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QUESTIONS:

1) Is an oil pipeline a simplex system, a half-duplex system, a full-duplex system, or none of the above? What about a river or a walkie-talkie-style communications? (2 Marks)

Answer: Like a single railroad track, it is half duplex. Oil can flow in either direction, but not both ways at once. A river is an example of a simplex connection while a walkie-talkie is another example of a half-duplex connection.

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2) A Modem constellation diagram has data points at the following coordinates: (1,1), (1, –1), (–1,1), and (–1, –1). How many bps can a modem with these parameters achieve at 1200 symbols/second? (2 Marks)

Answer: There are four legal values per baud, so the bit rate is twice the baud rate. At 1200 baud, the data rate is 2400 bps.

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3) Ten signals, each requiring 4000 Hz, are multiplexed onto a single channel using FDM. What is the minimum bandwidth required for the multiplexed channel? Assume that the guard bands are 400 Hz wide. (2 Marks)

Answer: There are ten 4000 Hz signals. We need nine guard bands to avoid any interference. The minimum bandwidth required is 4000(10) + 400(9) = 43,600 Hz.

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4) Explain where the following fit in the OSI reference model: (4 Marks)

a) A 4 kHz analog connection across the telephone network.

b) A 33.6 kbps modem connection across the telephone network.

c) A 64 kbps digital connection across the telephone network.

Answer:

a) Physical layer: the actual 4 kHz analog signal exists only in the physical layer of the OSI reference model.

b) Data-Link layer: a 33.6 kbps modem uses framing, flow-control, and error connection to connect a user to the switch.

c) Physical layer: the digital link across the network is controlled by many higher layer functions, but the 64 kbps signal that carries user information analogous to the 4 kHz signal used over the twisted pair that runs to the user’s premises.

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5) Why does a conventional telephone still work when the electrical power is out? (2 Marks)

Answer: The telephone company supplies each of its telephone lines with power at the central office. This power is stored in the form of wet batteries that can alternately be charged by a backup battery in the event of power failure at the central office. These huge batteries occupy entire floors in telephone offices.

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6) Suppose the signal has twice the power as a noise signal that is added to it. Find the SNR in decibels. Repeat if the signal has 10 times the noise power? 2n times the noise power? 10k times the noise power? (4 Marks)

Answer:

SNR dB = 10log10x2/n2 = 10log102 = 3.01 dB

SNR dB = 10log1010 = 10 dB

SNR dB = 10log102n = 10n log102 = 3.01n dB

SNR dB = 10log1010k = 10k dB

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6) A 10 kHz baseband channel is used by a digital transmission system. Ideal pulses are sent at the Nyquist rate and the pulses can take 16 levels. What is the bit rate of the system? (2 Marks)

Answer: Nyquist pulses can be sent over this channel at a rate of 20000 pulses per second. Each pulse carries log216 = 4 bits of information, so the bit rate is 80000 bits per second.

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7) Suppose that a low-pass communications system has a 1 MHz bandwidth. What bit rate is attainable using 8-level pulses? What is the Shannon capacity of this channel if the SNR is 20 dB? 40 dB? (2 Marks)

Answer: Nyquist pulses can be sent over this system at a rate of 2 million pulses per second. Eight level signaling carries 3 bits per pulse, so the bits rate is 6 Mbps.

The Shannon capacities are:

C = 1000000 log2(1+100) = 6.6 Mbps.

C = 1000000 log2(1+10000) = 13.3 Mbps.

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8) Suppose that a modem can transmit 8 distinct tones at distinct frequencies. Every T seconds the modem transmits an arbitrary combination of tones (that is, some are present, and some are not present). (2 Marks)

a) What bit rate can be transmitted using this modem?

b) Is there any relationship between T and the frequency of the signals?

Answer:

a) Each tone is either present or absent, hence there are 28 possible combinations of tones that can be transmitted every T seconds. The corresponding transmitted bit rate is 8/T bps.

b) Yes, there is a relationship. T must be long enough of each sinusoid can be observed to determine its frequency. This implies that the periods of all the sinusoids must be less than T.

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Question 1: (7 Marks)

Design (show only the network) a LAN system for small five-story building. One floor is dedicated to two mail servers and separated three database servers. Each of remaining floor has four computers with broadband access. Your design should meet the following restrictions and conditions: three-input hubs, one bridge, and unlimited Ethernet buses. The incoming broadband Internet access must be connected to a six-repeater ring, no bus LAN is allowed outside of a floor, and a traffic analyzer must be attached to the network.

Answer:

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Question 2 : (5 Marks)

What is the length of contention slot in CSMA/CD for (a) a 2-km twin-lead cable (signal propagation speed is 82% of the signal propagation speed in vacuum)?, and (b) a 40-km multimode fiber optic cable (signal propagation speed is 65% of the signal propagation speed in vacuum)?

Answer:

(a) Signal propagation speed in twin lead is 2.46 × 108 m/sec. Signal propagation time for 2 km is 8.13 μsec. So, the length of contention slot is 16.26 μsec. (b) Signal propagation speed in multimode fiber is 1.95 × 108 m/s. Signal propagation time for 40 km is 205.13 μsec. So, the length of contention

slot is 410.26 μsec.

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Question 3: (3 or 4 Marks)

Why do LANs tend to use broadcast networks? Why not use networks consisting of multiplexers and switches?

Answer:

The computers in a LAN are separated by a short distance (typically ................
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