Communication Engg. Qb

COMMUNICATION ENGINEERING EE T56

C.THIAGARAJAN-AP/ECE 1

UNIT-I

MODULATION SYSTEMS

1. What is communication?

The imparting or exchanging of information by speaking, writing, or using some

other medium. In other words sending or receiving information, such as telephone lines

or computers which uses a wire/wireless medium.

2. What is analog communication?

Analog Communication is a data transmitting technique in a format that utilizes

continuous signals to transmit data including voice, image, video, electrons etc

3. Draw the simple block diagram of communication system.

4. Define modulation.

Modulation is the process of varying one or more properties of a periodic

waveform, called the carrier signal, with a modulating signal which typically contains

information to be transmitted.

5. What is amplitude modulation?

Amplitude Modulation is the process of changing the amplitude of a relatively

high frequency carrier signal in proportion with the instantaneous value of the modulating

signal.

6. Define modulation index.

Modulation index is a term used to describe the amount of amplitude change

present in an AM waveform. It is also called as coefficient of modulation.

Mathematically modulation index is

m = Em/Ec

Where m = Modulation coefficient

Em = Peak change in the amplitude of the output waveform voltage.

Ec = Peak amplitude of the unmodulated carrier voltage.

Percent modulation gives the percentage change in the amplitude of the output

wave when the carrier is acted on by a modulating signal.



7. Draw the diagram of amplitude modulation process.

8. Define demodulation.

Demodulation is the act of extracting the original information-bearing signal from

a modulated carrier wave. Therefore it is called as demodulation.

9. What are the various advantages of AM Modulation?

It is simple to implement

it can be demodulated using a circuit consisting of very few components

AM receivers are very cheap as no specialized components are needed.

10. List the disadvantages of AM modulation.

It is not efficient in terms of its power usage

It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency

11. Define SSBM.

Single sideband modulation is widely used in the HF portion, or short wave

portion of the radio spectrum for two way radio communication.

12. Define radio communication.

Radio communication is defined as establishing end to end link through the

Electromagnetic radiation with lower frequencies and longer wavelengths than those of

microwaves, having frequencies lower than 300 megahertz and wavelengths longer than

1 meter.

13. What is the purpose of carrier insertion oscillator?

This generates a carrier signal that can be mixed with the incoming SSB signal,

thereby enabling the required audio to be recovered in the detector.



14. List the advantages of SSBM.

It has several advantages for two way radio communication that more than

outweigh the additional complexity required in the SSB receiver and SSB transmitter

required for its reception and transmission.

15. Define Frequency Modulation.

Frequency modulation (FM) is the encoding of information in a carrier wave by

varying the instantaneous frequency of the wave.

16. What are the advantages of FM?

It has resilience to noise.

Easy to apply modulation at a low power stage at the transmitter

It is possible to use efficient RF amplifier with frequency modulated signals.

17. Draw the block diagram of FM?

18. Define capture effect.

In telecommunications, the capture effect, or FM capture effect, is a phenomenon

associated with FM reception in which only the stronger of two signals at, or near, the

same frequency will be demodulated.

19. Draw the signal diagram of FM.

20. Define time domain.

Time considered as an independent variable in the analysis or measurement of

time-dependent phenomena.



21. Define frequency domain.

In electronics, control systems engineering, and statistics, the frequency domain

refers to the analysis of mathematical functions or signals with respect to frequency,

rather than time

22. What is signal?

A signal as referred to in communication systems, signal processing, and

electrical engineering "is a function that conveys information about the behavior or

attributes of some phenomenon

23. Define super-heterodyne receiver

Heterodyne means to mix two frequencies together in a nonlinear device or to

translate one frequency to another using nonlinear mixing. There are five sections to a

super heterodyne receiver: that are RF section, Mixer/converter section, IF section, Audio

detector section, and Amplifier section.

24. Define multiplexing

Multiplexing is the transmission of information from one or more source to one or

more destination over the same transmission medium.

25. Define Frequency division multiplexing

Frequency Division Multiplexing (FDM) allows engineers to utilize the extra

space in each wire to carry more than one signal. By frequency-shifting some signals by a

certain amount, engineers can shift the spectrum of that signal up into the unused band on

that wire.

26. What is Pulse code modulation?

In pulse code modulation, analog signal is sampled and converted to fix length,

serial binary number for transmission. The binary number varies according to the

amplitude of the analog signal.

27. Define Low level Modulation. In low level modulation, modulation takes place prior to the output element of the

final stage of the transmitter. For low level AM modulator class A amplifier is used.

28. Define High level Modulation. In high level modulators, the modulation takes place in the final element of the

final stage where the carrier signal is at its maximum amplitude. For high level modulator

class C amplifier is used.

29. What is the advantage of low level modulation?

An advantage of low level modulation is that less modulating signal power is

required to achieve a high percentage of modulation.



30. Distinguish between low level and high level modulation.

In low level modulation, modulation takes place prior to the output element of the

final stage of the transmitter. It requires less power to achieve a high percentage of

modulation. In high level modulators, the modulation takes place in the final element of

the final stage where the carrier signal is at its maximum amplitude and thus, requires a

much higher amplitude modulating signal to achieve a reasonable percent modulation.

31. Define image frequency.

An image frequency is any frequency other than the selected radio frequency

carrier that, if allowed to enter a receiver and mix with the local oscillator, will produce a

cross product frequency that is equal to the intermediate frequency.

32. Define Local Oscillator tracking.

Tracking is the ability of the local oscillator in a receiver to oscillate either above

or below the selected radio frequency carrier by an amount equal to the intermediate

frequency throughout the entire radio frequency band.

33. Define High side injection tracking.

In high side injection tracking, the local oscillator should track above the

incoming RF carrier by a fixed frequency equal to fRF +fIF.

34. Define Low side injection tracking.

In low side injection tracking, the local oscillator should track below the RF

carrier by a fixed frequency equal to fRF -fIF.

35. Define tracking error. How it is reduced.

The difference between the actual local oscillator frequency and the desired

frequency is called tracking error. It is reduced by a technique called three point tracking.

36. Define image frequency rejection ratio.

The image frequency rejection ratio is the measure of the ability of pre selector to

reject the image frequency.

37. Define Heterodyning.

Heterodyne means to mix two frequencies together in a nonlinear device or to

translate one frequency to another using nonlinear mixing.

38. What are the disadvantages of conventional (or) double side band full carrier system?

In conventional AM, carrier power constitutes two thirds or more of the total

transmitted power. This is a major drawback because the carrier contains no information;

the sidebands contain the information. Second, conventional AM systems utilize twice as

much bandwidth as needed with single sideband systems.



39. Define Single sideband suppressed carrier AM.

AM Single sideband suppressed carrier is a form of amplitude modulation in

which the carrier is totally suppressed and one of the sidebands removed.

40. Define AM Vestigial sideband.

AM vestigial sideband is a form of amplitude modulation in which the carrier and

one complete sideband are transmitted, but only part of the second sideband is

transmitted.

41. What are the advantages of single sideband transmission?

The advantages of SSBSC are

Power conservation: Normally, with single side band transmission, only one sideband is transmitted and the carrier is suppressed. So less power is required to

produce essentially the same quality signal.

