# The Nyquist Theorem In Data Communication Computer Science Essay

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The coder part of the codec and demodulator part of the codec are way too different from each other. Write on some aspects of the difference between bÂ­Â­oth of them. Ans :- Computer data sent over a modem are analog. The analog signals are digitized in the end office by a device called a codec coder - decoder producing a series of 8-bit numbers. The code makes 8000 codec is any technology for compressing and decompressing data.In telecommunications, short for coder/decoder a device that encodes or decodes a signal. For example, telephone companies use codecs

The demodulator takes the wavy analog signal from the line and converts it do a digital signal. The codec puts the transmitted data together from the data the demodulator spits out.

## The Nyquist theorem is one of the deciding factor in data communication. The fibre optics as well as the copper wires are communication mediums. Do you think the theorem is valid for the fiber optics or for the copper wires.

Ans: Nyquist theorem says that if you have a function whose Fourier spectrum does not contain any sines or cosines above f, then by sampling the function at a frequency of 2f you capture all the information there is. Thus, the Nyquist theorem is true for all mediaNyquist theorem is a sampling theory, doesn't matter what the media is, as long as the transmission is on a TDM system. The Nyquist theorem is not applicable to copper wire.Â Â Because it is applicable to noiseless channel. The bandwidth of a copper wire depends on the cable quality, length, and signal to noise ratio of data transmitted. As we see copper bandwidth is directly depends upon the signal to noise. In fiber optic cable we transmit the pulse of light and the light is producedÂ Â due to the motion of molecules throughout the system that creates much noise. To calculate the noise created per signal we uses the concept of signal to noise ratio that is given by shanon;s not by nyquiest.

## Q3. Noise affects all the signals which are there in air. There are some communicating modulation techniques. Noise affects which of the modulation technique the most.

Ans : Noise-reducing circuits were ineffective against random fluctuation noise, but in the presence of certain types of impulse interference, limiters and canceling circuits provided such great improvement in performance that it was possible to maintain satisfactory communication despite a 35-decibel reduction in carrier intensity. As a general principle, it appears that, whenever there is a characteristic difference between the wave forms or the spectra of the signal and the interference, the impairment of intelligibility by electrical interference may be reduced by employing amplitude-selective or frequency-selective circuits in the radio receiver.In order to study the effects of noise upon radio communication,an amplitude- modulation radio system was set up in the laboratory and provision was made for generating electrical interference and introducing it, together with the speech-modulated carrier from the transmitter, into the receiver. In addition, arrangements were made for producing ambient noise at the talkers' and listeners' positions, so that the separate effects of acoustic noise and radio noise could be compared and their combined effects could be studied. By means of word articulation tests, the intelligibility of speech heard over the radio system was determined as a function of signal-to-noise ratio for each of a number of types of noise.

Ans-

S.No.

Bits

Manchester

a)

1111

10101010

b)

0101

01100110

c)

0000

01010101

d)

1100

10100101

## Q5 The DC component plays a role in designing encoding scheme. Can we relate the DC component's effect on all the encoding schemes? Justify your answer by giving an example.

Ans:- DC components: After line coding, the signal may have zero frequency component in the spectrum of the signal, which is known as the direct-current (DC) component. DC component in a signal is not desirable because the DC component does not pass through some components of a communication system such as a transformer. This leads to distortion of the signal and may create error at the output. The DC component also results in unwanted energy loss on the line.

## Write the factors on which the combination is happening.

Ans: Multiplexing - The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side. the process of combining signals from multiple sources for transmission across a single link.

Channel - a portion of the path that carries a transmission between a mux and demux.

## Multiplexing Techniques

The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side

There are two predominant ways to multiplex:

â€¢Frequency Division Multiplexing

â€¢Time Division Multiplexing

## Frequency Division Multiplexing (FDM)

Frequency-division multiplexing (FDM) is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium. FDM can also be used to combine multiple signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal.

In FDM, multiple channels are combined onto a single aggregate signal for

transmission. The channels are separated in the aggregate by their frequency.

FDM was the first multiplexing scheme to enjoy wide scale network deployment, and

such systems are still in use today. However, Time Division Multiplexing is the preferred

approach today, due to its ability to support native data I/O (Input/Output) channels.

## Time Division Multiplexing

Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel. After the last sub-channel the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.

## Q8 While transferring the data from the transmission medium there are various aspects of your data getting tempered by other users. What in your opinion is the most secure and insecure transmission medium? Justify your answer with an example.

Ans:- There are three broad categories of media: Wire, fiber and wireless. A wire is a single, usually cylindrical, string of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various wire gauges. The term wire is also used more loosely to refer to a bundle of such strands, as in 'multistranded wire', which is more correctly termed a wire rope in mechanics, or a cable in electricity. Wireless network refers to any type of computer network that is wireless, and is commonly associated with a telecommunications network whose interconnections between nodes is implemented without the use of wires. Wireless telecommunications networks are generally implemented with some type of remote information transmission system that uses electromagnetic waves, such as radio waves, for the carrier and this implementation usually takes place at the physical level or "layer" of the network. On a very high level, it could be said that fiber is the most secure as it is the hardest to tap. Cable is a little more secure, as physical access is not hard to tap and sniff the passing traffic. Then there is wireless, it broadcasts point-to-point or well beyond the facility in all possible directions. Anyone that can pick up the signal may be able to sniff sensitive information. Although there is a second item we must consider, and that is the physical and technical controls that have been implemented. Wireless can be made more secure by using WPA, encryption or 802.1x. A cabled network can be fully switched, use encryption and have implemented VLANS. So the point would be that even seemingly weaker systems can have controls implemented to make their security more robust.