simultaneously.

After 30 years of underwater coaxial lines, the eighties brought with them radical changes, namely, digital transmission and optical fiber, which made it possible to transmit digitally over a robust bandwidth characterized by high quality and reliability.

The nineties introduced two breakthroughs for fiber-optic submarine connections, the optical amplifier and the dense wavelength-division multiplexing (DWDM), which made it possible to amplify light and send through the same optical fiber various high-speed signals in different “colors” (wavelengths) of light simultaneously.

Optical amplification and adequate management of the degrading effects of the optical fiber that deformed the pulses of information made it possible to eliminate the need for electronic regenerators, improving reliability and permitting changes in transmission speeds, without changing the characteristics of high-cost underwater repeaters.

The introduction of new technologies made it possible to lower costs dramatically, drawing the interest of increasingly more players in participating in new projects, because in the mid-nineties, there were very rapid investment returns. Voice circuit cost per year declined from US$40,000 in TAT-1 in 1956 to US$20 in TAT-12 in 1995.

These last breakthroughs led to explosive growth in capacity, more than meeting the extraordinary rise in data transmission due to the advent of Internet and introducing a positive synergy between supply and demand. For example, the capacity offered between 1988 and 1998 rose 64-fold.

At present, one million kilometers of underwater cables have been installed in the world, enough to go around the planet 30 times, forming a network of fiber-optic connections carrying large volumes of traffic between all continents.

The real feasibility of trans-oceanic systems, operating at 10 Gbit/s with 32 wavelengths, has been demonstrated. This means there is a capacity of 320 Gbit/s per fiber, which is enough to transmit more than 15 million voice circuits simultaneously on a typical underwater cable comprised of four pairs of optical fibers.

The boom in the international market of underwater cable networks took place in the decade from 1990 to 2000 and included considerable activity across the Pacific Ocean. The reason for the tremendous and urgent need for increasing trans-Pacific connectivity was the 100% growth rates of Internet traffic during those years, far higher than the 10% growth rate for telephony. Coupled with this increase, there was also a rise in the annual traffic of Intranet owing to globalization of business, including video-conferencing, real-time data transmission, and multimedia applications (video images, colored graphic images and high-fidelity sound).

One example of growth in business volume is the US$950-million China-USA Cable Network project, consisting of a 30,000-kilometer self-protecting ring, connecting China, Japan, and Korea to the United States, with 9 landing points and a branching to Guam and which could reach a global capacity of about 1.5 Terabits/s (1.5 trillion binary digits per second).

The excessive optimism about Internet growth and the integration of many new players in the projects inevitably led to an abrupt slowdown in the market of fiber-optic submarine cable systems after several years of overinvestment (US$28 billion up to 2002 worldwide), so that the only feasible short-term investments mainly involve upgrades of systems that have already been installed.

What is a fiber-optic submarine cable network?

A fiber-optic submarine cable network is comprised of connections made with fiber-optic cables that form rings permitting interconnections between cities within a continent and with other cities located on other continents. Normally, global connectivity in the world is obtained by interconnecting smaller-scale rings.

A fiber-optic submarine connection is comprised of two major parts: the dry terrestrial plant and the wet or undersea plant.

The wet plant is comprised of elements found undersea and which consist mainly of the cable that carries light signals of information from one station to another, the repeaters that make it possible to amplify the light signal as it deteriorates and the branching units that make it possible to integrate secondary stations to the main trunk route without jeopardizing the system’s reliability.

On the dry network can be found the components that make it possible to transmit, receive, and control the communications that are sent through underwater connection segments. These components are the terminal line equipment to transmit and receive information, the power generation equipment to feed the repeaters with electrical energy, the overland cable to connect the land station with the underwater cable, and the overland cable that makes it possible to close the electrical circuit through the sea.

To install submarine cables under the sea, a topographical survey of the sea floor has to be made so as to select the most appropriate route for the cable and avoid ocean trenches, mountains and other obstacles in the environment where it will be installed, for example, fishing/trawling activities, anchoring sites, and fish attacks. In those areas where this might be necessary, the cable can be buried and/or fastened to avoid any movements that might undermine its physical safety.