Bandwidth conservation: Single sideband transmission requires half as much bandwidth as conventional AM double side band transmission.

Noise reduction: Because a single side band system utilizes half as much bandwidth as conventional AM, the thermal noise power is reduced to half that of

a double side band system.

42. What are the disadvantages of single side band transmission?

1. Complex receivers: Single side band systems require more complex and

expensive receivers than conventional AM transmission.

2. Tuning Difficulties: Single side band receivers require more complex and

precise tuning than conventional AM receivers.

43. Define direct frequency modulation.

In direct frequency modulation, frequency of a constant amplitude carrier signal is

directly proportional to the amplitude of the modulating signal at a rate equal to the

frequency of the modulating signal.

44. Define indirect frequency Modulation.

In indirect frequency modulation, phase of a constant amplitude carrier directly

proportional to the amplitude of the modulating signal at a rate equal to the frequency of

the modulating signal.

45. Define instantaneous frequency deviation.

The instantaneous frequency deviation is the instantaneous change in the

frequency of the carrier and is defined as the first derivative of the instantaneous phase

deviation.



46. Define frequency deviation.

Frequency deviation is the change in frequency that occurs in the carrier when it

is acted on by a modulating signal frequency. Frequency deviation is typically given as a

peak frequency shift in Hertz (Df).The peak to peak frequency deviation (2Df) is

sometimes called carrier swing. The peak frequency deviation is simply the product of

the deviation sensitivity and the peak modulating signal voltage and is expressed

mathematically as Df=K1 Vm Hz

47. State Carson rule.

Carson rule states that the bandwidth required transmitting an angle modulated

wave as twice the sum of the peak frequency deviation and the highest modulating signal

frequency. Mathematically carsons rule is B=2(Df +fm) Hz.

48. Define Deviation ratio.

Deviation ratio is the worst case modulation index and is equal to the maximum

peak frequency deviation divided by the maximum modulating signal frequency.

Mathematically, the deviation ratio is

DR= Df (max)

fm(max)

49. Define Percentage modulation for an AM wave.

Percent modulation gives the percentage change in the amplitude of the output

wave when the carrier is acted on by a modulating signal.

50. What is the need for modulation?

Base band transmission has many limitations which can be overcome by using

modulation, since modulation shifts low frequency to high and reduce the size of the

antenna, avoid mixing of signals, increase the range and quality of communication

51. State the advantages of FM over AM.

Frequency modulation is one type of angle modulation this has the advantages

over amplitude modulation such as Noise reduction, improved system fidelity and more

efficient use of power.

52. Define standing wave ratio.

Standing wave the is ratio between the reflected wave and normal wave

components voltage/current at any point is known as SWR, the plot of resultant voltage

or current along the line length is termed as standing waves.



53. What are the advantages of the super heterodyne receiver?

Advantages in SH Receiver over TRF receivers are Improved the selectivity in

terms of adjacent channels, more uniform selectivity over frequency range, improved in

receiver stability, higher gain per stage and uniform bandwidth.

54. Explain the disadvantages of conventional/double side band full carrier system.

In conventional AM, carrier power constitutes two thirds or more of the total

transmitted power. This is a major drawback because the carrier contains no information;

the sidebands contain the information. Second, conventional AM systems utilize twice as

much bandwidth as needed with single sideband systems.

55. Define Single sideband suppressed carrier AM.

AM Single sideband suppressed carrier is a form of amplitude modulation in

which the carrier is totally suppressed and one of the sidebands removed.

56. Define AM Vestigial sideband.

AM vestigial sideband is a form of amplitude modulation in which the carrier and

one complete sideband are transmitted, but only part of the second sideband is

transmitted.

57. What are the advantages of single sideband transmission?

The advantages of SSBSC are

1. Power conservation: Normally, with single side band transmission, only one

sideband is transmitted and the carrier is suppressed. So less power is required to produce

essentially the same quality signal.

2. Bandwidth conservation: Single sideband transmission requires half as much

bandwidth as conventional AM double side band transmission.

3. Noise reduction: Because a single side band system utilizes half as much

bandwidth as conventional AM, the thermal noise power is reduced to half that of a

double side band system.

58. What are the disadvantages of single side band transmission?

1. Complex receivers: Single side band systems require more complex and

expensive receivers than conventional AM transmission.

2. Tuning Difficulties: Single side band receivers require more complex and

precise tuning than conventional AM receivers.

59. Define direct frequency modulation.

In direct frequency modulation, frequency of a constant amplitude carrier signal is

directly proportional to the amplitude of the modulating signal at a rate equal to the

frequency of the modulating signal.



60. Define indirect frequency Modulation.

In indirect frequency modulation, phase of a constant amplitude carrier directly

proportional to the amplitude of the modulating signal at a rate equal to the frequency of

the modulating signal.

61. Define critical frequency.

Critical frequency is the maximum frequency that the radio wave can be

transmitted vertically and still some of the frequency will be refracted back to the

transmitter.

62. Define instantaneous frequency deviation.

The instantaneous frequency deviation is the instantaneous change in the

frequency of the carrier and is defined as the first derivative of the instantaneous phase

deviation.

63. Define frequency deviation.

Frequency deviation is the change in frequency that occurs in the carrier when it

is acted on by a modulating signal frequency. Frequency deviation is typically given as a

peak frequency shift in Hertz (f).The peak to peak frequency deviation (2f) is sometimes called carrier swing. The peak frequency deviation is simply the product of

the deviation sensitivity and the peak modulating signal voltage and is expressed

mathematically as f=K1 x Vm Hz.

UNIT II

TRANSMISSION MEDIUM

1. Define transmission line. A transmission line is a metallic conductor system that is used to transfer electrical

energy from one point to another. A transmission line is two or more conductors separated by an insulator, such as a pair of wires or a system of wire pairs.

2. Define balanced transmission line. In balanced transmission line, both conductors carry current; one conductor carries

the signal and the other is the return. This type of transmission is called differential or balanced signal transmission.

3. Define unbalanced transmission line.

In unbalanced transmission line, one wire is at ground potential where as the other wire is at signal potential. This type of transmission is called single ended or unbalanced signal transmission.



4. Define Open wire transmission line.

An open wire transmission line is a two wire parallel conductor. It consists simply of two parallel wires, closely spaced and separated by air. Nonconductive spacers are placed at periodic intervals for support and to keep the dielectric between the conductors constant. The dielectric is simply the air between and around the two conductors in which the TEM wave propagates.

5. What are the advantages of open wire transmission line? a. Simple in construction b. Radiation losses are high c. It is susceptible to noise pickups.

6. Define twisted pair cable. It is formed by twisting two conductors. Pairs are generally stranded in units and the

units are cabled into cores. The cores are covered with various types of sheaths neighboring pairs are twisted with different pitch to reduce interference between pairs due to mutual conduction.

7. Define shielded cable transmission line.

In shielded cable transmission line, parallel two wire transmission lines are enclosed in a metallic conductive metal braid to reduce the radiation losses and interference. The metal braid is connected to ground acts as shield. The braid also prevents signal radiation from reaching the conductors.

