network hw2

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MODEL HOMEWORK # 2CAP306: Computer Networks

DOA: DOS:DOT: DOE:

HOMEWORK # 2

CAP306: Computer Networks

DOA:17/1/11 DOS:25/1/11

Homework Title / No. : _______homework-2____________________Course Code : 306_________ Course Instructor : ______________________ Course Tutor (if applicable) : __Rishi chopra___

Students Roll No._______RTB011A02______ Section No. : ___TB011____________

Declaration:I declare that this assignment is my individual work. I have not copied from any other students work or from any other source except where due acknowledgment is made explicitly in the text, nor has any

part been written for me by another person.

Students Signature : Seema Kumari

Evaluators comments:_____________________________________________________________________

Marks obtained : ___________ out of ______________________

Content of Homework should start from this page only:


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Part A

Q1 How the unguided media helps in data transmission. Especially the visible spectrum?

Transmission medium is the physical path between transmitter andreceiver in a data transmission system. Transmission media can beclassified or unguided. In both cases, communication is in the form of electromagnetic waves. With guided media, the waves are guided along asolid medium, such as copper twisted pair, copper coaxial cable, andoptical fiber. The atmosphere and outer space are examples of unguidedmedia that provide a means of transmitting electromagnetic signals butdo not guide them; this form of transmission is usually referred to aswireless transmission systems .

Fig 13.1: Transmission medium & physical layer.

In telecommunication, transmission media can be divided into two broadcategories which are as shown in Fig 13.2.

Fig 13.2 : Classes of transmission media.

The characteristics and quality of a data transmission are determined

both by the characteristics of the medium and the characteristics of thesignal. In the case of guided media, the medium itself is more important
http://www.engineersblogsite.com/wp-content/uploads/2010/04/14.jpghttp://www.engineersblogsite.com/wp-content/uploads/2010/04/13.jpg


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in determining the limitations of transmission.

Unguided TransmissionUnguided transmission is used when running a physical cable (either fiber or copper) between two end points is not possible. For example, running wiresbetween buildings is probably not legal if the building is separated by a publicstreet.

For unguided media, the bandwidth of the signal produced by the

transmitting antenna is more important than the medium in determiningtransmission characteristics. One key property of signals transmitted by antenna is directionality. In general, signals at lower frequencies areomni directional; that is, the signal propagates in all directions from theantenna. At higher frequencies, it is possible to focus the signal into adirectional beam.

In considering the design of data transmission systems, a key concern,generally, is data rate and distance: the greater the data rate anddistance, the better. A number of design factors relating to thetransmission medium and to the signal determine the data rate anddistance:

1. Bandwidth: All other factors remaining constant, the greater thebandwidth of a signal, the higher the data rate that can be achieved.

2. Transmission impairments: Impairments, such as attenuation,limit the distance. For guided media, twisted pair generally suffers moreimpairment than coaxial cable, which in turn suffers more than opticalfiber.

3. Interference: Interference from competing signals in overlappingfrequency bands can distort or wipe out a signal. Interference is of particular concern for unguided media, but it is also a problem withguided media. For guided media, interference can be caused by emanations from nearby cables. For example, twisted pair are oftenbundled together, and conduits often carry multiple cables. Interferencecan also be experienced from unguided transmissions. Proper shieldingof a guided medium can minimize this problem.

4. Number of receivers: A guided medium can be used to construct apoint-to-point link or a shared link with multiple attachments. In the


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Figure 2.2.7 Satellite Microwave Communication: point to- point Figure 2.2.8 Satellite Microwave Communication: Broadcast links

Q2 Which characteristics of an analog signal are changed to represent the low passanalog signal in each of the following analog to analog conversions?

1. AM2. FM3. PM

Ans:- In which Amplitude Modulation(AM) is represent the low pass analog signalin each of the following analog to analog conversion.

Amplitude modulation is a type of modulation where the amplitude of thecarrier signal is varied in accordance with the information bearing signal. Theenvelope, or boundary, of the amplitude modulated signal embeds theinformation bearing signal. The total power of the transmitted signal varies with


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the modulating signal, whereas the carrier power remains constant. A nonlinear device is used to combine the carrier and the modulating signal to generate anamplitude modulated signal. The output of the nonlinear device consists of

discrete upper and lower sidebands.The output of a nonlinear device does notvary in direct proportion with the input.

Amplitude Modulation is abbreviated AM

Characteristics of an Analog Signal

The maximum intensity of a wave is called the amplitude .

The distance between two crests is the wavelength .

The number of complete wave cycles every second is the frequency .

The phase difference measures (as an angle) how far ahead one wave iswhen compared to another wavAnalog-to-analog conversion is therepresentation of analog information by an analog signal.

One may ask why we need to modulate an analog signal; it is already analog.

Modulation is needed if the medium is bandpass in nature or if only abandpass channel is available to use.