How can high reliability of fiber-optic submarine cable networks be secured?

Since there is now far-reaching world connectivity through underwater cables forming rings, fiber-optic submarine cable networks normally have protection in the case of equipment failure or a cable break. Nevertheless, because of the huge amount of information that is carried, it is vital to re-establish the damaged equipment or cable, because as long as the fault has not been repaired, the system will be without any backup, which would produce a crisis of major proportions if a new failure occurs in another place of the ring.

To this end, the underwater cable stations are connected using a “working” ring and a “backup” ring. Under normal conditions, when there is no failure, priority traffic is carried by the working ring, whereas the protection ring carries low-priority traffic. In the case of failure, there will be segment switching (only between two stations and for equipment failure) or ring switching, which involves total loop reconfiguration in the shape of a “banana” (when there is a cable break). When there is switching using the protection ring, non-priority traffic is lost.

Ring technology also takes advantage of what was explained above as it is possible to send via the same optical fiber various high-speed signals in different “colors” of light simultaneously. Therefore if 16 colors are used and 4 pairs of fibers are available, it would be possible to build a total of 64 independent rings. (Afterwards this amount could be expanded, adding new “colors,” that is new wavelengths.)

Thanks to the number of available rings, the system’s flexibility and reliability can grow enormously, facilitating preventive and corrective maintenance.

In fiber-optic submarine cable networks, normally there is a remote center of operations that is in charge of supervising the network 24 hours a day and 365 days a year. Any corrective maintenance must have authorization and instructions from the network operation center so that nothing is ever done without adequate records or supervision.

Service communications (maintenance) between computers of different underwater cable stations are conducted through data transmission channels by means of routers using the TCP-IP protocol. These data channels are sent within the same high-speed signal emitted from the respective line terminal.

It is interesting to emphasize that, for purposes of preventive and corrective maintenance, it is necessary to undertake measurements and send orders to the repeaters under the sea. This is done by slightly varying (overmodulating) the light signal in keeping with the low-speed data signal, containing the messages sent to or received from the repeaters.

For the fast localization of the site of a cable break, an instrument (coherent optical reflectometer) is used to send light pulses and measures the time of return of these pulses when they are reflect off the break, showing errors that can be as low as 0.1% and at a range of up to 15,000 kilometers.

In some cases, there are electrical failures of the insulation on the underwater cable and the above-mentioned optical reflectometry methods cannot be used; in that case, there are electrical localization methods, but since they depend on many variable and particular parameters of each connection, they can show errors as large as 1%.

Importance of fiber-optic submarine cable networks

Fiber-optic submarine cable networks are probably most important for reducing the cost of long-distance communications, mainly as a result of the following:

• The extraordinary capacity of the technology, which lowers costs per circuit.

• The high supply of capacity at present owing to huge amounts of investment that were made.

• Stiff competition between the companies using the submarine cable networks.

The global importance of fiber-optic submarine cable networks stems from the possibility of creating, along with the fiber-optic overland networks, a powerful backbone that routes telecommunication signals throughout the planet without producing any bottlenecks or degradation in transmission quality (as would occur previously with the point-to-point communications via satellite). Regarding this, it should be emphasized that:

• All long-distance intercontinental signals and in many cases regional signals and whatever their origin might be (fixed or cell telephone, Internet data, private data, etc.) inevitably flow into the backbone composed of fiber-optic submarine and terrestrial networks.

• In view of the ring-based self-healing structure of fiber-optic submarine cable networks, there will always be a route through which information can reach its destination with high quality.

Fiber-optic submarine cable networks work silently and efficiently, ensuring very high and immediate availability, protected by a medium that shows no changes that might undermine their integrity. It is the responsibility of telecommunication administrations to understand and tap fully their huge technical and economic potential.

Miguel Angel De La Sotta Cerbino
Engineer MS