8. Define concentric transmission line.

Coaxial or concentric conductors are used for high frequency applications to reduce losses and to isolate transmission paths. The basic coaxial cable consists of a center conductor surrounded by a concentric conductor.

9. Describe the electrical and physical properties of a transmission line.

The electrical properties of a transmission line are wire conductivity and insulator

dielectric constant. The physical properties are wire diameter and conductor spacing.

10. List and describe the four primary constants of a transmission line.

The primary constants of a transmission line are series dc resistance, series

inductance, shunt capacitance, and shunt conductance. The primary constants are uniformly distributed throughout the length of the line and therefore are commonly called distributed parameters.

11. List the secondary constants of a transmission line. Secondary constants of a transmission line are

Characteristic impedance & Propagation constant



12. Define characteristic impedance for a transmission line.

Characteristic impedance is defined as the impedance seen looking into an infinitely long line or the impedance seen looking into a finite length of line that is terminated in a purely resistive load equal to the characteristic impedance of the line. It is also called as surge impedance.

13. Define propagation constant. Propagation constant is used to express the attenuation (signal loss) and the phase

shift per unit length of a transmission line. It is also called as propagation coefficient.

14. Define velocity factor for a transmission line.

Velocity factor (sometimes called velocity constant) is defined as the ratio of the

actual velocity of propagation through free space. Mathematically the velocity factor is Vf=vp/c, Where vf= velocity factor

Vp=actual velocity of propagation, C=velocity of propagation through free space

(3X108 m/s)

15. List the types of transmission line losses. Transmission line losses are conductor loss, radiation loss, dielectric heating loss,

coupling loss, and corona.

16. Define incident wave. An ordinary transmission line is bidirectional; power can propagate equally well in

both directions. Voltage that propagates from the source toward the load is called incident voltage

17. Define reflected wave. The voltage that propagates from the load toward the source is called reflected

voltage.

18. Define resonant line.

A transmission line with no reflected power is called a flat or resonant line.

19. Define non resonant transmission line. A transmission line is non resonant if it is of finite length or if it is terminated with a

resistive load equal in ohmic value to the characteristic impedance of the transmission line.

20. Define reflection coefficient.

The reflection coefficient is a vector quantity that represents the ratio of reflected voltage to incident voltage or reflected current to incident current .Mathematically, the reflection coefficient is gamma, , defined by

=Er/Ei (or) Ir/Ii



Where =reflection coefficient (unitless) Ei =incident voltage (volts)

Er =reflected voltage (volts) Ir = reflected current (amps), Ii =incident current (amps)

21. Define matched line.

When Zo=ZL, all the incident power is absorbed by the load .This is called a matched line. Where Zo=characteristic impedance ZL= load

impedance

22. Define unmatched line. When Zo ZL, some of the incident power is absorbed by the load and some is

returned to the source. This is called an unmatched or mismatched line. Where Zo=characteristic impedance

ZL= load impedance

23. Define standing wave.

In unmatched line, some of the incident power is absorbed by the load and some is

returned to the source. So there are two electromagnetic waves, traveling in opposite direction present on the line at the same time. The two traveling waves setup an interference pattern known as standing wave.

24. Define standing wave ratio.

The standing wave ratio is defined as the ratio of the maximum voltage to the minimum voltage (or) the maximum current to the minimum current of a standing wave on a transmission line.SWR is often called the voltage standing wave ratio (VSWR).

SWR= Vmax Vmin

25. Define ground wave propagation.

A ground wave is an electromagnetic wave that travels along the surface of earth. Therefore ground waves are sometimes called surface waves. Ground waves must be vertically polarized.

26. What are the disadvantages of ground wave propagation? 1. Ground waves require relatively high transmission power.

2. Ground waves are limited to very low, low, and medium frequencies, requiring large

antennas.

27. What are the advantages of ground wave propagation? 1. Ground waves are relatively unaffected by changing atmospheric conditions. 2. If the transmitted power is large enough, then ground wave propagation can be used to communicate between any two points in the world.



28. Define space wave propagation.

Space wave propagation of electromagnetic energy includes radiated energy that travels in the lower few miles of earths atmosphere. Space waves include both direct and ground reflected waves. Direct waves travel essentially in a straight line between the transmitting and receiving antennas. Space wave propagation with direct waves is commonly called line of sight transmission.

29. Define sky waves. Electromagnetic waves that are directed above the horizon level are called sky waves.

30. Define critical frequency. The critical frequency is defined as the highest frequency that can be propagated

directly upward and still be returned to earth by the ionosphere.

31. Define virtual height. Virtual height is the height above earths surface from which a refracted wave

appears to have been reflected.

32. Define maximum usable frequency.

Maximum usable frequency is the highest frequency that can be used for sky wave propagation between two specific points on earths surface.

33. What are baluns? It is special type of electrical transformer used to connect an unbalanced circuit to a

balanced one.

34. What are the various transmission losses? The various transmission line losses are conductor loss, radiation loss, dielectric heating loss, coupling loss, and corona.

35. What is meant by path loss? Path loss (or path attenuation) is the reduction in power density (attenuation) of an

electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system.

36. What is parallel conductor transmission line? A balanced line/parallel conductor is a transmission line consisting of two conductors

of the same type, and equal impedance to ground and other circuits. There are many formats of balanced lines; amongst the most common are twisted pair, star quad and twin-lead.

37. Define TOR. It is defined as ray travelling over path that involves a reflection from the surface of

ground.

38. What is fading? Fading is a statistical model for the effect of a propagation environment on a radio

signal, such as that used by wireless devices.



39. Define line of sight. Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic

wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line

40. What do you mean by free space? A zero particles in the location which does not have any radiation or wave

propagation etc. such space is referred as free space.

41. What is radio horizon? The horizon (or skyline) is the apparent line that separates earth from sky, the line

that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon.

42. What are the disadvantages of coaxial cable?

Very expensive due to fabrication.

It can be used only in unbalanced mode.

43. Define velocity constant. It defined as the ratio of the actual velocity of propagation through free space.

Therefore it is called as velocity constant.

44. What is coaxial cable? Coaxial cable, or coax is a type of cable that has an inner conductor surrounded by a

tubular insulating layer, surrounded by a tubular conducting shield.

45. Define fiber optic cable. An optical fiber cable is a cable containing one or more optical fibers that are used to

carry light. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed.

46. Define wired communication. Wired communication refers to the transmission of data over a wire-based

communication technology. Examples include telephone networks, cable television or internet access, and fiber-optic communication.

47. What is wireless communication? Wireless communication is the transfer of information between two or more points

that are not connected by an electrical conductor. The most common wireless technologies use radio. With radio waves distances can be short, such as a few meters for television or as far as thousands or even millions of kilometers for deep-space radio communications

48. What are the advantages of coaxial cable?

Immune to external radiation.

Less susceptible to noise.

Radiation is very small

Can operate at high frequencies



49. Define Gaussian noise. Gaussian noise is statistical noise having a probability density function (PDF) equal

to that of the normal distribution, which is also known as the Gaussian distribution. In other words, the values that the noise can take on are Gaussian-distributed.

