Hi Steve,..Re:.120,000 Leagues Under the Sea
Interesting article this week from IEEE's Spectrum.
Enjoy,
Lee
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120 000 leagues under the sea
The keys to the enormous expansion in the capacity of undersea cable are improved optical fibers plus enhanced electronics
By MEL MANDELL, Contributing Editor
IN 1988, ONLY 2 PERCENT OF THE WORLD'S TRANSOCEANIC FLOW of messages and data was carried by undersea cables. Satellites were the dominant carrier. Today, a mere dozen years later, cables carry 80 percent of a much bigger, ever-growing total. The reason: bandwidth, or more accurately, channel capacity, which is a function not only of bandwidth but of noise level as well.
The latest optical-fiber cables have at least 3000 times the capacity of their coaxial forebears, whereas today's satellites have improved only modestly over theirs. What's more, optical cables themselves are improving at an impressive rate. Current cables from Alcatel SA, Paris, for example, carry 10 Gb/s on each of 42 wavelengths for a total capacity of 420 Gb/s over a single fiber. And that is likely to grow to 68 wavelengths by next year.
Besides the enormous and continuing increases in capacity, cables enjoy other advantages over satellites: better longevity, security, and adherence to installation schedules (there are no booster rocket failures in the cable business), to name three. And undersea cables--all 580 000 km (120 000 leagues) of them--live a comparatively sheltered life compared with satellites, which are threatened by meteor showers, space debris, and sun spots.
In the hazardous region from the shore line out to the depths, the cables are usually heavily armored and buried beneath the sea floor. Out in the deep ocean, where the hazards are few and far between, they simply lie on the bottom, suspended occasionally over sharp depressions, in a calm, isothermal environment of about 2 øC, surrounded by the world's largest heat sink.
Continued capacity expansion
Four elements account for the remarkable and continuous capacity expansion of undersea cable: improved optical fibers, dense wavelength-division multiplexing (DWDM), enhanced electronics supporting higher transmission speeds, and the incorporation of more strands in each cable. For instance, the first optical-fiber cables made by Simplex Technologies Inc., Portsmith, N.H., the subsidiary of Tyco International Ltd., Pembroke, Bermuda, contained six strands, two pairs of which were operational, with one pair serving as a backup. The current Simplex cables contain eight strands, all operational. According to industry experts, these approaches are far from exhausted--that is, the capacity of submarine cables is still capable of substantial growth.
The suppliers of fiber strands are continuing their efforts to boost bandwidth. This past June, Lucent Technologies Inc., Murray Hill, N.J., introduced its TrueWave Submarine RS fibers. The great advantage of these fibers, according to Janice Haber, Lucent's vice president, technical fiber sales, is more constant dispersion across a broader operating range. Dispersion--the dependence of propagation speed on wavelength--causes pulsed signals to spread out and interfere with each other. In keeping the dispersion nearly constant across the entire bandwidth of a system, Lucent, in effect, has improved performance at the band edges, making the channels near the edge of the band more like those in the middle, thereby allowing the system to carry more wavelengths over longer distances.
Earlier, in October 1998, Corning Inc., Corning, N.Y., introduced its Submarine LEAF cable, which takes a different approach to increasing capacity. (LEAF is an acronym for large effective area fiber.) Instead of attempting to minimize dispersion and dispersion variation, LEAF exploits it, alternating lengths of fiber with different dispersion characteristics to increase the fiber's effective cross-sectional area by some 30 percent, compared with the company's prior fibers of the same diameter. Every fiber in a LEAF cable can support 16 wavelengths, each of which carries 10 Gb/s.
The longevity advantage
An important contributor to cable's growing cost advantage over satellites is its longevity. Undersea cables are designed to last 25 years; satellites, in the words of William E. Carter, president of the development subsidiary of Global Crossing Ltd., in Morristown, N.J., "run out of essential rocket fuel in 10 to 15 years." That total was confirmed by a spokesperson for Hughes Space & Communications Co., Los Angeles.
