Study On Wavelengths And Computer Networks Computer Science Essay

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As we know there are many layers in the environment in different top level of the ground. Ionosphere is the layer at the top level of the atmosphere ionized by solar radiation, and with the reflective properties of very low frequency waves to the ground. When these waves are reflected in accordance with soil from the surface and back to a longer range. The waves have a frequency range from "3kHz - 30kHz is very low frequencies, so that these waves requires enough power. Low-frequency waves are not used for data transmission in computer networks, is why these waves requires high power. But it does at lower data rates.

ITU-R frequencies less able to stay away from the interference between radio communication in several countries. Shorter frequencies has greater range and travels with the Earth's surface, so that the frequencies controlled transfer of many countries that use the same frequency can interfere with each other.Large frequencies are directional and unusual for the shorter distances so That a very rare opportunity for the two restriction interferes Each Other. ITU-R not only handles frequencies, but also develop standards for these frequencies and the frequencies used for scientific purposes. If they are not controlled by ITU-R spectrum can improve the speed.

Question 2.4:

What are the two different approaches in regulation regarding mobile phone systems in Europe and the US? What are the consequences?


The classical approach is based on EU standards and requirements prior to manufacture, or any items are available. This also helps to improve technology in the future. Europe ETSI synchronize the digital GSM technology to control at 900 MHz and 1800 MHz, while the U.S. FCC acceptable number of mobile technologies in the same frequency band around 850 MHz. EU takes a common standard for all mobile systems to help fully compatible with the new and existing equipment. It is the widespread appearance of the systems. Mobile companies do not follow any standards in the U.S. Thus, the system for mobile phones, including limited area. Americans have many incompatible independent standard AMPS, TDMA, CDMA and GSM - 1900 In the U.S., the Federal Communications Commission (FCC) regulates only similarity between different systems. When the wireless LAN, the American standards (802.11) appear dominant in relation to the European systems.

Question 2.4:

What are the two different approaches in regulation regarding mobile phone systems in Europe and the US? What are the consequences?


EU communication standards outlined by the European Institute for Telecommunications, ie ETSI. This also helps to improve future technology. This system uses a general standard for all mobile systems that provide interoperability with the new equipment available causing the entire system development. Except when the Federal Communications Commission (FCC), an organization created to monitor all forms of communication. Mobile companies do not follow the strict rules in the U.S. for a limited range of mobile phones.

Two different schemes are listed below

• (TBRs) technical basis for a regulation drawn up by TRAC

• (ETS), developed the European Telecommunications Standards of the European (ETSI) Telecommunications Standards Institute.

A more European standard for networking systems for mobile radio, that use is (TETRA), Terrestrial Trunked Radio System. The purpose of this network is to provide both data and voice services for public and private, professional applications for mobile radio

use digital technology. The purpose is to develop more spectrum efficiency and performance of existing analog technology.

There are different types of consequences:

• quality and bandwidth provision.

• The main issue is heat

• Radiation, ionized / Non - ionizing radiation

Question 2.5:

Why is the international availability of the same ISM bands important?


Communication agree with all the interference generated by ISM equipment.ISM used for reserved and the special purpose task as Bluetooth, microwaves, etc., and this band is too low data rate communications. ISM does not require any license from any of the Company or any of the organization. ISM band at 2.4 GH is very important because it is used in the latest technologies such as Bluetooth and wireless communications. Today, in almost many electronic devices and Bluetooth and wireless start working.ISM need around the world available for free to mobile users.

Question 2.8:

What are the main problems of signal propagation? Why do radio waves not always follow a straight line? Why is reflection both useful and harmful?


The biggest problems with the signal propagation characteristics are: dispersion, attenuation, reflection, refraction and diffraction. These problems except the damping can lead to deflection of a straight line of waves from the original path.

Radio waves do not follow a straight line for the following reasons:

1: A Radio waves do not follow a straight line because of gravitational effects. If they travel in a vacuum, without gravity effect that can be spread in a straight line.

2: A medium has great effect on the feel of its density.

3: The change in refractive index

If those considerations did not exist when the transfer will be possible in the populated area, because it is (LOS) view. There is also a need for multi-path transmission.

Question 2.11:

Why, typically, is digital modulation not enough for radio transmission? What are general goals for digital modulation? What are typical schemes?


The purpose of digital modulation is to transmit a digital bit stream over an analog bandpass channel design, because it is a digital modulation techniques and data in the form of "0" or "1" is decoded into an analog signal for transmission. The need for digital modulation requires where there is a digital data to be transferred, but the medium allows only an analog broadcast channel. The purpose of digital modulation toughness against interference, efficiency and fading.

Case for digital modulation are:

1: Baseband transmission is used.

2: small antennas used.

Most commonly used modulation

1: Amplitude Shift Keying a (ASK):

2: A Phase Shift Keying (PSK):

3: Frequency Shift (FSK):

Question 2.13:

What are the main benefits of a spread spectrum system? How can spreading be achieved? What replaces the guard space in Figure 2.31 when compared to Figure 2.32? How can DSSS systems benefit from multipath propagation?


It provides a secure connection, fault resistance and avoid being detected easily. CDMA uses spread spectrum, and is also called "spread spectrum systems". It is very consistent from interference for CDMA.

Spread Spectrum is a method where a signal is modulated with a frequency higher than the original sending data can be obtained by XORing the original data with a predefined code known as a chipping code. Sender and receiver both know that the chipping code to encode and decode data streams correctly.

Spread may occur via two different ways.

Direct Sequence Spread Spectrum (DSSS): There is a communication tool used in the Lawn transmissions where a data signal at the sending station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. It takes a power down a bit and make a (XOR) with a so-called chipping sequence.

Frequency hopping spread spectrum (FHSS): It separates the total available bandwidth into many channels with less bandwidth plus space appears between the two channels. Transmitters and receivers stay at one of these channels for a while, then jump to another channel. The pattern of channel use is called the Hopping Sequence.

FHSS is available in two options, slow and fast jumps. In the slow hopping transmitter uses a frequency for a number of bit periods. Fast Hopping system asks the sender to change the rate several times during the broadcast of a single piece. The slow hopping systems are usually cheaper and has relaxed tolerances.

DSSS systems typically search receiver that combines the signals traveling along different paths. Recombination provides a stronger signal in relation to the strongest signal alone.

Question 2.15:

What limits the number of simultaneous users in a TDM/FDM system compared to a CDM system? What happens to the transmission quality of connections if the load gets higher in a cell, i.e., how does an additional user influence the other users in the cell?


FDM / TDM no limit users

• Speed synchronization between transmitter

• Bandwidth

• Time slots

• (ISI) noise power constraints

FDM / TDM systems have a hard / set a higher limit for concurrent users. The user assigns a time slot on a particular frequency. The user request is rejected when all shall be scheduled in advance. Flexible capacity is the most important element in the CDM. Normally, the CDM system signal to noise ratio reduces the number of simultaneous users. The system always has the option to accept an additional user. But the noise level is so high that the transfer will prove to be impossible. FDM / TDM users do not affect existing users when they varied in frequency and time. In CDM, by reducing the supply of signal-users the quality of the transfer of existing users.