50. What is white Gaussian noise? White noise is not the noise source. It is the classification of noise. The noise which

has Gaussian distribution and has flat spectral density over a wide range of frequencies is called white noise

51. Define wave propagation. Wave propagation is any of the ways in which waves travel. With respect to the

direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves. For electromagnetic waves, propagation may occur in a vacuum as well as in a material medium.

52. What is scattering loss?

The part of the transmission (power) loss that results from scattering within a transmission medium or from roughness of a reflecting surface.

53. What is radiation loss? Radiation losses often determine the maximum achievable energy in a charged

particle accelerator.

54. What is conductor loss? It occurs due to the distributed resistance of the conductor. This loss is directly

proportional to the square of the current flow. It can be reduced by increasing the diameter of the conductor.

55. Define dielectric heating loss. The two conductors have some voltage difference. They are isolated by dielectric

material. This voltage difference causes heating of the line. It is called as dielectric heating loss.

56. What is coupling loss? It is the loss in the signal strength when two separate pieces of transmission line are

connected together. The mechanical connection is basically discontinuity in the line. Therefore the discontinuity causes line to heatup, radiate energy and dissipates power.

57. What is corona? When the potential difference between the two conductors exceeds breakdown

voltage of the dielectric insulator, corona effect is said to have occurred. Due to this effect heavy cross current flow between the two conductors and line is damaged.

58. Define Rayleigh scattering. The scattering of light by particles in a medium, without change in wavelength, this

phenomenon is called Rayleigh scattering.



59. Define Vmax. It is defined as a maximum voltage that exists at the voltage reaches the peak.

60. What is Vmin? It is defined as a minimum voltage that exists at the voltage reaches the zero.

61. What are tunable detectors? The tunable detectors are used to demodulate the signal and couple the required

output to high frequency scope analyzer.

62. What is radiation pattern?

Radiation pattern is a representation of radiation characteristics of an antenna which is a function of elevation angle azimuth angle for a constant radial distance and frequency

63. What is radiation efficiency?

Radiation efficiency is defined as the ratio of total power radiated to total power accepted at its input.

64. How do you measure the polarization?

The polarization of an antenna is measured using transmitting mode and probing the polarization by a dipole antenna in which the dipole is rotated in the plane of polarization and the received voltage pattern is recorded.

65. Define polarization. Polarization is a property of waves that can oscillate with more than one orientation.

66. What is Bandwidth? A range of frequencies within a given band, in particular that used for transmitting a

signal is called as Bandwidth.

67. Define population inversion. It is defined as a transposition in the relative numbers of atoms, molecules, etc.

occupying particular energy levels.

68. Define broadcast frequencies. The VHF band is further divided into two frequency ranges: VHF low band (Band I)

between 54 and 88 MHz, containing channels 2 - 6, and VHF high band (Band III) between 174 and 216 MHz, containing channels 7 - 13.

69. Define spectral density estimation. In statistical signal processing, the goal of spectral density estimation (SDE) is to

estimate the spectral density of a random signal from a sequence of time samples of the signal.

70. Define reflected ray. It is nothing but the reflected ray is the reflected version of the incident ray. Hence it

is denoted as reflected ray.



UNIT III

DIGITAL COMMUNICATION

1. What are the advantages of digital transmission?

The advantage of digital transmission over analog transmission is noise immunity.

Digital signals are better suited to processing and multiplexing than analog signals.

Digital transmission systems are more noise resistant than the analog transmission systems.

Digital systems are better suited to evaluate error performance.

2. What are the disadvantages of digital transmission? The transmission of digitally encoded analog signals requires significantly more

bandwidth than simply transmitting the original analog signal. Analog signal must be converted to digital codes prior to transmission and converted

back to analog form at the receiver, thus necessitating additional encoding and decoding circuitry.

3. Define pulse code modulation.

In pulse code modulation, analog signal is sampled and converted to fixed length, serial binary number for transmission. The binary number varies according to the amplitude of the analog signal.

4. What is the purpose of the sample and hold circuit? The sample and hold circuit periodically samples the analog input signal and converts those samples to a multilevel PAM signal.

5. What is the Nyquist sampling rate?

Nyquist sampling rate states that, the minimum sampling rate is equal to twice the highest audio input frequency.

6. Define fold over distortion.

The minimum sampling rate(fs) is equal to twice the highest audio input frequency(fa).If fs is less than two times fa, distortion will result. The distortion is Called aliasing or fold over distortion.

7. What are the causes of fold over distortion? The side frequencies from one harmonic fold over into the sideband of another harmonic. The frequency that folds over is an alias of the input signal hence, the names aliasing or fold over distortion.

8. Define overload distortion. If the magnitude of sample exceeds the highest quantization interval, overload distortion occurs.



9. Define quantization. Quantization is a process of approximation or rounding off. Assigning PCM codes to absolute magnitudes is called quantizing.

10. Define dynamic range.

Dynamic range is the ratio of the largest possible magnitude to the smallest possible magnitude. Mathematically, dynamic range is

DR= Vmax

Vmin 11. Define coding efficiency.

Coding efficiency is the ratio of the minimum number of bits required to achieve a certain dynamic range to the actual number of PCM bits used. Mathematically, coding efficiency is

Coding efficiency= Minimum number of bits (including sign bit) X 100

Actual number of bits(including sign bit) 12. Define companding.

Companding is the process of compressing, then expanding. With companded systems, the higher amplitude analog signals are compressed prior to transmission, and then expanded at the receiver.

13. Define slope overload. How it is reduced.

The slope of the analog signal is greater than the delta modulator can maintain, and is called slope overload. Slope overload is reduced by increasing the clock frequency and by increasing the magnitude of the minimum step size.

14. Define granular noise. How it is reduced?

When the original input signal has relatively constant amplitude, the reconstructed signal has variations that were not present in the original signal. This is called granular noise. Granular noise can be reduced by decreasing the step size.

15. Define adaptive delta modulation.

Adaptive delta modulation is a delta modulation system where the step size of the AC is automatically varied depending on the amplitude characteristics of the analog input signal.

16. Define peak frequency deviation for FSK. Peak frequency deviation ( f) is the difference between the carrier rest frequency and either the mark or space frequency and either the mark or space frequency.

( f)=_fm-fs_2

17. Define modulation index for FSK. The modulation index in FSK is defined as h = (_f) fa

Where h= FM modulation index called the h factor in FSK, fa = fundamental frequency of

the binary modulating signal ( f) = Peak frequency deviation (hertz)



18. Define bit rate.

In digital modulation, the rate of change at the input to the modulator is called the bit rate (fb) and has the unit of bits per second (bps).

19. Define Baud rate. The rate of change at the output of the modulator is called baud.

20. Define QAM. Quadrature amplitude modulation is a form of digital modulation where the digital

information is contained in both the amplitude and phase of the transmitted carrier.

21. Write the relationship between the minimum bandwidth required for an FSK system and the bit rate.

The minimum bandwidth can be approximated as B=2 f

+2fb Where B=minimum bandwidth (hertz)

f=minimum peak frequency deviation (hertz) Fb=bit rate

22. What are the two fold effects of quantizing process? 1. The peak-to-peak range of input sample values subdivided into a finite set of decision

levels or decision thresholds 2. The output is assigned a discrete value selected from a finite set of representation

levels are reconstruction values that are aligned with the treads of the staircase.

