Page 54 - DCAP406_DCAP_207_Computer Networks

Unit 4: Physical Layer-2

Two transmitters cannot share the same frequency band because of mutual interference and Notes
therefore band usage is regulated.

Did u know? International use of the radio spectrum is regulated by the International
Telecommunication Union (ITU). Domestic use of the radio spectrum is regulated by
national agencies such as Wireless Planning and Coordination (WPC) in India. WPC
assigns each transmission source a band of operation, a transmitter radiation pattern, and
a maximum transmitter power.
Omni directional or directional antennas are used to broadcast radio waves depending upon
band. The transceiver unit, which is consisted of transmitter and receiver along with the antenna,
determines the power of RF signal. Other characteristics of radio waves is that in vacuum all
electromagnetic waves or radio waves travel at the same speed i.e. at the speed of light which is
equal to 3 × 108 meter per seconds. In any medium this speed gets reduced and also becomes
frequency dependent. In case of copper the speed of light becomes approximately two thirds of
the speed of light.

The basic features of the radio waves are that:
They are easy to generate
They have same velocity in vacuum

They may traverse long distances
They are omni directional
They can penetrate building easily so they find extensive use in communication both
indoor and outdoor

They are frequency dependent. At low frequency they can pass through obstacles well but
the power falls off sharply with distance from the source, as power is inversely proportional
to cube of the distance from the source. At HF they travel in straight lines and bounce off
obstacles.

4.1.2 Very Low Frequency (VLF)

The VLF method takes advantage of electromagnetic radiation generated in the low frequency
band of 3-30 KHz by powerful radio transmitters used in long-range communications and
navigational systems. At large distances from the source, the electromagnetic field is planar and
horizontal and the electric component E lies in a vertical plane perpendicular to the H component
in the direction of propagation and follow the ground. AM uses VLF band. This band of frequencies
cannot be used for data transfer because they offer relatively low bandwidth.

4.1.3 Microwave Transmission

Microwave radio, a form of radio transmission that uses ultra-high frequencies, developed out
of experiments with radar (radio detecting and ranging) during the period preceding World
War II. There are several frequency ranges assigned to microwave systems, all of which are in
the Giga Hertz (GHz) range and the wavelength in the millimeter range. This very short
wavelength gives rise to the term microwave. Such high frequency signals are especially
susceptible to attenuation and, therefore must be amplified or repeated after a particular distance.

In order to maximize the strength of such a high frequency signal and, therefore, to increase the
distance of transmission at acceptable levels, the radio beams are highly focused. The transmit

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