Transmission Media

Guided Media

  • This refers to transmission media that carries a signal through a conductor.
  • As a result, it requires a physical linkage between sender and receiver.
  • In telecommunications, this linkage is often in the form of cables.

Twisted Pairs

  • These are simply two insulated copper wires that have been twisted in a spiral pattern.
  • Usually many of these pairs are bundled together into a single cable, which is encased in a protective sheath.
  • This interweaving is done to reduce electrical noise that could be introduced from outside sources.
  • Twisted pairs are among the most commonly used cables due to its low cost and flexibility.
  • It is also thin, allowing for more wires to be laid down in the same area.
  • Its frequency ranges up to 1MHz, and up to 10MHz for shorter distances.
  • It is easy to join, and most systems could work with it.
  • It has a moderate attenuation rate, with an amplitude loss of 0.5 per kilometre.
  • Also, normal twisted pair cables only offer moderate shielding from interference.
    • This can be improved by adding a layer of foil on the outside sheath.
    • Doing so makes it a shielded twisted pair.

Coaxial Cables

  • This is a cable composed of a conducting copper core, which is surrounded by an insulator and a metallic shield. The shielding is usually surrounded by an outer jacket which protects the cable.
  • This mixture of a metallic shield and insulation means it is much less susceptible to external noise than twisted pairs.
  • Frequency range up to 100MHz, and 1GHz for short distances.
    • Higher frequency range makes it more suited for modulated signals such as TV or high speed digital signals.
  • The shielding used is usually either a copper weave or a double sheathe to preserve the flexibility of the cable.
  • Yet its attenuation is worse, with a rate more than double that of twisted pairs.
  • It requires special systems to work and is more difficult to join.
  • Thus it could be used for weak signals that are susceptible to interference.

Optic Fibres

  • This is covered in detail here.
  • Optical fibres are practically immune to noise, as the only type of interference it can receive is light from outside sources. With the way it's cabled, this is close to impossible.
  • Frequency range of up to 320 THz (320 000 000 MHz) and thus allows for very large bandwidths.
  • Attenuation rate is amazing low (0.5 dB/km, less than 1/5 of Twisted cables).
  • Its major downfall lies in its maintenance. Damaged optical fibres are extremely difficult to repair and often require expensive equipment.
  • It exclusively uses binary, a digital system, to transfer information.

Unguided Media

  • This refers to media that is not physically connected, such as microwaves etc.