Cables generally last much longer than 25 years. The coax cables laid over 40 years ago for the U.S. Navy's system for monitoring ship traffic off our coasts still work. And after cables are retired, they can be put to other uses. For instance, an abandoned AT&T cable (it was severed by a trawler in 1989) laid between Hawaii and California back in 1964 has been given to a group of seismologists to study undersea disturbances 1600 km east of Hawaii. (At the time the cable was severed, it was 25 years old and not worth repairing. Newer cables that are worth repairing can be quickly hauled up and fixed--at a lot lower cost than a zapped satellite, assuming the latter could be fixed at all.)
Cable has still another edge over satellites: no quarter-second delay in responses in phone conversations. Indeed, even the longest cable systems, such as those that cross the Pacific, are but a small fraction of the length of the 71 000-km journey that such conversations must take if transmitted by synchronous satellite.
Some of the carriers that use undersea cable also operate satellites. As a result, they can and do somewhat mitigate the effects of the quarter-second one-way delay of satellite voice transmissions by carrying voice in one direction by satellite and the other direction by cable.
Another plus with cables is that they are no burden at all on the increasingly crowded radio spectrum.
The security advantage
Besides its immunity to space debris, sun spots, and meteor showers, cable also has several security benefits. Some 20 years ago, U.S. Navy submarines, in a herculean, super-secret espionage effort, tapped the coaxial undersea cables leading from Soviet naval bases without detection. Optical-fiber cables are immune to such external tapping.
Satellite transmissions, on the other hand, can be fairly easily intercepted. In a war, communications (and spy) satellites could be shot down by missiles. While undersea cables could be cut, the practice of burying the in-shore segments makes this difficult; the mid-ocean portions are hard to find without a map and help from shore-based monitoring stations.
Adherence to installation schedules is another huge asset. Other than delays of a week or so due to storms at sea, installations of undersea cable systems are nearly always completed on schedule. In contrast, many communication (and spy) satellites have been destroyed during failed launchings, or, if launched, have failed to reach the desired orbit or orientation. Even after reaching orbit, they may drift out of the desired orientation.
Once in orbit, failing satellites can be repaired only at great expense, such as the unlikely dispatch of a space shuttle. In contrast, a severed, damaged, or failing undersea cable can be hauled up, repaired, and relaid. This effort can sometimes be done extremely quickly--in less than a week--if a cable-laying ship happens to be nearby. The ship's crew knows in advance exactly where the cable is severed or which repeater is failing since it knows the cable's exact route and can learn where in the cable the problem is located from shore-based technicians. Those technicians operate equipment that monitors and analyzes samples of the output of the cable's repeaters, which are extracted by loopback couplers. For each repeater, a different wavelength is monitored, which simplifies fault location.
Metal still rules
Glass may carry the data, but the latest undersea cables still carry a lot more metal than glass. In making an optical-fiber cable, the process begins with a core of copper-coated steel called a kingwire around which pairs of fiber strands are wrapped [Fig. 1]. That assemblage is shrouded in a protective Hytrel/Nylon sheath and then hermetically sealed in a welded sheath of thin copper, which not only protects the glass strands from contamination but also carries electrical power to the cable's repeaters. (The current's return path is through the sea.) Manufacturing the kingwire assembly is an impressive business carried out in a narrow building several football fields long.
Next, the copper-sheathed assembly is further encased in liquid polyethylene into which high-strength steel cables are embedded before the plastic can harden. Mid-ocean cable segments contain just those thin steel cables, to keep them as light as possible. In-shore segments are protected by additional heavy steel cables. As a result, they may weigh 16 times as much as mid-ocean cable, and obviously cost much more.
After cable is fabricated in segments as long as 80 km, it is wound on huge spools about 5 meters in diameter. If the cable is designed for transoceanic runs, the necessary repeaters are inserted between cable segments as the cable segments are unwound from the spools prior to being loaded onto a cable-laying ship. Repeater-to-cable couplings [Fig. 3] are attached to the terminations of each segment. These very large connectors encased in tough beryllium-copper alloy weigh as much as 300 kg. The assembled cable is run down a long chute, at the end of which a cable-laying ship is docked. There the cable is manually spooled in horizontal layers by strong-armed crews inside the ship's holds--the exact same way the first transoceanic cable was stored in 1857!
