v  Transmission and reception are achieved by means of an antenna

v  Unguided media are natural parts of the existing environment that can be used as physical paths to carry electrical signals. Earth’s atmosphere and outer space are examples of unguided media that are commonly used to carry signals. These media can carry such electromagnetic signals as microwave and infrared light waves.

v  Unguided/non-physical/wireless/unbounded media have no physical link between sender and receiver.

v  The unguided media is the wireless media. It simply transports electromagnetic waves without using any physical conductor. Signals are normally broadcast through the air and thus are available to any one who has the device capable of receiving them.

v  Unguided signals can be travelled from source to the destination in several ways. These ways include ground propagation, sky propagation and line of sight propagation.

v  In the ground propagation, the radio waves travel through the lowest portion of atmosphere, hugging the earth. These very low frequency signals emanate in all directions from transmitting antenna and follow the curvature of planet. In sky propagation, the higher frequency radio waves radiate upward into the ionosphere, where they are reflected back to the earth.

v  In the line of sight propagation, very high frequency signals are transmitted in straight lines directly from the antenna to antenna. Antennas must be directional, facing each other and either tall enough or close enough together not to be affected by curvature of the earth. The line of sight propagation is tricky as radio transmissions can not be completely focused. Infra red waves are used for the short range communication such as those between a PC and the peripheral device.

v  Some people believe that the future holds only two kinds of communication: fiber and wireless. All fixed (i.e., nonmobile) computers, telephones, faxes, and so on will use fiber, and all mobile ones will use wireless.

v  Wireless has advantages for even fixed devices in some circumstances. For example, if running a fiber to a building is difficult due to the terrain (mountains, jungles, swamps, etc.), wireless may be better. It is noteworthy that modern wireless digital communication began in the Hawaiian Islands, where large chunks of Pacific Ocean separated the users and the telephone system was inadequate.

v  Wireless media include radio frequencies, microwave, satellite, and infrared.

v  Deployment of wireless media is faster and less costly than deployment of cable, particularly where there is little or no existing infrastructure. Wireless is also useful where environmental circumstances make it impossible or cost-prohibitive to use cable (e.g., in the Amazon, in the Empty Quarter in Saudi Arabia, on oil rigs).

 

Radio Wave

Figure: Radio-based LANs use portable transmitters and receivers at each LAN device

 

RF Propagation

v  A network that uses electromagnetic radio waves operates at radio frequency and its transmissions are called RF transmissions.

v  Each host on the network attaches to an antenna, which can both send and receive RF.

 

Characteristics of radio

v              Signal carried in electromagnetic spectrum

v No physical “wire”

v Multi-directional

v              Propagation environment effects:

          Reflection

          Obstruction by objects

          Interference

There are three types of RF (radio frequency) propagation:

 

Ground Wave

v  Ground wave propagation follows the curvature of the Earth. Ground waves have carrier frequencies up to 2 MHz.

v  AM radio is an example of ground wave propagation.

Ionosphere

v  Ionosphere propagation bounces off of the Earth’s ionosphere layer in the upper atmosphere. It is sometimes called double hop propagation. It operates in the frequency range of 30 – 85 MHz. Because it depends on the Earth’s ionosphere, it changes with the weather and time of day. The signal bounces off of the ionosphere and back to earth.

v  Ham radios operate in this range.

Line of Sight (LOS)

v  Line of sight propagation transmits exactly in the line of sight. The receive station must be in the view of the transmit station. It is sometimes called space waves or troposphere propagation. It is limited by the curvature of the Earth for ground-based stations (100 km, from horizon to horizon). Reflected waves can cause problems. Examples of line of sight propagation are: FM radio, microwave and satellite.

 

Microwave

v  Microwaves operate at high operating frequencies of 3 to 10 GHz. This allows them to carry large quantities of data due to their large bandwidth.

v  Terrestrial (Focused Beam) Microwave

Types of antenna for microwave communication

 

Characteristics of radio Terrestrial (Focused Beam) Microwave

v Fewer amplifiers required than with coax

v              Traverse rough/impassable terrain

v              Large number of channels per antenna

v              Velocity of propagation of signal = c

v              Affected by rain for frequencies > 10GHz

v              Interference with radio

v              Long-haul communications

v              Higher freq =) higher data rates

Satellite Microwave

v  Satellite (‘Bird’) as relay station:

       Receives on one frequency, amplifies or reflects signal

       Transmits on another frequency/another direction

v  Used for:

       Television

       Long distance

       Telephone

       Space exploration

       Private business networks

 

Advantages:

v  They require no right of way acquisition between towers.

v  They can carry high quantities of information due to their high operating frequencies.

v  Low cost land purchase: each tower occupies only a small area.

v  High frequency/short wavelength signals require small antennae.

 

Disadvantages:

v  Attenuation by solid objects: birds, rain, snow and fog.

v  Reflected from flat surfaces like water and metal.

v  Diffracted (split) around solid objects.

v  Refracted by atmosphere, thus causing beam to be projected away from receiver.

Infrared

v  Infrared technology uses the invisible portion of the light spectrum with wavelengths just a little less than those of red light. These frequencies are very high offering nice data transfer rates. Modern infrared LANs can achieve throughput at 16 Mbps with potential for better. We are used to seeing infrared technology utilized for our television or VCR remotes.

v  Characteristics of Infrared

          Modulated non-coherent infrared light

          Line of Sight (or reflection)

          Blocked by IR-opaque objects (walls, et cetera)

v  Examples:

          TV remote control

          IRD port

          Alarm sensor systems

v  There are a few disadvantages associated with wireless, however. Historically, wireless solutions support much lower data rates than do wired solutions, although with new developments in wireless broadband, that is becoming less of an issue (see Part IV, “Wireless Communications”).

v  Wireless is also greatly affected by external impairments, such as the impact of adverse weather, so reliability can be difficult to guarantee. However, new developments in laser-based communications—such as virtual fiber—can improve this situation.

v  Of course, one of the biggest concerns with wireless is security: Data must be secured in order to ensure privacy.