23. What is meant by idle channel noise? Idle channel noise is the coding noise measured at the receiver output with zero

transmitter input.

24. What is meant by prediction error?

The difference between the actual samples of the process at the time of interest and the predictor output is called a prediction error.

25. Define delta modulation.

Delta modulation is the one-bit version of differential pulse code modulation.

26. Name the types of uniform quantizer?

1. Mid tread type quantizer. 2. Mid riser type quantizer.

27. Define mid tread quantizer? Origin of the signal lies in the middle of a tread of the staircase.

28. Define mid-riser quantizer?

Origin of the signal lies in the middle of a riser of the staircase



29. Define quantization error? Quantization error is the difference between the output and input values of quantizer.

30. What you mean by non-uniform quantization?

Step size is not uniform. Non-uniform quantizer is characterized by a step size that increases as the separation from the origin of the transfer characteristics is increased. Non-uniform quantization is otherwise called as robust quantization

31. What are the different types of companding? law companding & A law companding

32. Define Time division multiplexing.

Time Division Multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel.

33. What are the advantages of pulse code modulation? Pulse code modulation requires large bandwidth Pulse code modulation can be used in storing the data. PCM can encode the data also.

34. What are the disadvantages of pulse code modulation?

Pulse code modulation receivers are cost effective when we compared to other modulation receivers.

Channel bandwidth should be more for digital encoding. Decoding also needs special equipments and they are also too complex.

35. List the application of PCM.

Pulse code modulation is used in mobile phones, normal telephones etc.

Remote controlled cars, planes, trains use pulse code modulations.

Pulse code modulation is used in telecommunication systems, air traffic control systems etc.

36. Define T-carrier system. A dedicated phone connection supporting data rates of 1.544Mbits per second. A T-1 line

actually consists of 24 individual channels, each of which supports 64Kbits per second. Each 64Kbit/second channel can be configured to carry voice or data traffic.

37. Define radio system.

It is defined as a high quality digital FM radio system that utilizes the subcarrier band to generate digital audio signals. The hybrid FM radio system utilizes the subcarrier band to transmit the digital FM signals that are synchronized to the analog FM signals.

38. Define frequency shift keying.

Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK)



39. What is phase shift keying? Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing,

or modulating, the phase of a reference signal (the carrier wave).

40. Define AFSK. Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is

represented by changes in the frequency (pitch) of an audio tone, yielding an encoded signal suitable for transmission via radio or telephone.

41. What is BER? The bit error rate or bit error ratio (BER) is the number of bit errors divided by the total

number of transferred bits during a studied time interval. BER is a unit less performance measure, often expressed as a percentage.

42. Give some example for simple channel model. Binary symmetric channel (used in analysis of decoding error probability in case

of non-bursty bit errors on the transmission channel) Additive white gaussian noise (AWGN) channel without fading.

43. Name any one of the BER distance measuring technique.

A most general way of measuring the number of bit errors is the Levenshtein distance.

44. What is redundancy? It is the error detecting mechanism, which means a shorter group of bits or extra bits may

be appended at the destination of each unit.

45. List out the available detection methods. There are 4 types of redundancy checks are used in data communication. a) Vertical redundancy checks (VRC). b) Longitudinal redundancy checks (LRC). c) Cyclic redundancy checks (CRC). d) Checksum.

46. Write short notes on VRC. The most common and least expensive mechanism for error detection is the vertical

redundancy check (VRC) often called a parity check. In this technique a redundant bit called a parity bit, is appended to every data unit so, that the total number of 0s in the unit (including the parity bit) becomes even.

47. Write short notes on LRC.

In longitudinal redundancy check (LRC), a block of bits is divided into rows and a redundant row of bits is added to the whole block.

48. Write short notes on CRC.

The third and most powerful of the redundancy checking techniques is the cyclic redundancy checks (CRC) CRC is based on binary division. Here a sequence of redundant bits, called the CRC remainder is appended to the end of data unit.



49. Write short notes on CRC generator. A CRC generator uses a modulo-2 division.

In the first step, the 4 bit divisor is subtracted from the first 4 bit of the dividend. Each bit of the divisor is subtracted from the corresponding bit of the dividend

without disturbing the next higher bit. 50. Write short notes on CRC checker.

A CRC checker functions exactly like a generator. After receiving the data appended with the CRC it does the same modulo-2 division. If the remainder is all 0s the CRC is dropped and the data accepted. Otherwise, the received stream of bits is discarded and the dates are present.

UNIT IV

DATA COMMUNICATION AND NETWORK PROTOCOL

1. Define data communication codes.

Data communication codes are prescribed bit sequences used for encoding characters and symbols.

2. Define error detection.

Error detection is simply the process of monitoring the received data and determining when a transmission has occurred.

3. Define Echoplex. Echoplex is a relatively simple type of error detection scheme that is used almost

exclusively in data communications systems where human operators are used to enter the data manually from a keyboard.

4. Describe serial interface. Serial interface is used to ensure an orderly flow of data between the line control unit

and the modem.

5. Define parallel interface. Parallel interfaces transfer data between two devices eight or more bits at a time. That

is one entire data word is transmitted at a time .Parallel transmission is sometimes referred to as serial by word transmission.

6. What are the advantages of parallel transmission?

The advantage of parallel transmission is data are transmitted much faster than with

serial transmission because there is a transmission path for each bit of the word. In parallel interface there is no need to convert data from parallel to serial or vice versa.

7. What is the purpose of data modem?

The primary purpose of data modem is to interface computers, computer networks,

and other digital terminal equipment to analog communication lines and radio terminals.



8. Classify data modems.

Data modems are generally classified in to synchronous and asynchronous data modems.

9. Define OSI.

The term open system interconnection is the name for a set of standards for communications among computers. The primary purpose of OSI standards is to serve as a structural guideline for exchanging information between computers.

10. What are the advantages of bus topology?

a. The bus topology is easy to understand, install, and use for small networks. b. The cabling cost is less as the bus topology requires the least amount of cable to

connect the computers. c. The bus topology is easy to expand by joining two cables with a BNC barrel

connector. d. In the expansion of bus topology repeaters are used to boost the signal and increase

the distance.

11. What are the disadvantages of star topology?

One disadvantage of a star topology is that the network is only as reliable as the central node. When the central node fails, the entire system fails.

12. Describe LAN.

A local area network is usually a privately owned and links the devices in a single office, building or campus of up to a few kilometers in size.

13. Define LAN topology. The topology or physical architecture of a LAN identifies how the stations are interconnected.

14. What are the seven layers of open system interconnection? The seven layers of open system interconnection are

Physical layer Data link layer Network layer Transport layer Session layer

Presentation layer Application layer

15. What is mean by data communication? Data communication is the exchange of data (in the form of 1s and 0s) between two

devices via some form of transmission medium (such as a wire cable).



16. What are the three criteria necessary for an effective and efficient network? The most important criteria are performance, reliability and security. Performance of the

network depends on number of users, type of transmission medium, the capabilities of the

connected h/w and the efficiency of the s/w. Reliability is measured by frequency of failure, the

time it takes a link to recover from the failure and the networks robustness in a catastrophe.