About 12 years ago, electronic repeaters were superseded by optical amplifiers. Their great advantages are simplicity and flexibility. As linear amplifiers, they simply boost the entire optical signal propagating down the fiber [Fig. 2]. If operators change the number or nature of the signals, the amplifiers will handle the new signals so long as their design limits are not exceeded.
In contrast, electronic repeaters do not have enough bandwidth to simply boost the entire signal in one fell swoop. They must separate it into its different optical components, demodulate them, separately regenerate each of the digital modulating signals, modulate new optical carriers, combine the optical carriers, and then couple them into the outgoing fiber. That process makes it difficult to adjust a repeater to changing conditions. Basically, it can only handle the number of channels at the data rate per channel for which it was designed.
Laying it down
A few months before a cable is to be laid, its route along the ocean floor is surveyed by a high-resolution side-scanning sonar. The surveyors' objective is to map a route that avoids shipwrecks, jagged rocks, and underwater volcanoes.
Laying begins in one of two ways. The first approach was detailed by Maurice Kordahi, director of undersea lightwave jointing, installation, and maintenance for Tyco's Submarine Systems. The in-shore segment, anchored to a beach manhole, is floated out to the offshore cable-laying ship. (A beach manhole is a concrete vault, buried near the shoreline, through which the buried end of the undersea cable [see 'Cable's underwater saga"] is linked to a buried terrestrial cable leading to the nearest land-line cable terminal.)
In the second approach, the cable-laying ship, loaded with its cargo, takes a position as close to the cable's termination point on land as possible, as dictated by the depth of the sea and ship's draft. The end of the cable is played out and floated on buoys toward the shore. At the landing site, a husky crew in wet suits wrestles the end onto the shore where it is buried [Fig. 4].
To protect the cable segments closest to shore, where conditions are harshest, the heavily armored cable is often buried by being laid through a hole in a huge steel plow pulled by the ship. This not only prevents fraying against rocks and sand by tides, surf, currents, and storms, but also protects against trawler nets and ship anchors. According to Global Crossing's Carter, the decline in fishing off U.S. coasts has reduced the threat from trawlers. However, in East Asia, where fishing fleets are far more active, trawlers are a much greater threat.
Where shoreline conditions prohibit burying, the most heavily armored cable is chosen, according to Tyco's Kordahi. This cable is tough enough to withstand damage from nets drawn along the bottom.
Global Crossing always has its transoceanic cables laid in roughly parallel but widely separated pairs, according to Carter. Only half the capacity of each cable is used. In the rare event of a failure of one cable, its mate is fully used to maintain capacity. The switch-over takes only 300 ms. Laying of pairs of complementary cables is characteristic of the east-west cables, which bear very heavy traffic. North-south cables, which are not required to carry as much traffic, are generally singletons, according to Tim McInerny, a vice president with Concert, Atlanta, Ga., the 50-50 venture to which AT&T Corp. and British Telecommunications PLC, London, have assigned all their transoceanic communications systems, including satellites.
Coast-wise cables
While undersea cable's key role is linking the continents and well-populated islands, it has taken on a useful secondary role in linking cities on the same coastline. The first coastal cable system was laid off the Brazil coast to link its largest cities, Rio de Janeiro and SÆo Paulo. Another system is planned for West Africa to link such coastal cities as Lagos, in Nigeria, and Dakar, in Senegal. At present, a phone call between any of those cities has to be routed thousands of kilometers through France at high cost.
No-cost rights of way are not the only reason for choosing undersea cable. For example, added Concert's McInerney, in Brazil "coast-wise cable was selected because there are very few north-south highways to expedite the laying of terrestrial cable."
In general, undersea cable requires much less maintenance than terrestrial cable because it's designed and constructed so carefully, claimed Carter. It must be admitted, however, that repairing a coast-wise cable could set a company back as much as a million dollars, while repairing a land line could cost as little as a few hundred dollars.
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spectrum.ieee.org
Spectrum editor: Michael J. Riezenman
IEEE Spectrum April 2000 Volume 37 Number 4 |
