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MD. Saidur Rahman KohinooR

Data Communication MID-TERM Examination SPRING-2016 1. (a)

A digital data stream 0010110101 is passing through a communication system. As we know data stream will be converted to signal using various schemes. Draw the signal of given bit stream using NRZ-I, Manchester and Psuedoternary scheme.

(b) Suppose a signal travels from sender Rahim to receive, Rina. At Rahim's side the signal power is 350 W, at Rina's side power is 230 W. What is the attenuation in dB?

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2. (a) Contrast the OSI model described with the TCP/IP reference model, Using a diagrarm show the correspondence between relevant protocol layers in the two models.

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(b) In OSI model, why are the flow control and error control functionalities implemented both at the data link layer and transport layer?

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3. (a) Explain the different transmission impairments that affect data communications?

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(b) "Any composite signal is a combination of simple sine waves will different frequencies, amplitude and phases ". Justify the statement.

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4. (a) Draw three sine waves with the same amplitude and frequency using phases 900 ,1800 And 2700. (b) Comparing different techniques of sampling for PCM? Find out which one is best and why?

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5. (a) Differentiate between Synchronous and Asynchronous transmission. Why we need proper Synchronization during transmission? Give an appropriate example. (b) Draw a hybrid topology with a ring backbone with three bus networks and two mesh networks? Discuss the consequences if a connection fails in the backbone.

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6. (a) A signal with 300 microwatts power passes through 10 devices, each with an average noise of 3 microwatts. Find SNR and SNRdB.

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(b) Write short note (with example):i. Logical Address. ii. Port Address. iii. Protocol and Standard.

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Q-1: (a) A digital data stream 0010110101 is passing through a communication system. As we know data stream will be converted to signal using various schemes. Draw the signal of given bit stream using NRZ-I, Manchester and Psuedoternary scheme. Ans:


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Q-1: (b) Suppose a signal travels from sender Rahim to receive, Rina. At Rahim's side the signal power is 350 W, at Rina's side power is 230 W. What is the attenuation in dB? Ans: Given, Power P1 = 350 W Power P2 = 230 W .`. We know that, Attenuation dB = 10 log10 (P2/P1) = 10 log10 (230/350) = -1.8234 Q-2: (a) Contrast the OSI model described with the TCP/IP reference model, Using a diagrarm show the correspondence between relevant protocol layers in the two models. Ans:

Q-2: (b) In OSI model, why are the flow control and error control functionalities implemented both at the data link layer and transport layer? Ans: The errors between the nodes can be detected by the data link layer control, but the error at the node (between input port and output port) of the node cannot be detected by the data link layer. For checking this error, another error control function need in transport layer. On the other hand, flow control of data link layer work until the sender receives feedback from the receiver, it will not send next data. Flow control of transport layer controls the data rate for sending next data. Q-3: (a) Explain the different transmission impairments that affect data communications? Ans: TRANSMISSION IMPAIRMENT: Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. There are three causes of impairment are attenuation, distortion, and noise.


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 Attenuation  Loss of Energy  When a signal travels through a medium, it looses some of its energy in order to overcome the resistance of the medium  That is why wire carrying electric signals get hot  Some of the electrical energy in the signal is converted to heat  To compensate for this loss Amplifiers are used to amplify the signal to heat  Distortion  Distortion means that the signal changes its form or shape  Distortion occurs in a composite signal  Signal changes its form or shape  Occurs in a composite signal, made of different frequencies  Each signal component has its own speed  NOISE  Thermal Noise: Due to random motion of electrons in a wire that creates an extra signal not originally sent by TX  Induced Noise: Comes from sources like Motors and Appliances  The Formula for finding Noise, SNR = Average Signal power / Average Noise power Q-3: (b) "Any composite signal is a combination of simple sine waves with different frequencies, amplitude and phases ". Justify the statement. Ans: Composite signal: A composite signal is made of many simple sine waves, where sine waves can vary in frequencies, amplitude and phases. According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases. As an example, If the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies; if the composite signal is non periodic, the decomposition gives a combination of sine waves with continuous frequencies.

Q-4: (a) Draw three sine waves with the same amplitude and frequency using phases 90 0 ,1800 And 2700. Ans:


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Q-4: (b) Comparing different techniques of sampling for PCM? Find out which one is best and why? Ans: Different techniques of sampling for PCM: There are three sampling methods-ideal, natural, and flat-top. In ideal sampling, pulses from the analog signal are sampled. In natural sampling, a high-speed switch is turned on for only the small period of time when the sampling occurs. In flat-top sampling, a circuit is used and this is the most common sampling method.


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Best one: Flat top sampling is best among others because flat-top samples creates by using a circuit. As a result, The sampling process is sometimes referred to as pulse amplitude modulation (PAM). We need to remember, however, that the result is still an analog signal with nonintegral values. Q-5: (a) Differentiate between Synchronous and Asynchronous transmission. Why we need proper Synchronization during transmission? Give an appropriate example. Ans: Synchronous and Asynchronous: Synchronous Asynchronous Send bit one after another without any start Send one start bit (0) at the beginning and one stop bit (1) at the end of each byte. or stop bit. Needs a clock signal between the sender and Does not need a clock signal between the the receiver. sender and the receiver. Supports high data transfer rate. Slower data transfer rate Requires master/slave configuration. There may be a title between each byte. Why we need proper Synchronization during transmission: In synchronous transmission, the bit stream is combined into longer frames which may contain multiple bytes. Each byte is introduced onto the transmission link without a gap between it and the next one. That means, data are transmitted as an unbroken string of 1s and 0s, and the receiver separates that string into the bytes or characters. Here is the example-


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Q-5: (b) Draw a hybrid topology with a ring backbone with three bus networks and two mesh networks? Discuss the consequences if a connection fails in the backbone. Ans:

Q-6: (a) A signal with 300 microwatts power passes through 10 devices, each with an average noise of 3 microwatts. Find SNR and SNRdB. Ans: Given, Average power of noise = 3 µW Number of devices = 10


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.`. Average power of signal = 300/10 = 30 µW So, SNR = 30 / 3 = 10 .`. SNRdB = 10 log10 10 = 10 Q-6: (b) Write short note (with example):i. Logical Address. ii. Port Address. iii. Protocol and Standard. Ans: Logical Address: The logical address is a 32-bit IP address that is not embedded in the network card, but it is assigned to it for the purpose of routing between networks. This type of address operates at Layer 3 I mean Network Layer of the OSI Model. Port address: There are many applications running on the computer. Each application runs with a port number (logically) on the computer. This port number is called port address. A port address is a 16-bit address. This type of address operates at Transport Layer of the OSI Model. Protocol and Standard: Protocol is a set of rules that govern all aspects of data communication between computers on a network. The key elements of a protocol are syntax, semantics and timing. Standards are developed by cooperation among standards creation committees, forums, and government regulatory agencies. Some Standards Creation Committees are: ISO, ANSI, IEEE, EIA etc..

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