Transoceanic Communications Cables
Friday, January 13, 2012 at 9:09AM Permalink 
Map of undersea cabling (Photo: cablemap.info)
If the idea of laying cables thousands of miles long at the bottom of the deepest oceans on the planet in order to transmit data around the globe at near the speed of light sounds like science fiction, you may be surprised to learn that this feat was first achieved by Samuel Morse back in 1842. Morse's proof of concept project involved transmitting a signal across the New York Harbor through a wire insulated with tarred hemp and India rubber.
The first commercial line was run across the English Channel in 1850 by the Anglo-French Telegraph Company and over the next fifty years the British dominated submarine cabling industry built an impressive network spanning the globe - including the first trans-Pacific line linking the mainland USA with Hawaii laid in 1902.
While the mere feat of laying undersea cable spanning the Pacific in 1902 is impressive, the engineers of the day faced huge challenges in managing the flow of electricity across the huge lengths of cables without the use of repeaters and other modern technologies. The electrical resistance associated with the length of the cable diminshed the bandwidth and limited the data rate to 10-12 words per minute.
Underseas Cabling in Modern Times
Today over 99% of transoceanic data communication is conducted via the network of undersea cables (the rest being transmitted via less reliable and lower bandwidth satellite connections). The multi-terabit per second, highly redundant system is highly reliable but laying the cables is an extremely expensive undertaking - several hundreds of millions per transoceanic cable typically.
During the 1980's and 90's projects were often undertaken by consortia of international carriers to limit the outlay by individual corporations. TAT-8 for example - the first undersea cable to use fiber optics - was built in 1988 by a consortium of 35 companies led by AT&T, France Telecom and British Telecom and cost $335m to construct.
TAT-8 comprised two working pairs of optical fibers with a third reserved as a redundant pair. The signal on each optical fiber was modulated at 295.6 Mbits/s that was capable of carrying 40,000 circuits (simultaneous phone calls) (this was before the internet).
Since the TAT-8 used fiber as opposed to coaxial, the electrical interference shielding from the high voltage supply lines that powered the repeaters was removed. Electrical interference has no affect on fiber signal - which is transmitted by light as opposed to electrical pulses - but it did happen to have an affect on passing sharks. Highly sensitive to electric fields, sharks would be whipped into a feeding frenzy and would attack the line until the voltage killed them.
Due to the numerous prolonged outages, shark shielding was deployed alongside subsequent trans-Atlantic cables.
Operation Ivy Bells - Wiretapping Soviet Undersea Cables
During the early 1970's the United States learnt of the existence of an undersea cable spanning the Soviet controlled Sea of Okhotsk, linking two major Soviet naval bases.
The cable was protected by sound detectors on the sea bed designed to detect intruders and the sea was heavily trafficked by Soviet vessels, but the United States Navy managed to submarine in some divers who attached a 20ft long device around the cable - without piercing the casing or leaving any evidence of tampering. The device was also designed to detach if the cable was ever raised for repair.
The divers would then retrieve and replace tapes each month, which the NSA and other agencies would then process for intelligence. Apparently the Soviet's were so confident that the cable was secure that the data transmitted across the wire was unencrypted.
Later in the conflict the United States tapped other cables with more advanced nuclear powered devices that could store up to a year's worth of data.
Transoceanic Cabling FAQ's
My guest today is a 25 year veteran in the networking world with extensive experience building packet and transport infrastructure for the large carriers and large enterprises. He currently works for a major vendor planning the next generation core packet-optical devices. In a previous role he was involved in designing a transatlantic system for a major carrier. He agreed to answer a few questions on the condition of anonymity.
Are the cables just laying on the seabed, buried, suspended?
In the open ocean, the cable is allowed to settle directly on the ocean floor. Near the coast (where damage is likely due to anchors, fishing, and construction) the cable is buried under the ocean floor (via pneumatically powered underwater trenching machines).
