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How do undersea internet cables work?

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Quick answer

Undersea internet cables carry data as rapid pulses of light through glass fibers on the ocean floor. Landing stations convert electrical signals to light, which travels thousands of kilometers. Optical repeaters boost the light every 50–80 kilometers. At the far end, the light is converted back to electrical data for local networks.

Undersea internet cables work by carrying data as pulses of light through hair-thin glass fibers laid on the ocean floor. Repeaters along the cable boost the light signal every 50–80 kilometers to keep it strong across entire oceans.

Step by step

  1. Data from your device is converted into electrical signals and routed to a coastal landing station.
  2. At the landing station, a laser encodes the data into pulses of light.
  3. The light pulses enter a hair-thin glass fiber inside the undersea cable.
  4. The light reflects along the fiber, but weakens over distance.
  5. Optical repeaters along the cable amplify the light every 50–80 kilometers.
  6. At the receiving landing station, the light is converted back into electrical data.

Key things to understand

  1. Undersea cables are bundles of glass fibers that guide laser light across ocean floors.
  2. At landing stations, internet traffic is converted from electrical signals into pulses of light.
  3. The light pulses travel through the fiber, reflecting internally to stay inside the glass.
  4. Every 50–80 kilometers, optical repeaters amplify the dimming light to prevent signal loss.
  5. At the far end, receivers convert the light back into electrical data for local networks.

Labelled diagram

  1. Landing station

    Converts electrical data into pulses of laser light.

  2. Optical fiber

    Hair-thin glass strand guides light using total internal reflection.

  3. Repeater

    Every 50–80 km, amplifies light to keep signal strong.

  4. Receiving station

    Converts light pulses back into electrical data.

How data travels through an undersea cable

Real-world examples

  • The Marea cable connects Virginia Beach in the US to Bilbao, Spain, spanning over 6,600 kilometers across the Atlantic.
  • The SEA-ME-WE 3 cable links Southeast Asia, the Middle East, and Western Europe, serving many countries along its route.
  • A single undersea cable can carry terabits per second, enough for millions of simultaneous video calls.
  • In 2006, a magnitude 7.0 earthquake near Taiwan damaged several undersea cables, disrupting internet across Asia.

Common mistake

✗ Myth: Undersea internet cables are mainly threatened by sharks biting them.

✓ Fact: Shark bites are extremely rare and not a significant threat. The main dangers are fishing trawlers, ship anchors, undersea landslides, and earthquakes, which is why cables are heavily armored and often buried in shallow waters.

People also ask

How deep are undersea internet cables?
Undersea cables generally lie on the ocean floor at depths ranging from about 200 meters near the coast to over 8,000 meters in deep ocean trenches. In shallow waters, they are often buried under the seabed for protection, while in deep ocean they rest directly on the seafloor.
How many undersea cables are there?
There are roughly 400 to 500 active undersea cables worldwide, spanning over 1.2 million kilometers. They carry about 99% of international internet traffic. The exact number changes as new cables are laid and old ones retired.
Can undersea cables be tapped?
Yes, undersea cables can be tapped, but it is difficult. The signals are often encrypted, and physically accessing the cable requires sophisticated equipment. Most modern cables use encryption, so even if tapped, the data remains secure. Governments and organizations use various methods to protect against tapping.
How are undersea cables laid on the ocean floor?
Special cable-laying ships slowly unreel the cable from large drums while moving forward. The cable sinks to the seabed, following a route surveyed to avoid trenches, volcanoes, and fishing zones. In shallow waters, ploughs or remotely operated vehicles may bury it for protection.
What happens when an undersea cable breaks?
Internet traffic is automatically rerouted through other cables, so most users may not notice. However, if many cables in one region fail, speeds can drop and connections become unreliable. Repair ships then locate the break and splice in a new section of cable.
How do undersea cables get power for repeaters?
The cable itself carries a high-voltage direct current through a copper conductor. This powers the optical repeaters spaced along the cable. The voltage is kept constant from shore, and the current returns through the seawater and the cable's outer conductor.
Are undersea cables safe from sharks and other threats?
Shark bites are rare and not a major threat; cables are heavily armored. The main risks are fishing trawlers, ship anchors, undersea landslides, and earthquakes. Cables are also designed with multiple layers of protection to withstand the deep ocean environment.

Part of the guide: How the Internet Works →