Sunday, December 21, 2008

Undersea Fibre Optic Cable Technology

OK, in light of yet another cable cut in Europe that severely disrupt my office email system, I thought I post what little I know about undersea submarine fibre optic cables. This is what I found out from having to do Matrix Cable System in 2007-2008.

Note: I will make this quick and dirty for now, and add revisions/details later if anyone is interested, so post your comment and tell me what information I should put in this.

What is Undersea Fibre Optic Cable System

Undersea Fibre Optic Cable System are a system of fibre optic cable laid/buried on/under the ocean floor for the purpose of carrying large capacity data transmission. It is by far the most efficient way to carry data from point to point, and the only one capable of coping with the world's hunger for data transmission.

As you may know, Fibre Optic carries data by guiding laser light from one point to another. Furthermore, a single fibre optic can carry multiple color laser wavelength. Almost all recent cable system can easily carry over 1 Tbps (Tera bits per second = 1,000,000 Mbps) of data transmission at each sections. Thus a Fibre Optic Cable System can carry more (200-1000 times) data capacity than a satellite system, and faster too.

Now these fibre optic cables are then laid on the ocean floor, primarily because it has less probability of being disturbed than on land. For shallow waters (where fishing vessels passes, and ship anchors), the cables are usually steel-armored (Light Armor, Single Armor, Double Armor, etc) and buried 1-2 meters under the ocean floor. For deep water > 1000 meter, the cable can just be left with polyeurethane jacket and laid on the ocean bed.

Fibre Optic Cable Systems are usually build along a main line, with branches serving multiple landing points. SEA-ME-WE3, (South East Asia, Middle East, Western Europe) the world longest cable system, has 39 landing points from Perth, Australia, to Norden, Germany.

Why Submarine Fibre Optic Cable System


Fibre optic carries large quantity of data in the form of laser lights. This technology is quickly evolving, from 2.5Gbps per wavelength (light color) in 1990s to 10Gbps in mid 2000s to 40Gbps right now (2008). Note that the technology usually uses a pair of fibre (Fibre Pair = 2 fibre core) for a single duplex (2 way) transmission.

Furthermore, as lights carried in a fibre optic cable has little interference with each other, many wavelength can be carried inside the same fibre, each carrying its own 10Gbps data, in a method called Dense Wavelength Division Multiplexing. The current technology easily support 64 10Gbps carrier wavelengths for a 640Mbps (64 x STM-64) data transmission PER FIBRE PAIR. Thus an 8 Fibre Pair DWDM system (like Unity Cable) can carry above 5Tbps of data transmission capacity.

For comparison, The a latest digital satellite, Shin-corp's IP Star satellite can only carry a maximum downstream capacity of about 4.5Gbps. The previous generation satellite usually carry only 50-100 transponders with only 45Mbps capacity per transponder. On the other hand, though, a satellite can be accessed at almost any point in its region of coverage.

So, in comparison to a satellite transmission. the fibre optic cable system can deliver three order of magnitude (1000x) more bandwidth for the same price as a satellite, but can only deliver them to specific points, not a large area. Thus for backhaul, fibre optic is preferred, while for "last mile", satellite is much better.


Another reason for submarine fibre optic over satellite is latency (delay in transmission). A satellite in Geosynchronous (Geostationary) Orbit is about 35,000 km above sea level. To transmit anywhere, the data must go up, and then down = over 70,000 km distance. The earth circumference is only 40,000km. Thus to go halfway around the world on fibre optic, you'd need at most 20,000km. As both uses light that travels at about 300,000 km/s, you can see that submarine fibre optic will get you there faster, especially in short distances.


Recent news notwithstanding, Submarine fibre optic cable system (one properly built, that is) is pretty robust. The picture above shows a Single Armor and a Double Armor fibre optic cable. As you can see, a submarine fibre optic cable is mostly steel (and cotton and tar, for waterproofing).

The Light Armor (the middle portions of above) cable has the following layers:

  • fibre optic cores inside a loose-fitting tube, filled with waterproofing gell
  • Copper power transmission cable
  • Light gauge steel wire twisted around the core above.
  • Thick Polyeurethane jacket (good enought to withstand shark bites)

The Single Armor (on left) cable add the following layer on top of the Light Armor.

  • One layer of heavy gauge steel wires twisted around the light armor cable
  • tar-soaked cotton yarn woven around the outside of the armor layer.

The Double Armor (on right) cable add the following layer on top of the Light Armor.

  • Two layer of heavy gauge steel wires twisted around the light armor cable
  • Tar-soaked cotton yarn woven around the outside of the armor layer.
  • Total diameter about the size of an adult human wrist.
Incidentally, a double armor cable cost about US$50,000 per km, while a light armor one maybe about US$14,000 per km.

You only need Double Armor cable when the cable is exposed on the ocean floor in shallow waters. Single Armor or Light Armor is used in shallow water with 1-2m burial. You only need Light Armor if you deploy the cable in deep ocean > 1000m.


A DWDM Submarine Fibre Optic Cable System usually is built for long haul data transmission. The system is most likely require a repeater every 100-150km which refocus and strengthen the laser lights so that the data can still be read at the other end. The repeaters are powered by generator at the landing stations. A properly design system should be built with enough redundancy to surivive any one landing site power failure.

The repeater technology is the one limiting the transmission capacity of a single fibre optic cable system. The latest Tyco Gen-3 repeater system can only accomodate enough repeater for 8 fibre pair in a single submarine repeater. Each repeater can only repeat laser light for about 64 x 10Gbps wavelength. (add tech detail?). As repeater technology improves to repeat wider spectrum of laser lights, the capacity per fibre will increase. 96x10Gbps and ??x40Gbps technologies are being field tested now.

Note, a short haul cable system of less than 300km can be repeaterless. In this situation, usually high fibre core count (24-144 fibre core) can be utilized. With this many fibre, an SWDM system (single Wavelegth) is usually used. A long haul cable system can be built using multiple short haul segments, but good quality landing stations must be built along the way to retransmit the data. Failure of one single landing station will cause the whole system to go down.

1 comment:

n9986 said...

Interesting and informative. Could you provide information on how such high bandwidth data is routed at end points and what kind of hardware is required to process such tremendous amount of data?