Telecommunication
Telecommunication is the transmission of signals (i.e. data) over a long distance for the purpose of communication.
Multiplexing
Sometimes, data coming from different sources still need to go through the same physical cable. To allow that, we can combine different signals into one: it's called multiplexing. The single signal is then demultiplexed into the original ones and send to their respective destinations.
There is different ways to multiplex signals. Let's take human communication as an example.
- Space-division: Each channel can use its own part of the space exclusively. If people talking together are far apart from each other, the conversations won't interfere.
- Frequency-division: Different channels use the different frequencies. If bat and humans talk in the same time, the conversations won't interfere.
- Time-divison: Different channels use different time slots. People don't interfere with each other if they don't talk in the same time
- Code division: When signals are encrypted, they are not coded the same. People speaking different languages don't interfere.
Transatlantic cables
To pass data from a continent to another, we use submarine transatlantic cables. It began around 1860 with telegraph cables, then coaxial cables in mid 20th century. Now, every cable has amplified optical fiber.
Some typical underseas cable properties:
- Length: can reach over 8000 km
- Fiber optical: around 20 pairs
- Capacity: aournd 500 Terabits per second (about 15'500 movie every second).
- Multiplexing: wavelength division (i.e. colors of light)
- Material: multiple layers
- Steel wire for mechanical
- Copper tubing for power conduction
- Polyethylene insulation for water proofing
- …etc.
- Maintenance: about 100 cable faults per year (mostly due to human actions)