page_title: Submarine communications cable
section_title: Optical telecommunications cables
hierarchical_section_title: Submarine communications cable / Modern history / Optical telecommunications cables
context_page_description: A submarine communications cable is a cable laid on the sea bed between land-based stations to carry telecommunication signals across stretches of ocean and sea. The first submarine communications cables laid beginning in the 1850s carried telegraphy traffic, establishing the first instant telecommunications links between continents, such as the first transatlantic telegraph cable which became operational on 16 August 1858. Subsequent generations of cables carried telephone traffic, then data communications traffic. Modern cables use optical fibre technology to carry digital data, which includes telephone, Internet and private data traffic.
  Modern cables are typically about 25 millimetres in diameter and weigh around 1.4 tonnes per kilometre for the deep-sea sections which comprise the majority of the run, although larger and heavier cables are used for shallow-water sections near shore. Submarine cables first connected all the world's continents when Java was connected to Darwin, Northern Territory, Australia in 1871 in anticipation of the completion of the Australian Overland Telegraph Line in 1872 connecting to Adelaide, South Australia, and thence to the rest of Australia.
context_section_description: In the 1980s, fibre-optic cables were developed. The first transatlantic telephone cable to use optical fibre was TAT-8, which went into operation in 1988. A fibre-optic cable comprises multiple pairs of fibres. Each pair has one fibre in each direction. TAT-8 had two operational pairs and one backup pair.
  Modern optical fibre repeaters use a solid-state optical amplifier, usually an Erbium-doped fibre amplifier. Each repeater contains separate equipment for each fibre. These comprise signal reforming, error measurement and controls. A solid-state laser dispatches the signal into the next length of fibre. The solid-state laser excites a short length of doped fibre that itself acts as a laser amplifier. As the light passes through the fibre, it is amplified. This system also permits wavelength-division multiplexing, which dramatically increases the capacity of the fibre.
  Repeaters are powered by a constant direct current passed down the conductor near the centre of the cable, so all repeaters in a cable are in series. Power feed equipment is installed at the terminal stations. Typically both ends share the current generation with one end providing a positive voltage and the other a negative voltage. A virtual earth point exists roughly halfway along the cable under normal operation. The amplifiers or repeaters derive their power from the potential difference across them. The voltage passed down the cable is often anywhere from 3000 to 15,000VDC at a current of up to 1,100mA, with the current increasing with decreasing voltage; the current at 10,000VDC is up to 1,650mA. Hence the total amount of power sent into the cable is often up to 16.5kW.
  The optic fibre used in undersea cables is chosen for its exceptional clarity, permitting runs of more than 100 kilometres (62 mi) between repeaters to minimize the number of amplifiers and the distortion they cause. Unrepeated cables are cheaper than repeated cables however their maximum transmission distance is limited, however their maximum transmission distance has increased over the years; in 2014 unrepeated cables of up to 380km in length were in service; however these require unpowered repeaters to be positioned every 100km.
  The rising demand for these fibre-optic cables outpaced the capacity of providers such as AT&T. Having to shift traffic to satellites resulted in lower-quality signals. To address this issue, AT&T had to improve its cable-laying abilities. It invested $100 million in producing two specialized fibre-optic cable laying vessels. These included laboratories in the ships for splicing cable and testing its electrical properties. Such field monitoring is important because the glass of fibre-optic cable is less malleable than the copper cable that had been formerly used. The ships are equipped with thrusters that increase manoeuvrability. This capability is important because fibre-optic cable must be laid straight from the stern, which was another factor that copper-cable-laying ships did not have to contend with.
  Originally, submarine cables were simple point-to-point connections. With the development of submarine branching units (SBUs), more than one destination could be served by a single cable system. Modern cable systems now usually have their fibres arranged in a self-healing ring to increase their redundancy, with the submarine sections following different paths on the ocean floor. One reason for this development was that the capacity of cable systems had become so large that it was not possible to completely backup a cable system with satellite capacity, so it became necessary to provide sufficient terrestrial backup capability. Not all telecommunications organizations wish to take advantage of this capability, so modern cable systems may have dual landing points in some countries (where back-up capability is required) and only single landing points in other countries where back-up capability is either not required, the capacity to the country is small enough to be backed up by other means, or having backup is regarded as too expensive.
  A further redundant
caption: Diagram of an optical submarine cable repeater