A transmission medium can be broadly defined as anything that can carry information from a source to a destination.  The transmission medium is usually free space, metallic cable, or fiber-optic cable.

Transmission media can be generally categorized as either unguided or guided.

Guided Media

In the guided media, the data signals are sent along a specific path, through a wire or a cable. Copper wire and optical fibers are the most commonly used guided media that transmits data as electric signals.

Copper wires offer low resistance to current signals, facilitating signals to travel longer distances.

Types of guided media:

  • Twisted Pair
  • Coaxial Pair
  • Optical fiber

1. Twisted Pair Cable

A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together.

One of the wires is used to carry signals to the receiver, and the other is used only as a ground reference. 

  • Twisting tends to decrease the crosstalk
  • Crosstalk is the interference due to the magnetic filed of 2 wires nearby
  • Used to transmit both analog and digital transmission

The two basic types of twisted-pair transmission lines specified are unshielded twisted pair (UTP) and shielded twisted pair (STP).

STP cable has an extra layer of metal foil between the twisted pair of copper wires and the outer covering. The metal foil covering provides additional protection from external disturbances. However, the covering increases the resistance to the signal and thus decreases the length of the cable. STP is costly and is generally used in networks where cables pass closer to devices that cause external disturbances.

UTP is the most commonly used medium for transmission over short distances up to 100m. Out of the four pairs of wires in a UTP cable, only two pairs are used for communication.

2. Coaxial Cable

Coaxial cable has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and as the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover.

  • Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair
  • Coaxial cable is a versatile transmission medium, used in a wide variety of applications, including: Television distribution - aerial to TV systems

3. Fiber Optic Cable

 A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. An optical fiber cable consists of

  1. Core - optical fiber conductor (glass) that transmits light,
  2. Cladding - an optical material that surrounds the core to prevent any light from escaping the core, and
  3. Jacket - outer covering made of plastic to protect the fiber from damage.

Optical fibers are being used for transmission of information over large distances more cost effectively than the copper wire connection. Communication systems are now unthinkable without fiber optics. 

Unguided Media

Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them. Radio, microwave and satellite transmissions fall into this category.

1. Radio Transmission

The electromagnetic radio waves that operate at the audio frequency are also used to transmit computer data. This transmission is also known as Radio Frequency (RF) transmission. The computers using RF transmission do not require a direct physical connection like wires or cable. Each computer attaches to an antenna hat can both send and receive radio transmission.

 2. Microwave Transmission

Microwave transmission refers to the technique of transmitting information over a microwave link. Microwaves have a higher frequency than radio waves. Microwave transmission can be aimed at a single direction, instead of broadcasting in all directions (like in radio waves). Microwaves can carry more information than radio waves but cannot penetrate metals. Microwaves are used where there is a clear path between the transmitter and the receiver.

Microwave transmission has the advantage of not requiring access to all contiguous land along the path of the system, since it does not need cables. 

Disadvantages:  a) Needs expensive towers and repeaters, and

  1. b) They are subject to interference from passing airplanes and rain.

3. Satellite Transmission

The communication across longer distances can be provided by combining radio frequency transmission with satellites. Geosynchronous satellites are placed in an orbit synchronized with the rotation of the earth at a distance of 36,000 km above the surface of the earth. Geosynchronous satellites appear to be stationary when viewed from the earth. The satellite consists of transponder that can receive RF signals and transmit them back to the ground at a different angle. A ground station on one side of the ocean transmits signal to the satellite which in turn sends the signal to the ground station on the other side of the ocean.