Editing Submarine communications cable (section)

===Optical telecommunications cables===
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[[File:Submarine cable map umap.png|thumb|upright=1.35|World map showing submarine cables in 2015]]

In the 1980s, [[optical fiber|fiber-optic cables]] were developed. The first transatlantic telephone cable to use optical fiber was [[TAT-8]], which went into operation in 1988. A fiber-optic cable comprises multiple pairs of fibers. Each pair has one fiber in each direction. TAT-8 had two operational pairs and one backup pair. Except for very short lines, fiber-optic submarine cables include repeaters at regular intervals.

Modern optical fiber repeaters use a solid-state [[optical amplifier]], usually an [[Optical amplifier#Doped fiber amplifiers|erbium-doped fiber amplifier]] (EDFA). Each repeater contains separate equipment for each fiber. These comprise signal reforming, error measurement and controls. A solid-state laser dispatches the signal into the next length of fiber. The solid-state laser excites a short length of doped fiber that itself acts as a laser amplifier. As the light passes through the fiber, it is amplified. This system also permits [[wavelength-division multiplexing]], which dramatically increases the capacity of the fiber. EDFA amplifiers were first used in submarine cables in 1995.<ref>{{cite book | url=https://books.google.com/books?id=rutYPgFCwaUC&dq=submarine+wet+plant&pg=PA163 | title=Core Networks and Network Management | isbn=978-90-5199-497-1 | last1=Faulkner | first1=D. W. | last2=Harmer | first2=Alan | date=May 10, 1999 | publisher=IOS Press }}</ref>

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 (PFE) 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 ground|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 16.5&nbsp;kW.<ref>{{cite web|url=https://www.networkworld.com/article/2235353/the-incredible-international-submarine-cable-systems.html|title=The Incredible International Submarine Cable Systems|first=Michael|last=Morris|date=April 19, 2009|website=Network World}}</ref><ref>{{cite conference|url= https://www.suboptic.org/wp-content/uploads/2014/10/255_Poster_EC_04.pdf|title= Very Compact and High Voltage Power Feeding Equipment (PFE) for Advanced Submarine Cable Network|conference= SubOptic|date= 2010|first1= Tomoyuki|last1= Kaneko|first2= Yoshinori|last2= Chiba|first3= Kaneaki|last3= Kunimi|first4= Tomotaka|last4= Nakamura|access-date= 2020-08-08|archive-date= 2020-08-08|archive-url= https://web.archive.org/web/20200808071549/https://www.suboptic.org/wp-content/uploads/2014/10/255_Poster_EC_04.pdf|url-status= dead}}</ref>

The optic fiber used in undersea cables is chosen for its exceptional clarity, permitting runs of more than {{Convert|100|km|}} between repeaters to minimize the number of amplifiers and the distortion they cause. Unrepeated cables are cheaper than repeated cables, and their maximum transmission distance is limited. This transmission distance has increased over the years; in 2014 unrepeated cables of up to {{Convert|380|km|}} in length were in service; however these require unpowered repeaters to be positioned every 100&nbsp;km.<ref name = art>{{cite conference|url= https://www.suboptic.org/wp-content/uploads/2014/10/We2.19.pdf|title= Unrepeatered Systems: State of the Art Capability|first1= Nicolas|last1= Tranvouez|first2= Eric|last2= Brandon|first3= Marc|last3= Fullenbaum|first4= Philippe|last4= Bousselet|first5= Isabelle|last5= Brylski|conference= |access-date= 2020-08-08|archive-date= 2020-08-08|archive-url= https://web.archive.org/web/20200808070142/https://www.suboptic.org/wp-content/uploads/2014/10/We2.19.pdf|url-status= dead}}</ref>

[[File:Submarine cable repeater SVG.svg|thumb|upright=1.35|left|Diagram of an optical submarine cable repeater]]

The rising demand for these fiber-optic cables outpaced the capacity of providers such as AT&T.{{when|date=June 2014}} 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&nbsp;million in producing two specialized fiber-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 fiber-optic cable is less malleable than the copper cable that had been formerly used. The ships are equipped with [[Bow thruster|thrusters]] that increase maneuverability. This capability is important because fiber-optic cable must be laid straight from the stern, which was another factor that copper-cable-laying ships did not have to contend with.<ref name="keith">{{cite news |last1=Bradsher |first1=Keith |date=15 August 1990 |title=New Fiber-Optic Cable Will Expand Calls Abroad, and Defy Sharks |work=[[The New York Times]] |url=https://www.nytimes.com/1990/08/15/business/business-technology-new-fiber-optic-cable-will-expand-calls-abroad-defy-sharks.html |access-date=14 January 2020}}</ref>

