How are undersea cables laid in the oceans, Advantages over Satellite

Dhawal, was just doing some googling and stumbled upon some thing that might interest a lot of you.

Ever wondered how the trans-oceanic, inter-continental undersea cables are laid?

Well, it is a very tedious job to lay cables across the oceans across earth, it’s also fascinating to know how the undersea cables that are laid across the world.

Here is a website that show the undersea Cable Map.


Isn’t the undersea cable network map awesome? looks brilliant isn’t it? We’ll it even looks complex and require real men and women, spend long hours and very hard work to make it possible.

Submerged cables are laid down by using specially modified ships and sometimes they are even built for this purpose as they have to endure harsh conditions as well as huge pressures, these ship carry the submarine cable on-board and slowly lay it out on the sea bed as per the plans given by the cable operator. The ships can carry with them up to 2,000 kms of cable in length.

These cable-ships used various techniques to plough etc. also depending on the sea conditions and the ocean-bed where the cable is being laid-down, cable ships can lay anywhere between 100-150 kms of cable per day.

The ships are commonly referred to as cable-layers or cable-ships.

Types of cable used for undersea laying

  1. Polyethylene
  2. “Mylar” tape
  3. Stranded metal (steel) wires
  4. Aluminum water barrier
  5. Polycarbonate
  6. Copper or aluminum tube
  7. Petroleum jelly
  8. Optical fibers

Depending on where the cable is being laid, it might differ in thickness. Thinner cable systems are used for shallow ocean depths, whilst thicker cables are used for deep ocean beds, typical up to 20,000 feet.

Such cables are able to withstand pressure from 12,000 lbs/square-inch to 22,000 lbs/square-inch (this is necessary because of the extreme pressures in the deep ocean beds.)

These cables use DWDM (Dense Wavelength Division Multiplexing), now to learn about DWDM see this Wikipedia Article, signals to carry packets (including TCP/IP). Obviously they use repeater to re-generate and strengthen the signal which attenuates over long distance.

They have 10 years of lifespan and a typical costs for projects are anywhere from USD 100 Million to USD 500 Million.

Why don’t we use satellite communication instead

1. Satellites aren’t used because they can’t carry terabytes of data for less than a billion dollars per communication line.

2. The bandwidth available using a single fiber optic cable and a laser beam is much much greater than you can get from a single satellite radio channel. This is due to the higher frequency and shorter wavelength of light compared to microwaves. The higher the frequency, the greater the bandwidth.

3. An undersea cable is a bundle many fiber optic cables. Consider each fiber cable as a channel. You can have more channels, each with a higher capacity, than you can build radio channels into a satellite.

4. The uplinks and downlinks cost  and putting the satellite in space is a huge huge ask and far more risky.

5.  The delay for satellite communications would be around 255ms both uplink and downlink. For continuous traffic this not to a bad price to pay. But for burst traffic (like voice) you pay for the delay at each pause. The Rule of Thumb is 10MS per 1000 miles so Rule of Thumb to Europe on say TAT-8 would be about 75MS vs 510MS for satellite.

6. Finally, you can fix a broken cable. Once you launch the satellite you don’t get a chance to fix it if it gets broke.


Okay, so here is how the Actual Laying process of Undersea cabling works

Submarine cable laying process starts from the Landing Station, where a long cable section is connected to the landing-point and then extended out to a few kilometers in the sea. This end is connected to the cable on the ship and then the ship starts its cable laying process.

This is how the Landing Station looks like, notice the Ship in the horizon of the sea.

The ships, which are specialized, are almost all owned by the submarine cable consortium or manufacturers.  These ships are stationed at various points along where the cable extends to ensure that in the event of a cable-cut, the ships can set sail immediately for cable repairs.

Cable laying ship port

Cable coiled up in the cargo-hold, this coiling of 100 of Kms of cable is a proces that can take between 3 to 4 weeks to complete. It is coiled very carefully, see the next pic


Landing Install ship (that connects the landing and ocean cables)

The part of the ship from where the cables are lowered into the sea

 Cable turntable – the turntable is slowly unwinding the cable and lowering it on to the ocean-floor.

A cable laying ship at sea. Notice the cable being lowered onto the sea bed, on the right hand side (starboard) (white portion) at the rear end of the ship.

Here is a picture of a repeater being launched into the sea (which is placed every 40-60 kilometers) to fix and strengthen the fiber-optic signal and to amplify it


The cable is not simply left to just sit on the ocean bed, but is actually being fed into a plough, that is laying the cable in the trench.

Here is the process of laying submarine cable with a plough

Cable plough on shore being pulled to the ocean via a Ship

Some plough even have hydraulics and water jets to clean the trench.

Smaller plough

Cable diver inspecting the cable

When cables are damaged, either divers or specialized small sized submersible submarines with cameras and lights are sent down to the seabed to investigate where the cuts are. Then either divers of with the use of the robotic arms of the submersible ring the two ends of the cable onto the surface, where they are re-spliced and joined again.


How awesome did you feel the undersea cabling is?

The DNetWorks Team


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