17. What are the three fundamental characteristics of the communication system? The effectiveness of the data communication system depends on three

Fundamental characteristics:

Delivery: The system must deliver data to the correct destination.

Accuracy: The system must deliver data accurately.

Timeliness: The system must deliver data in a timely manner.

18. What are the advantages of distributed processing? Advantages of distributed processing include security/encapsulation, distributed

databases, faster problem solving, security through redundancy and collaborative processing.

19. Why are protocols needed? In networks, communication occurs between the entities in different systems. Two

entities cannot just send bit streams to each other and expect to be understood. For

communication, the entities must agree on a protocol. A protocol is a set of rules that govern data

communication.

20. Why are standards needed? Co-ordination across the nodes of a network is necessary for an efficient communication.

If there are no standards, difficulties arise. A standard provides a model or basis for development

to which everyone has agreed.

21. For n devices in a network, what is the number of cable links required for a mesh and ring topology?

Mesh topology n (n-1)/2

Ring topology n

22. What is the difference between a passive and an active hub? An active hub contains a repeater that regenerates the received bit patterns before sending

them out. A passive hub provides a simple physical connection between the attached devices.

23. Distinguish between peer-to-peer relationship and a primary-secondary relationship. Peer-to-peer relationship: All the devices share the link equally. Primary-secondary relationship:

One device controls traffic and the others must transmit through it.

24. Assume 6 devices are arranged in a mesh topology. How many cables are needed? How many ports are needed for each device?

Number of cables=n (n-1)/2=6(6-1)/2=15

Number of ports per device=n-1=6-1=5



25. Group the OSI layers by function. The seven layers of the OSI model belonging to three subgroups. Physical, data link and

network layers are the network support layers; they deal with the physical aspects of moving data

from one device to another. Session, presentation and application layers are the user support

layers; they allow interoperability among unrelated software systems. The transport layer ensures

end-to-end reliable data transmission.

26. What are header and trailers and how do they get added and removed? Each layer in the sending machine adds its own information to the message It receives

from the layer just above it and passes the whole package to the layer just below it. This

information is added in the form of headers or trailers. Headers are added to the message at the

layers 6,5,4,3, and 2. The transport layer creates a communication between the source and

destination.

27. What are the three events involved in a connection? Creating a connection involves three steps: connection establishment, data transfer and

connection release.

28. What is the DC component? Direct current is a zero-frequency signal with constant amplitude.

29. How does NRZ-L differ from NRZ-I? In the NRZ-L sequence, positive and negative voltages have specific meanings: positive

for 0 and negative for 1. In the NRZ-I sequence, the voltages are meaningless. Instead, the

receiver looks for changes from one level to another as its basis for recognition of 1s.

30. Using HDB3, encode the bit stream 10000000000100. Assume the number of 1s so far is odd and the first 1 is positive.

31. What are the functions of a DTE? What are the functions of a DCE? Data terminal equipment is a device that is an information source or an information sink.

It is connected to a network through a DCE. Data circuit-terminating equipment is a device used

as an interface between a DTE and a network.

32. What does the electrical specification of EIA-232 describe? The electrical specification of EIA-232 defines that signals other than data must be sent

using OFF as less than -3 volts and ON as greater than +3 volts. The data must be transmitted

using NRZ-L encoding.

33. Discuss the mode for propagating light along optical channels. There are two modes for propagating light along optical channels, multimode and single mode.

Multimode: Multiple beams from a light source move through the core in different paths.

Single mode: Fiber with extremely small diameter that limits beams to a few angles, resulting in

an almost horizontal beam.



34. What is refraction? The phenomenon related to the bending of light when it passes from one medium to another.

35. How are the guided media differing from unguided transmission media? Guided transmission media Unguided transmission media

1. Guided indicate, medium is contained 1.Unguided medium does not

have any within physical boundary Physical boundary

2. Transmission takes place through wire. 2. It is a wireless transmission.

36. What are the criteria used to evaluate transmission medium? The criteria used to evaluate transmission medium are

Throughput Propagation speed Propagation time Wavelength

37. Give the relationship between propagation speed and propagation time? Propagation time = distance / propagation speed

The time required for a signal or a bit to travel from one point to another is called

Propagation time. Propagation speed is the distance, a signal or a bit travel through a medium in

one second.

38. Explain cross talk and what is needed to reduce it? Effect of one wire on another is called as cross talk. One wire will be the sending antenna

and the other wire will be the receiving antenna. We can use the shielded twisted pair cable or

coaxial cable for transmission, which contains metal foil to reduce cross talk.

39. What are the responsibilities of data link layer? Specific responsibilities of data link layer include the following.

a) Framing

b) Physical addressing

c) Flow control

d) Error control

e) Access control

40. Mention the types of errors. There are 2 types of errors

a) Single-bit error.

b) Burst-bit error.



41. Define Single bit error. The term single bit error means that only one bit of a given data unit (such as byte character/data

unit or packet) is changed from 1 to 0 or from 0 to 1.

42. Define Burst error.

It is defined as 2 or more bits in the data unit have changed from 1 to 0 or from 0 to 1.

43. What is redundancy? It is the error detecting mechanism, which means a shorter group of bits or extra bits may be

appended at the destination of each unit.

44. List out the available detection methods. There are 4 types of redundancy checks are used in data communication.

a) Vertical redundancy checks (VRC).

b) Longitudinal redundancy checks (LRC).

c) Cyclic redundancy checks (CRC).

d) Checksum.

45. Write short notes on VRC. The most common and least expensive mechanism for error detection is the vertical

redundancy check (VRC) often called a parity check. In this technique a redundant bit called a

parity bit, is appended to every data unit so, that the total number of 0s in the unit (including the

parity bit) becomes even.

46. Write short notes on LRC. In longitudinal redundancy check (LRC), a block of bits is divided into rows and a redundant

row of bits is added to the whole block.

47. Write short notes on CRC. The third and most powerful of the redundancy checking techniques is the cyclic

redundancy checks (CRC) CRC is based on binary division. Here a sequence of redundant bits,

called the CRC remainder is appended to the end of data unit.

48. Write short notes on CRC generator. A CRC generator uses a modulo-2 division.

In the first step, the 4 bit divisor is subtracted from the first 4 bit of the dividend.

Each bit of the divisor is subtracted from the corresponding bit of the dividend without disturbing the next higher bit.

49. Write short notes on CRC checker. A CRC checker functions exactly like a generator. After receiving the data appended

with the CRC it does the same modulo-2 division. If the remainder is all 0s the CRC is dropped

and the data accepted. Otherwise, the received stream of bits is discarded and the dates are

resent.



50. Give the essential properties for polynomial. A polynomial should be selected to have at least the following properties.

a) It should not be

b) It should be divisible by(x+1).

51. Define checksum. The error detection method used by the higher layer protocol is called checksum. Checksum is

based on the concept of redundancy.

52. What are the steps followed in checksum generator? The sender follows these steps

The units are divided into k sections each of n bits.

All sections are added together using 2s complement to get the sum.

The sum is complemented and become the checksum.

The checksum is sent with the data.

53. List out the steps followed is checksum checker side. The receiver must follow these steps

a) The unit is divided into k section each of n bits.

b) All sections are added together using 1s complement to get the sum.

c) The sum is complemented.

d) If the result is zero.