Originally, submarine cables were simple point-to-point connections. With the development of [[submarine branching unit]]s (SBUs), more than one destination could be served by a single cable system. Modern cable systems now{{when|date=June 2014}} usually have their fibers arranged in a [[self-healing ring]] to increase their redundancy, with the submarine sections following different paths on the [[seabed|ocean floor]]. One reason for this development was{{when|date=June 2014}} that the capacity of cable systems had become so large that it was not possible to completely back up 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 [[Cable landing point|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.{{citation needed|date=December 2024}}

A further redundant-path development over and above the self-healing rings approach is the [[Mesh networking|mesh network]] whereby fast switching equipment is used to transfer services between network paths with little to no effect on higher-level protocols if a path becomes inoperable. As more paths become available to use between two points, it is less likely that one or two simultaneous failures will prevent end-to-end service.{{citation needed|date=December 2024}}

As of 2012, operators had "successfully demonstrated long-term, error-free transmission at 100&nbsp;Gbps across Atlantic Ocean" routes of up to {{convert|6000|km|-2|abbr=on}},<ref>{{cite web |url=http://www.submarinenetworks.com/systems/trans-atlantic/hibernia-atlantic/hibernia-atlantic-trials-100g-transatlantic |title=Submarine Cable Networks&nbsp;– Hibernia Atlantic Trials the First 100G Transatlantic |publisher=Submarinenetworks.com |access-date=2012-08-15 |url-status=dead |archive-url=https://web.archive.org/web/20120622130600/http://submarinenetworks.com/systems/trans-atlantic/hibernia-atlantic/hibernia-atlantic-trials-100g-transatlantic |archive-date=2012-06-22 }}</ref> meaning a typical cable can move tens of [[terabits]] per second overseas. Speeds improved rapidly in the previous few years, with 40&nbsp;Gbit/s having been offered on that route only three years earlier in August 2009.<ref>{{cite web |url= http://www.lightreading.com/document.asp?doc_id=180473 |title=Light Reading Europe&nbsp;– Optical Networking&nbsp;– Hibernia Offers Cross-Atlantic 40G&nbsp;– Telecom News Wire |publisher=Lightreading.com |access-date=2012-08-15 |url-status=live |archive-url= https://web.archive.org/web/20120729194852/http://www.lightreading.com/document.asp?doc_id=180473 |archive-date=2012-07-29 }}</ref>

Switching and all-by-sea routing commonly increases the distance and thus the round trip latency by more than 50%. For example, the round trip delay (RTD) or latency of the fastest transatlantic connections is under 60&nbsp;ms, close to the theoretical optimum for an all-sea route. While in theory, a [[great circle route]] (GCP) between London and New York City is only {{convert|5600|km|-2|abbr=on}},<ref>{{cite web |url=http://www.gcmap.com/mapui?P=NYC-LCY&DU=km |title=Great Circle Mapper |publisher=Gcmap.com |access-date=2012-08-15 |url-status=dead |archive-url=https://web.archive.org/web/20120725194235/http://www.gcmap.com/mapui?P=NYC-LCY&DU=km |archive-date=2012-07-25 }}</ref> this requires several land masses (Ireland, [[Newfoundland (island)|Newfoundland]], Prince Edward Island and the isthmus connecting [[New Brunswick]] to [[Nova Scotia]]) to be traversed, as well as the extremely tidal [[Bay of Fundy]] and a land route along [[Massachusetts]]' north shore from [[Gloucester, Massachusetts|Gloucester]] to [[Boston]] and through fairly built up areas to [[Manhattan]] itself. In theory, using this partial land route could result in round trip times below 40&nbsp;ms (which is the speed of light minimum time), and not counting switching. Along routes with less land in the way, round trip times can approach [[speed of light]] minimums in the long term.

The type of optical fiber used in unrepeated and very long cables is often PCSF (pure silica core) due to its low loss of 0.172dB per kilometer when carrying a 1550&nbsp;nm wavelength laser light. The large chromatic dispersion of PCSF means that its use requires transmission and receiving equipment designed with this in mind; this property can also be used to reduce interference when transmitting multiple channels through a single fiber using [[Wavelength-division multiplexing|wavelength division multiplexing]] (WDM), which allows for multiple optical carrier channels to be transmitted through a single fiber, each carrying its own information.<ref>{{Cite web|url=https://www.fujitsu.com/global/documents/about/resources/publications/fstj/archives/vol42-4/paper07.pdf|title=High-Performance Submarine Line Terminal Equipment for Next-Generation Optical Submarine Cable System: FLASHWAVE S650}}</ref> WDM is limited by the optical bandwidth of the amplifiers used to transmit data through the cable and by the spacing between the frequencies of the optical carriers; however this minimum spacing is also limited, with the minimum spacing often being 50&nbsp;GHz (0.4&nbsp;nm). The use of WDM can reduce the maximum length of the cable although this can be overcome by designing equipment with this in mind.{{citation needed|date=December 2024}}

