TYPES OF UNGUIDED MEDIA

Unguided Media

     In unguided media transmission, messages or electromagnetic signals travel through air or space. Cables, connectors and other solid conductors are not required in unguided media systems. Wireless communication is particularly useful in those regions where cables are difficult to implement or install. The main types of unguided media are, Radio waves, Microwaves and Infrared waves.
The three main types of wireless media are:
  • Radio waves
  • Microwaves
  • Infrared waves

Radio Wave Links

     Electromagnetic waves in the frequency range of 3 kHz to 1 GHz are commonly known as radio waves. There are basically two types of configuration for electromagnetic wave transmission and reception, directional and omni-directional. In directional configuration, an antenna radiates electromagnetic energy as a focused beam in a particular direction. The receiving antenna should be aligned in that direction for receiving the signals. In omni-directional configuration, an antenna radiates electromagnetic energy equally in all directions. This implies that transmitting antenna and receiving antenna do not have to be aligned as in directional configuration. Also here electromagnetic waves from a transmitting antenna can be received by many users.
Radio waves are basically omni-directional in nature. They use omni-directional antennas for transmission and reception of electromagnetic signals. Radio waves are used in AM and FM broadcast systems, television, cordless phones, paging and a wide variety of other multi-casting applications.

Terrestrial Microwave Links

     Frequencies in the range of about 1 GHz to 300 GHz are commonly known as microwaves. Microwaves are narrow beams and hence they are directional in nature. Microwaves are usually transmitted through air between two stations. Thus in microwave systems, the transmitting antenna and receiving antenna should be aligned. Microwave propagation is also termed as line-of-sight propagation. This is because transmitting and receiving antenna should be in visible contact with each other. Terrestrial microwave communication is commonly used in situations where cables are difficult to install. Such situations include communication across sea, mountains, large buildings and remote areas. Microwaves are widely used in point-to-point communication systems such as mobile communication, satellite systems and wireless LANs.




Infrared Links

     Frequencies in the range from 300 GHz to 400 THz are the infrared waves. They are used for short-range communications such as data transfer between two cell phones in one room, TV remote control operation and data transfer between a computer and a cell phone that resides in the same closed area. Line-of-sight propagation mode is employed in all applications of infrared waves. That means, transmitter and receiver must be aligned with respect to each other so that nothing obstructs the path of infrared wave.
Characteristics
  • Potentially large bandwidth available (about 400 THz) and hence data rate will be very high.
  • Infrared waves cannot penetrate walls. Thus infrared communication in one room is not affected by communication in the nearby rooms.
  • Infrared communication guarantees better security with minimum interference.
  • License from standard authorities is not required.
  • Computers within a room or building can be configured to communicate with each other using microwaves. Such an arrangement is commonly known as indoor wireless LAN.
  • Infrared waves are unreliable for communication outside a building as the sun rays may interfere with the infrared waves.


Satellite Microwave Links

     Satellite microwave links are used for long distance communication. Communication satellites are launched by humans to run a wide variety of applications including commercial, military, governmental and private. They are basically microwave repeaters. The functional parts of a microwave repeater are receiver, transmitter, amplifier, re-generator, filter, multiplexer, demultiplexer and antenna. A Satellite radio repeater is termed as a transponder. A communication satellite includes multiple transponders. Microwave signal frequencies from one earth station is received, amplified and redirected to another earth station by the transponders.
Applications
  1. Television and radio transmission
  2. Air navigation
  3. Long-haul telephone transmission
  4. Global Positioning Systems (GPS)
  5. Video conferencing and multimedia applications
  6. Virtual Private Networks (VPN)
  7. Very Small Aperture Terminal System (VSAT)

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