TE SubCom animation depicting use of submersible plow to bury cable:
How are cable breaks handled? What causes them? How can you tell which section of the line is affected?
Cable breaks occur several times a year (there are hundreds of active cables around the world). Typically a cable is cut due to anchors (large ships), earthquakes, fishing equipment, or construction (such as dredging).
A device called an OTDR (Optical Time Domain Reflectometer) allows the operator to pin-point the break. It basically sends an optical pulse along the fiber and times the reflection that occurs at the break. Knowing the speed of light in fiber (roughly .67c) and the time the pulse travels to the break and back, allows calculation of the distance.
TE SubCom animation depicting cable repair process:
How does reflection occur at a break?
The break in cable usually breaks the fiber as a near perpendicular cleave (perpendicular the the axis of the core of the fiber). This creates a face on the end of the fiber. The angle of incidence of the light propagating down the fiber is nearly 90 degrees to the face of the break in the fiber. Since the index of refraction of the fiber is significantly different from the outside medium (air or seawater), the internal face of the fiber at the break acts like a mirror.
How are the repeaters powered?
The amplifiers (not really repeaters) are placed every 40 to 60 km (depending on cable design). Inside the sealed case are amplifier modules (typically EDFAs)- one for each fiber. The pump lasers and control electronics receive power through a copper conductor in the cable. This power is supplied by the landing stations as high voltage DC (can be 10,000s V- depending on the length of cable).
Is there a NOC specifically for monitoring the submarine system?
There is not necessarily a special NOC for the submarine systems. For the carrier I worked for, monitoring the submarine cable was little different than terrestrial optical systems. For the most part, the systems are self-monitoring and notify the NOC operators when a fault occurs or some parameter exceeds a threshold. If there is truly a fault, the NOC personnel are generally not capable of "fixing" the system - as in most cases, something serious has happened such as a fiber cut or amplifier failure. These types of outages require dispatching a repair crew.
How secure are the repeaters against wire tapping? I believe I read somewhere that it happened during the Cold War, has any new technology been employed to secure the data crossing transoceanic lines?
I have heard of the cold-war events - but do not know if they are true. Theoretically, you can "tap" any of the existing fiber systems whether terrestrial or submarine. In fact, most equipment actually has "tap ports" on it to allow diagnostic equipment to analyze the light signals. In any event, there is nothing magical about securing data across any network - it must be strongly encrypted at the end-points. If you are worried about security, then you must assume that your data will be intercepted - no matter what the intervening technology.
What's the average depth of the line? Are they purposely routed across the most shallow points?
The cables follow the undersea topology, there is little attempt to restrict the depth. It is more important to minimize the length of the cable.
What advice would you give to an engineer sitting in an enterprise NOC if he or she wanted to get involved in a project like this?
There are many aspects to designing, installing, operating and maintaining optical networks. Depending on specifically what an individual is interested in doing, they might want to focus on specific things.
For example, if the person is interested in the actual cable construction, you would need to work for a vendor as the cable construction is done at the vendor's facility (Alcatel is a big one). The construction of the cable is a custom job for every cable system built and requires people of many different skill sets - everything from optical engineers, to mechanical engineers, to electrical engineers, etc. Most of the people involved in the cable design are experienced engineers - many of them have PhD.
If you were interested in the installation or maintenance aspect of a submarine cable (maybe you fancy being a pirate on the high seas), then getting experience in the maritime trades via the navy, coast guard, or shipping fleets might be a good start. I suppose the ships are sometimes looking for basic "grunts" and "deck hands". However, submarine cable laying is not a large industry and the work is probably not continuous. There just are not that many submarine cables being built at any given time.
If you want to be involved in the architecture part, you would need to be in the architecture group in a service provider that was planning on paying for a new submarine cable. Being a network architect usually takes many years of experience in designing and running many types of networks before you can acquire the broad skills necessary to do architectural work. The architects and designers in carriers contract with the vendors to build and install the submarine cable systems.
Network architecture is the type of work I have been doing for many years.
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