Optical post amplifiers, used to increase the strength of the signal generated by the optical transmitter often use a diode-pumped erbium-doped fiber laser. The diode is often a high power 980 or 1480&nbsp;nm laser diode. This setup allows for an amplification of up to +24dBm in an affordable manner. Using an erbium-ytterbium doped fiber instead allows for a gain of +33dBm, however again the amount of power that can be fed into the fiber is limited. In single carrier configurations the dominating limitation is self phase modulation induced by the [[Kerr effect]] which limits the amplification to +18 dBm per fiber. In WDM configurations the limitation due to crossphase modulation becomes predominant instead. Optical pre-amplifiers are often used to negate the thermal noise of the receiver. Pumping the pre-amplifier with a 980&nbsp;nm laser leads to a noise of at most 3.5&nbsp;dB, with a noise of 5&nbsp;dB usually obtained with a 1480&nbsp;nm laser. The noise has to be filtered using optical filters.{{citation needed|date=December 2024}}

[[Raman amplification]] can be used to extend the reach or the capacity of an unrepeatered cable, by launching 2 frequencies into a single fiber; one carrying data signals at 1550&nbsp;nm, and the other pumping them at 1450&nbsp;nm. Launching a pump frequency (pump laser light) at a power of just one watt leads to an increase in reach of 45&nbsp;km or a 6-fold increase in capacity.{{citation needed|date=December 2024}}

Another way to increase the reach of a cable is by using unpowered repeaters called remote optical pre-amplifiers (ROPAs); these still make a cable count as unrepeatered since the repeaters do not require electrical power but they do require a pump laser light to be transmitted alongside the data carried by the cable; the pump light and the data are often transmitted in physically separate fibers. The ROPA contains a doped fiber that uses the pump light (often a 1480&nbsp;nm laser light) to amplify the data signals carried on the rest of the fibers.<ref name = art/>

WDM or wavelength division multiplexing was first implemented in submarine fiber optic cables from the 1990s to the 2000s,<ref>{{Cite web|url=https://www.itu.int/dms_pub/itu-t/opb/tut/T-TUT-HOME-2022-1-PDF-E.pdf|title=GSTR-SDM}}</ref> followed by DWDM or [[wavelength division multiplexing#Dense WDM|dense wavelength division mulltiplexing]] around 2007. Each fiber can carry 30 wavelengths at a time. SDM or [[spatial division multiplexing]] submarine cables have at least 12 fiber pairs which is an increase from the maximum of 8 pairs found in conventional submarine cables, and submarine cables with up to 24 fiber pairs have been deployed.<ref>{{Cite web|url=https://www.datacenterdynamics.com/en/news/nec-to-build-worlds-highest-capacity-submarine-cable-for-facebook-shuttling-500tbps-from-us-to-europe/|title=NEC to build world's highest capacity submarine cable for Facebook, shuttling 500Tbps from US to Europe|first=Sebastian Moss|last=Comment|date=October 12, 2021|website=www.datacenterdynamics.com}}</ref><ref>{{Cite web|url=https://sumitomoelectric.com/sites/default/files/2023-04/download_documents/E96-07.pdf|title=Optical Fibers for High Fiber Count Submarine Cable Systems}}</ref><ref>{{Cite web|url=https://subtelforum.com/stf-mag-feature-next-generation-transponder-technology-to-align-with-subsea-sdm-cables/|title=STF Mag Feature: Next Generation Transponder Technology to Align with Subsea SDM Cables|date=September 27, 2022}}</ref> The type of modulation employed in a submarine cable can have a major impact in its capacity.<ref>{{Cite web|url=https://topconference.com/wp-content/uploads/1Towards-Maximizing-Data-Throughput-on-Subsea-Fiber-Optic-Cables-Geoff-Bennett-Infinera.pdf|title=Maximizing Performance on All Types of Submarine Cable}}</ref><ref>{{Cite web|url=https://2023.apricot.net/assets/files/APPS314/sdm-a-new-subsea-par_1677480490.pdf|title=Spatial Division Multiplexing A New (Subsea) Cable Paradigm}}</ref> SDM is combined with DWDM to improve capacity.<ref>{{Cite web|url=https://www.lightwaveonline.com/optical-tech/transmission/article/16667943/google-subcom-to-deploy-space-division-multiplexing-on-dunant-submarine-cable|title=Google, SubCom, to deploy space-division multiplexing on Dunant submarine cable|first=Stephen|last=Hardy|date=April 10, 2019|website=Lightwave}}</ref>