54. Write short notes on error correction. It is the mechanism to correct the errors and it can be handled in 2 ways.

a) When an error is discovered, the receiver can have the sender retransmit the entire data unit.

b) A receiver can use an error correcting coder, which automatically corrects certain errors.

55. What are the 2 error-correcting methods? a) Single bit error correction

b) Burst error correction.

56. What is the purpose of hamming code? A hamming code can be designed to correct burst errors of certain lengths. So the simple

strategy used by the hamming code to correct single bit errors must be redesigned to be

applicable for multiple bit correction.

57. Define LAN. A Local Area Network (LAN) is a data communication system that allows a number of

independent devices to communicate directly with each other in a limited geographic area.



58. Mention the various architecture in a LAN. LAN is dominated by 4 architectures.

Ethernet

Token bus

Token ring

Fiber distributed data interface (FDDI)

59. Define Gateway. A device used to connect two separate networks that we different communication

protocols.

60. What is LSP? In link state routing, a small packet containing routing information sent by a router to all

other router by a packet called link state packet.

UNIT V

SATELLITE AND OPTICAL FIBER COMMUNICATIONS

1. Define satellite.

Satellite is a celestial body that orbits around a planet. In aerospace terms, a satellite

is a space vehicle launched by humans and orbits earth or another celestial body.

2. State Keplers first law. Keplers first law states that a satellite will orbit a primary body following an

elliptical path.

3. State Keplers second law. Keplers second law states that for equal time intervals of time a satellite will sweep

out equal areas in the orbital plane, focused at the bary center.

4. State Keplers third law. The third law states that the square of the periodic time of orbit is proportional to the

cube of the mean distance between the primary and the satellite.

5. Define orbital satellite.

Orbital satellites are also called as nonsynchronous satellite. Nonsynchronous satellites rotate around earth in an elliptical or circular pattern. In a circular orbit, the speed or rotation is constant however in elliptical orbits the speed depends on the height the satellite is above the earth.



6. Define prograde orbit. If the satellite is orbiting in the same direction as earths rotation and at an angular velocity greater than that of earth, the orbit is called a prograde (or) posigrade orbit.

7. Define retrograde orbit. If the satellite is orbiting in the opposite direction as the earths rotation or in the same direction with an angular velocity less than that of earth, the orbit is called a

retrograde orbit.

8. Define Geo synchronous satellite. Geo synchronous or geo stationary satellites are those that orbit in a circular

pattern with an angular velocity equal to that of Erath. Geosynchronous satellites

have an orbital time of approximately 24 hours.

9. Define apogee. The point in an orbit which is located farthest from the earth is called apogee.

10. Define Perigee. The point in an orbit which is located closest to earth is called perigee.

11. Define angle of inclination. The angle of inclination is the angle between the earths equatorial plane and the orbital plane of a satellite measured counterclockwise at the point in the orbit where it

crosses the equatorial plane traveling from south to north.

12. Define Decending node. The point where a polar or inclined orbit crosses the equatorial plane traveling

from south to north. This point is called descending node.

13. Define ascending node.

The point where a polar or inclined orbit crosses the equatorial plane traveling

from north to south is called ascending node.

14. Define line of nodes. The line joining the ascending and descending nodes through the center of earth is

called line of nodes.

15. Define angle of elevation.

Angle of elevation is the vertical angle formed between the directions of travel of an

electromagnetic wave radiated from an earth station antenna pointing directly toward

a satellite and the horizontal plane.

16. Define Azimuth angle

Azimuth is the horizontal angular distance from a reference direction, either the

southern or northern most point of the horizon.



17. What are the advantages of Optical fiber communication? It offers Greater information capacity, immunity to cross talk, immunity to static

interference, safety and security

18. Define a fiber optic system. An optical communications system is an electronic communication system that uses

light as the carrier of information. Optical fiber communication systems use glass or

plastic fibers to contain light waves and guide them in a manner similar to the way

electromagnetic waves are guided through a waveguide.

19. Define refractive index.

The refractive index is defined as the as the ratio of the velocity of

propagation of light ray in free space to the velocity of propagation of a light ray in a

given material. Mathematically, the refractive index is n = c/ where c = speed of light in free space, = speed of light in a given material

20. Define critical angle. Critical angle is defined as the minimum angle of incidence at which a light ray may

strike the interface of two media and result in an angle of refraction of 90or greater.

21. Define single mode and multi mode propagation. If there is only one path for light to take down the cable, it is called single mode. If

there is more than one path, it is called multimode.

22. Define acceptance angle. It defines the maximum angle in which external light rays may strike the Air/fiber

interface and still propagate down the fiber with a response that is no greater than 10

dB below the maximum value.

23. Define numerical aperture. Numerical aperture is mathematically defined as the sine of the maximum angle a

light ray entering the fiber can have in respect to the axis of the fiber and still

propagate down the cable by internal reflection.

24. Define modal dispersion. Modal dispersion or pulse spreading is caused by the difference in the propagation

times of light rays that take different paths down a fiber. Modal dispersion can occur

only in multimode fibers. It can be reduced by using single mode step index fibers

and graded index fibers.

25. What are the advantages of heterojunction LEDs? a. The increase in current density generates a more brilliant light spot. b. The smaller emitting area makes it easier to couple its emitted light into fiber. c. The small effective area has a smaller capacitance, which allows the planar

heterojunction LED to be used at higher speeds.



26. What are the disadvantages of injection laser diode? ILDs are typically on the order of 10 times more expensive than LEDs

Because ILDs operate at higher powers, they typically have a much shorter life time than LEDs. ILDs are more temperature dependent than LEDs.

27. What are step index and graded index fibers? In the case of graded index fiber, the refractive index of a core is a constant and is

larger than the refractive index of the cladding. The light propagation is mainly by

meridional rays. In the case of graded index fiber (GRIN fiber) the refractive index of

the core varies parabolically from the centre of the core having maximum refractive

index to the core cladding interface having constant minimum refractive index. Here

the light propagation is by skew rays.

28. Why do we prefer step index fiber for long distance communication? Step index single mode fiber has a) low attenuation due to smaller core diameter b)

higher bandwidth and c) very low dispersion.

29. Define WDM. In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology

which multiplexes a number of optical carrier signals onto a single optical fiber by using

different wavelengths (i.e., colors) of laser light. This technique enables bidirectional

communications over one strand of fiber, as well as multiplication of capacity.

30. What is meant by Rayleigh scattering? The index variation causes a Rayleigh type of scattering of light. Rayleigh scattering in

glass in the same phenomenon that scatters light from sun in the atmosphere, giving rise to blue

sky.

31. What is the measure of information capacity in optical waveguide? It is usually specified by bandwidth distance product in MHz. For a step index fiber the

various distortion effects tend to limit the bandwidth distance product to 20MHz.

32. Mention the losses responsible for attenuation in optical fibers. Absorption losses, Scattering losses and bending losses

33. What do you meant by extrinsic absorption? Absorption phenomena due to impurity atoms present in the fiber.

34. Define microscopic bending. Fibers losses occur due to small bending arise while the fiber is inserted into a cable.

35. What is the function of an optical amplifier? The main function of the optical amplifier is to strengthen the weak signal without being

any conversion.

