Early long-distance submarine telegraph cables exhibited formidable electrical problems. Unlike modern cables, the technology of the 19th century did not allow for in-line repeater amplifiers in the cable. Large voltages were used to attempt to overcome the electrical resistance of their tremendous length but the cables' distributed capacitance and inductance combined to distort the telegraph pulses in the line, severely limiting the data rate for telegraph operation. Thus, the cables had very limited bandwidth.
As early as 1823,[citation needed] Francis Ronalds had observed that electric signals were retarded in passing through an insulated wire or core laid underground, and the same effect was noticed by Latimer Clark (1853) on cores immersed in water, and particularly on the lengthy cable between England and The Hague. Michael Faraday showed that the effect was caused by capacitance between the wire and the earth (or water) surrounding it. Faraday had noted that when a wire is charged from a battery (for example when pressing a telegraph key), the electric charge in the wire induces an opposite charge in the water as it travels along. As the two charges attract each other, the exciting charge is retarded. The core acts as a capacitor distributed along the length of the cable which, coupled with the resistance and inductance of the cable limits the speed at which a signal travels through the conductor of the cable.
Early cable designs failed to analyze these effects correctly. Famously, E.O.W. Whitehouse had dismissed the problems and insisted that a transatlantic cable was feasible. When he subsequently became electrician of the Atlantic Telegraph Company he became involved in a public dispute with William Thomson. Whitehouse believed that, with enough voltage, any cable could be driven. Because of the excessive voltages recommended by Whitehouse, Cyrus West Field's first transatlantic cable never worked reliably, and eventually short circuited to the ocean when Whitehouse increased the voltage beyond the cable design limit.
Thomson designed a complex electric-field generator that minimized current by resonating the cable, and a sensitive light-beam mirror galvanometer for detecting the faint telegraph signals. Thomson became wealthy on the royalties of these, and several related inventions. Thomson was elevated to Lord Kelvin for his contributions in this area, chiefly an accurate mathematical model of the cable, which permitted design of the equipment for accurate telegraphy. The effects of atmospheric electricity and the geomagnetic field on submarine cables also motivated many of the early polar expeditions.
Thomson had produced a mathematical analysis of propagation of electrical signals into telegraph cables based on their capacitance and resistance, but since long submarine cables operated at slow rates, he did not include the effects of inductance. By the 1890s, Oliver Heaviside had produced the modern general form of the telegrapher's equations which included the effects of inductance and which were essential to extending the theory of transmission lines to higher frequencies required for high-speed data and voice.
Bandwidth problems
Transatlantic telephony
In 1942, Siemens Brothers of Charlton, London in conjunction with the United Kingdom National Physical Laboratory, adapted submarine communications cable technology to create the world's first submarine oil pipeline in Operation Pluto during World War II.
TAT-1 (Transatlantic No. 1) was the first transatlantic telephone cable system. Between 1955 and 1956, cable was laid between Gallanach Bay, near Oban, Scotland and Clarenville, Newfoundland and Labrador. It was inaugurated on September 25, 1956, initially carrying 36 telephone channels.
In the 1960s, transoceanic cables were coaxial cables that transmitted frequency-multiplexed voiceband signals. A high voltage direct current on the inner conductor powered the repeaters. The first-generation repeaters are among the most reliable vacuum tube amplifiers ever designed.[7] Later ones were transistorized. Many of these cables are still usable, but abandoned because their capacity is too small to be commercially viable. Some have been used as scientific instruments to measure earthquake waves and other geomagnetic events.[8]
Optical telephone cables
Modern optical fiber repeaters use a solid-state optical amplifier, usually an Erbium-doped fiber amplifier. 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.
Repeaters are powered by a constant direct current passed down the conductor near the center 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 half way along the cable under normal operation. The amplifiers or repeaters derive their power from the potential difference drop across them.
The optic fiber used in undersea cables is chosen for its exceptional clarity, permitting runs of more than 100 kilometers between repeaters to minimize the number of amplifiers and the distortion they cause.
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 fibers arranged in a self-healing ring to increase their redundancy, with the submarine sections following different paths on the ocean floor. One driver for this development was 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 back-up 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 back-up is regarded as too expensive.
A further redundant-path development over and above the self-healing rings approach is the "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, the less likely it is that one or two simultaneous failures will prevent end-to-end service.
Cable repair
Cables can be broken by fishing trawlers, anchoring, undersea avalanches and even shark bites. Breaks were common in the early cable laying era due to the use of simple materials and the laying of cables directly on the ocean floor rather than burying the cables in trenches in more vulnerable areas. Cables were also sometimes cut by enemy forces in wartime. Cable breaks are by no means a thing of the past, with more than 50 repairs a year in the Atlantic alone,[9] and significant breaks in 2006 and 2008.
To effect repairs on deep cables, the damaged portion is brought to the surface using a grapple. Deep cables must be cut at the seabed and each end separately brought to the surface, whereupon a new section is spliced in. The repaired cable is longer than the original, so the excess is deliberately laid in a 'U' shape on the seabed. A submersible can be used to repair cables that lie in shallower waters.
A number of ports near important cable routes became homes to specialised cable repair ships. Halifax, Nova Scotia was home to a half dozen such vessels for most of the 20th century including long-lived vessels such as the CS Cyrus West Field, CS Minia and CS Mackay-Bennett. The latter two were contracted to recover victims from the sinking of the RMS Titanic. The crews of these vessels developed many new techniques and devices to repair and improve cable laying, such as the "plough
Intelligence gathering
Underwater cables, which cannot be kept under constant surveillance, have tempted intelligence-gathering organizations since the late 19th century. Frequently at the beginning of wars nations have cut the cables of the other sides in order to shape the information flows into cables that were being monitored. The most ambitious efforts occurred in World War I, when British and German forces systematically attempted to destroy the others' worldwide communications systems by cutting their cables with surface ships or submarines.[10] During the Cold War the United States Navy and National Security Agency (NSA) succeeded in placing wire taps on Soviet underwater communication lines in Operation Ivy Bells.
Notable events
The Newfoundland earthquake of 1929 broke a series of trans-Atlantic cables by triggering a massive undersea avalanche. The sequence of breaks helped scientists chart the progress of the avalanche.
In July 2005, a portion of the SEA-ME-WE 3 submarine cable located 35 kilometres (22 mi) south of Karachi that provided Pakistan's major outer communications became defective, disrupting almost all of Pakistan's communications with the rest of the world, and affecting approximately 10 million Internet users.[11][12][13]
The 2006 Hengchun earthquake on December 26, 2006 rendered numerous cables near Taiwan inoperable.
In March, 2007, pirates stole an 11 kilometres (6.8 mi) section of the T-V-H submarine cable that connected Thailand, Vietnam, and Hong Kong, affecting Vietnam's Internet users with far slower speeds. The thieves attempted to sell the 100 tons of illicit cargo as scrap.[14]
The 2008 submarine cable disruption was a series of cable outages, two of the three Suez Canal cables, two disruptions in the Persian Gulf, and one in Malaysia. It caused massive communications disruptions to India and the Middle East.
Submarine communications cable
The first submarine communications cables carried telegraphy traffic. Subsequent generations of cables carried first telephony traffic, then data communications traffic. All modern cables use optical fiber technology to carry digital payloads, which are then used to carry telephone traffic as well as Internet and private data traffic. They are typically 69 millimetres (2.7 in) in diameter and weigh around 10 kilograms per meter (7 lb/ft), although thinner and lighter cables are used for deep-water sections.[1]