Transponders are used to send data through the cable. The open cable concept allows for the design of a submarine cable independently of the transponders that will be used to transmit data through the cable.<ref>{{Cite journal|title=Design, Acceptance and Capacity of Subsea Open Cables|journal=Journal of Lightwave Technology|date=February 2021 |volume=39 |issue=3 |pages=742–756 |doi=10.1109/JLT.2020.3045389 |last1=Rivera Hartling |first1=Elizabeth |last2=Pilipetskii |first2=Alexei |last3=Evans |first3=Darwin |last4=Mateo |first4=Eduardo |last5=Salsi |first5=Massimiliano |last6=Pecci |first6=Pascal |last7=Mehta |first7=Priyanth |bibcode=2021JLwT...39..742R |doi-access=free }}</ref><ref>{{Cite journal|title=Design, Acceptance and Capacity of Subsea Open Cables|first1=Elizabeth|last1=Rivera Hartling|first2=Alexei|last2=Pilipetskii|first3=Darwin|last3=Evans|first4=Eduardo|last4=Mateo|first5=Massimiliano|last5=Salsi|first6=Pascal|last6=Pecci|first7=Priyanth|last7=Mehta|date=February 14, 2021|journal=Journal of Lightwave Technology|volume=39|issue=3|pages=742–756|doi=10.1109/JLT.2020.3045389|bibcode=2021JLwT...39..742R |doi-access=free}}</ref><ref>{{Cite web|url=https://web.asn.com/media/data/files_user/72/SDM1/How_to_Open_Cable_The_Guidelines_and_the_Gotchas_-_04-07-2019_R1.pdf|archive-url=https://web.archive.org/web/20220413060821/https://web.asn.com/media/data/files_user/72/SDM1/How_to_Open_Cable_The_Guidelines_and_the_Gotchas_-_04-07-2019_R1.pdf|url-status=dead|title=Subsea Open Cables: A Practical Perspective on the Guidelines and Gotchas|archive-date=2022-04-13}}</ref><ref>{{Cite web|url=https://comfutures2020.ieee-comfutures.org/wp-content/uploads/sites/101/2020/02/ComFutures2020-Ses1-SubseaCom-Kovsh.pdf|title=Subsea Communications}}</ref> SLTE (Submarine Line Terminal Equipment) has transponders and a ROADM ([[Reconfigurable optical add-drop multiplexer]]) used for handling the signals in the cable<ref name="auto">{{Cite web|url=https://www.itu.int/en/ITU-D/Regional-Presence/AsiaPacific/SiteAssets/Pages/Events/2017/Submarine%20Cable/submarine-cables-for-Pacific-Islands-Countries/Ciena%20Subsea%20PITA%20Aug2017%20v2.pdf|title=International High Capacity Connectivity through Submarine Cables}}</ref><ref>{{cite web | url=https://subtelforum.com/the-open-road-to-submarine-capacity/ | title=The Open Road to Submarine Capacity | date=January 26, 2021 }}</ref> via software control. The ROADM is used to improve the reliability of the cable by allowing it to operate even if it has faults.<ref>{{cite book | url=https://books.google.com/books?id=KvMQCgAAQBAJ&dq=SLTE+ROADM&pg=PA354 | isbn=978-0-12-804395-0 | title=Undersea Fiber Communication Systems | date=November 26, 2015 | publisher=Academic Press }}</ref> This equipment is located inside a cable landing station (CLS). C-OTDR (Coherent Optical Time Domain Reflectometry) is used in submarine cables to detect the location of cable faults.<ref name="auto"/> The wet plant of a submarine cable comprises the cable itself, branching units, repeaters and possibly OADMs ([[Optical add-drop multiplexer]]s).<ref>{{Cite web|url=https://www.fujitsu.com/global/documents/about/resources/publications/fstj/archives/vol35-1/paper05.pdf|title=WDM Optical Submarine Network Systems}}</ref><ref>{{cite book | url=https://books.google.com/books?id=2KDSDwAAQBAJ&dq=submarine+wet+plant&pg=PA399 | isbn=978-3-030-39445-5 | title=Advances in Information and Communication: Proceedings of the 2020 Future of Information and Communication Conference (FICC), Volume 1 | date=February 24, 2020 | publisher=Springer }}</ref>