36. List the different types of filter. The different types of filters,



Fixed filter

Tunable filter. Fixed filter:-

Gratings

Bragg gratings Tunable filter:-

Fabry perot filter.

Mach zehnder interferometer.

37. What is the function of couplers? A coupler is a device which is used to combine and split signals in an optical network.

38. What are the different types of optical couplers? Different types of couplers are

Directional coupler,

Star coupler,

Fused fiber coupler,

2 x 2 coupler.

39. What are the different types of multiplexing techniques?

Time division multiplexing (TDM)

Optical time division multiplexing (OTDM)

Wavelength division multiplexing (WDM)

40. What is an optical network? An optical network is not necessarily all-optical: the transmission is certainly optical,

but the switching could be optical, or electrical, or hybrid Also, an optical is not

necessarily packet-switched.

41. What are the advantages of optical network?

Huge bandwidth,

Low signal attenuation (as low as 0.2 dB/km),

Immunity to electromagnetic interference,

High security of signal because of no electromagnetic radiation.

42. Give the 2 analysis that are used to ensure system performance?

The 2 analysis that are used to ensure system performance are:

link power budget analysis

rise time budget analysis



43. Explain briefly about link power budget analysis?

In the optical power loss model for a pt-to-pt link, the optical power received at

the photo detector depends on the amount of light coupled into the fiber & losses

occurring in the fiber at the connectors & splices. The link loss budget is derived from

the sequential loss contribution of each element in the link

44. Give the range of system margin in link power budget?

The system margin is usually (6-8) dB. A positive system margin ensures proper

operation of the circuit. A negative value indicates that insufficient power will reach the

detector to achieve the required bit error rate, BER.

45. What are connectors?

The connectors are used to join the optical sources as well as detectors to the optical

fiber temporarily. They are also used to join two optical fibers.

46. What are the different types of connectors used in fiber?

The 2 major types of connectors are:

Lens type expanded beam connector

Ferrule type connector

47. What are the uses of optical fiber?

To transmit the information which are in the form of coded signals of the telephone communication.

To transmit the optical images

To act as a light source at inaccessible places.

48. What is LED?

A light-emitting diode (LED) is a two-lead semiconductor light source. It is a pn junction diode, which emits light when activated, when a suitable voltage is applied to the

leads.

49. What are the different types of LED? The different types of LED are active and opto electronic types

50. Mention the important semiconductors used in LED. GaAs,

InP,

InGaAs

GaAlAs

51. Explain the radiation pattern from surface emitting laser diode or LED. The emission pattern is isotropic with a 120 degree half-power beam width, i.e.,

source is equally bright when viewed from any direction, but the power diminishes as

cos, where is the angle between the viewing direction and normal to the surface



52. Mention the important semiconductors used in laser and LEDs. GaAs,

InP,

InGaAs GaAlAs

53. Define internal quantum efficiency of a LED The internal quantum efficiency in the active region is the fraction of electron hole

pairs that recombine radiatively. Its given by i=Rr/ (Rr+ Rnr)

i is the internal quantum efficiency

Rris the radiative recombination per unit volume.

54. Why do we prefer laser diodes over LEDs for communication applications? 1. High intensity radiation

2. Narrow spectral width of the laser source is the preferable features of

Laser compared to LED.

55. Define optical communication. Optical communication, also known as optical telecommunication, is

communication at a distance using light to carry information. It can be performed visually

or by using electronic devices.

56. What is meant by propagation of light? A light ray is a line or curve that is perpendicular to the light's wave fronts (and is

therefore collinear with the wave vector). Light rays bend at the interface between two

dissimilar media and may be curved in a medium in which the refractive index changes.

Geometric optics describes how rays propagate through an optical system

57. What are the advantages of LED?

LEDs are less complex circuits than Laser diodes.

Fabrication is easier.

They have long life.

58. What are the two types of LED configuration?

Homo junction

Single and double hetero junction.

59. What are the advantages of Photodiodes?

Small size

Suitable material

High sensitivity

Fast response time



60. What are the types of photodiode? PIN photo detector Avalanche photodiode (APD)

PART-B (11 MARKS)

UNIT-I

MODULATION SYSTEMS

1. Draw the block diagram of super heterodyne receiver and explain the

significance of each block. Compare it with other types of receiver circuits. (11)

Hint & Key:

Block diagram of super heterodyne receiver

Explanation about each block

Description about the super heterodyne receiver.

Comparative statement about the other receiver types

2. (a) Explain a method of generation of amplitude modulated signal and sketch

the time domain waveform of message, carrier and modulated signals. (8)

Hint & Key:

Diagram of Amplitude modulation

Explanation about the diagram

Working principle of AM


(b) Compare the amplitude modulation and frequency modulation. (3)

Hint & Key:

Comparison/ Difference between AM & FM

3. Explain in detail about the frequency division multiplexing with all

necessary diagrams. (11)

Hint & Key:

Diagram of FDM

Explanation about the diagram & FDM Working principle

Description about the super heterodyne receiver.


4. Draw the block diagram of PWM and explain its modulation concepts. (11)

Hint & Key:

Block diagram of PWM

Explanation about each block

Process of PWM

Application, advantages & Disadvantages.



5. Discuss about the time and frequency domain representation of signals. (11)

Hint & Key:

Explanation about time representation of signals

Explanation about the frequency representation of signals.

Diagrammatic representation of representation of signals.

Application, advantages & Disadvantages

6. Explain in detail about the frequency modulation & demodulation. (11)

Hint & Key:

Diagram of Frequency modulation


Working principle of FM


7. Explain in detail about the Amplitude modulation & demodulation. (11)

Hint & Key:

Block Diagram of Amplitude modulation & demodulation


Working principle of Modulator & Demodulator

8. Explain in detail about the frequency division multiplexing concept. (11)

Hint & Key:

Diagram of Frequency division multiplexing


Working principle of FDM

Comparative statement about the other Multiplexing technique

9. Describe the modulation technique in detail. (11)

Hint & Key:

Diagram of modulation


Types of modulation

10. Explain in detail about the representation of signals. (11)

Hint & Key:

Introduction about the signals

Types of representation of signals.

Schematic diagram

Explanation about the diagrams



UNIT-II

TRANSMISSION MEDIUM

1. (a) Discuss the different types of parallel conductor transmission lines. (6)

Hint & Key:

Explanation about transmission lines

Types of parallel conductor

Description about all the types.

Advantages, disadvantages & applications of parallel conductor transmission lines.

(b) What are the different losses in transmission line? How are they reduced? (5)

Hint & Key:

Explanation about transmission losses

Types of transmission losses

Description about all the types.

Methods to overcome the transmission losses

Advantages, disadvantages & applications of parallel conductor transmission losses

2. Explain the relevant diagram, the principle of ground wave and space wave

propagation of electromagnetic waves and also compare them. (11)

Hint & Key:

Explanation about wave propagation

Types of wave propagation

Description about space and ground wave propagation

Comparison about the two wave propagation methods.

3. Explain in detail about the transmission lines concept. (11)

Hint & Key:

Explanation about transmission lines

Communication Engg. Qb

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