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# Final exam sol

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This is the solution to the final exam for the 2012-2 data communication course

This is the solution to the final exam for the 2012-2 data communication course

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• 1. Name: Dec 7 2012Student ID: 11:00-12:15Q1. An image is 1024x768 pixels with 3 bytes/pixel. Assume the image is uncompressed.How long does it take to transmit it over a 56-kbps modem channel?Sol: The size of the image is 1024 x 768 x 3 x 8 = 18874368 bits.For a 56-kbps modem, it takes 18874368/56000  337 seconds.Q2. A digital signaling system is required to operate at 9600bps. If a signal element encodesa 4-bit word, what is the minimum required bandwidth of the channel if we assume a noisefree system?Sol: Because we assume noise free, we can use Nyquist’s equation: C = 2B log2M.We have C = 9600 bps log2M = 4, because a signal element encodes a 4-bit word. (So, actually M=16). we have B = 1200 HzQ3. Wireless Lan uses the following algorithm for access control a. CSMA/CD b. CSMA/CA (ans) c. Pure aloha d. None of theseQ4. Consider that there are two hosts: Host1 and Host 2. Host1 is situated at east Korea.Host 2 is situated at west Korea. The two hosts are connected by a channel with atransmission Rate of 1 Gbps. The size of packet is 1000 bytes.(a) What is the time needed to transmit the packet? Calculate transmission delay(b) What is the time needed for the packet to travel from Host A to Host B, assume thatpacket travels at speed of light? Calculate Propagation Delay(c) Assume that there is no queuing delay, then how much is the end-to-end delay? (a)+(b)
• 2. Q5. For the following data stream, draw the encoding for NRZI, Machester, and DifferentialManchester.Datastream: 1010010110. 1 0 1 0 0 1 0 1 1 0 High voltage NRZI Low voltage High voltage Bipolar- Zero voltage AMI Low voltage High voltage Manchester Low voltage High voltageDifferentialManchester Low voltageQ6. . For the following illustration, Host A has a file of size 5 MB to send to host B. But, themaximum size of the file that the host A can send at one time is 500 bytes. a) How many frames should host A send to host B to complete the transmission? b) How many maximum numbers of the frames can be on the link maximum?
• 3. Q7. In the exponential back off algorithm, if a collision happen for the 15 th time, thealgorithm randomly chooses K from {0,1,.......?}? 1023Q8. Draw the Frequency Domain Plot of the illustrated signalQ9. What is the channel capacity for a teleprinter channel with a 300-Hz bandwidth and asignal-to-noise ratio of 3 dB?Ans. Use Shannon capacity : C = B log2 (1+(S/N))