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Amplitude Modulation

The total bandwidth for AM can be determined from the bandwidth of the audiosignal :

Figure 1


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Q.3 Explain total internal reflection. How it is used in optical fiber ? How opticalfiber is used to multiplex the signals?Ans:-

Total Interal Reflection

Total internal Reflection is an optical phenomenon that occurs when a Ray of Lightstrikes a medium boundary at an angle larger than the Critical Angle with respect tothe normal to the surface. If the Refractive Index is lower on the other side of theboundary no light can pass through, so effectively all of the light is reflected. Thecritical angle is the Angle of Incidence above which the total internal reflectionoccurs. When light crosses a boundary between materials with different refractiveindices, the light beam will be partially refracted at the boundary surface, andpartially reflected. However, if the angle of incidence is greater (i.e. the ray is closer to being parallel to the boundary) than the critical angle--the angle of incidence atwhich light is refracted such that it travels along the boundary--then the light will stopcrossing the boundary altogether and instead totally reflect back internally. This canonly occur where light travels from a medium with a higher refractive index to onewith a lower refractive index. For example, it will occur when passing from Glass toair, but not when passing from air to glass.

A fiber optic is a glass "hair" which is so thin that once light enters one end, it cannever strike the inside walls at less than the critical angle. The light undergoes totalinternal reflection each time it strikes the wall. Only when it reaches the other end isit allowed to exit the fiber.


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Fiber optic cables are used to carry telephone and computer communications.Advantages over electrical wired include:

1. Fiber optics can carry much more information in a much smaller cable.2. No interference from electromagnet fields result in "clearer" connections.

3. No electrical resistance.4. No hazard of electrocution if cable breaks.

A WDM system uses a multiplexer at the transmitter to join the signals together, anda demultiplexer at the receiver to split them apart. With the right type of fiber it ispossible to have a device that does both simultaneously, and can function as anoptical add-drop multiplexer. The optical filtering devices used have traditionallybeen etalons, stable solid-state single-frequency FabryProt interferometers in theform of thin-film-coated optical glass.

Part B

Q1 In CRC , show the relationship between the following entities (size meansthe number of bits

A. The size of data word and size of codeword.B. The size of divisor and size of reminder.C. The degree of polynomial generator and size of divisor.D. The degree of polynomial generator and size of reminder.

Ans:-

A. The size of data word and size of codeword.

n=k+r

n=size of codeword

k=size of data word

r=size of reminder

B. The size of divisor and size of reminder.Sol. Reminder is always one bit smaller than divisor


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reminder : the reminder is the length of check bitC. The degree of polynomial generator and size of divisor.

Sol: The degree of polynomial is one bit less than that the divisor.e.gCRC 32 generator with the polynomial degree 32 will have a divisor 33bit.

D. The degree of polynomial generator and size of reminder.Sol. The degree of polynomial generator is same as the size of reminder.eg If we have 32 bit CRC then we will have 32 bit reminder.

Q2 Obtain the 4 bit CRC code word for the data bit sequence 111110000(leftmost bit is the LSB) using the generator polynomial X 4 + X2 + 1

Ans:-Data word : 1111100000

Divisor : X 4 +x2+1

: X4+ 0.X 3+ x 2+0.x+1 =10101

The no. of Data bit(m) =10

The no. of bit in code word =N

Dividend = Data Word +(N-m)numbers of zeros

1100010110101| 111110000000000 |

10101

01010010101000010000

1010100101001010100001000

In CRC the required code word is obtained by writing the data word followed bythe reminder

111110000000000

1000

111110000001000


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Code Word = 111110000001000

Q3 How is a data checked for error during the transmission using paritymethod?

Ans:-

1. The Parity checking at the receiver can detect the presence of an error if theparity of the received signal is different from expected parity. That means if it isknown that the parity of transmitted signal is always going to be even and if the received signal has an odd parity then the receiver can conclude that thereceived signal is not correct.2. When a single error os an odd number of error occur during transmission theparity of the code word changes. Parity of the code word is checked at thereceiver and change in parity indicates that error is present in the received word

In the Parity Check error detection scheme, a parity bit is addedto the end of a block of data. The value of the bit is selected sothat the character has an even number of 1s (even parity) or anodd number of 1s (odd parity). For odd parity check, thereceiver examines the received character and if the totalnumber of 1s is odd, then it assumes that no error hasoccurred. If any one bit (or any odd number of bits) is

erroneously inverted during transmission, then the receiver willdetect an error.The errors can be divided into two types: Single-bit error and Burst error.

single-bit ErrorThe term single-bit error means that only one bit of given data unit (such as a

byte, character, or data unit) is changed from 1 to 0 or from 0 to 1.

Burst ErrorThe term burst error means that two or more bits in the data unit havechanged from 0 to 1 or vice-versa. Note that burst error doesnt necessarymeans that error occurs in consecutive bits.


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P mesg bits Parity Receivers

decision

Trasmitted code 0 10010110 Even correctword

Received code 0 00010110 Odd Incorrect Word

Note: The under line number is error in code

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