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Dec 7 2012

11:00-12:15

Q1. 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 encodes

a 4-bit word, what is the minimum required bandwidth of the channel if we assume a noise

free 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 Hz

Q3. Wireless Lan uses the following algorithm for access control

a. CSMA/CD

b. CSMA/CA (ans)

c. Pure aloha

d. None of these

Q4. 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 a

transmission 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 that

packet 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)

Q5. For the following data stream, draw the encoding for NRZI, Machester, and Differential

Manchester.

Datastream: 1010010110.

1 0 1 0 0 1 0 1 1 0

NRZI

High voltage

Low voltage

High voltage

High voltage

High voltage

Low voltage

Low voltage

Low voltage

Bipolar-

AMI

Manchester

Differential

Manchester

Zero voltage

Q6. . For the following illustration, Host A has a file of size 5 MB to send to host B. But, the

maximum 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?

Q7. In the exponential back off algorithm, if a collision happen for the 15th time, the

algorithm randomly chooses K from {0,1,.......?}? 1023

Q8. Draw the Frequency Domain Plot of the illustrated signal

Q9. What is the channel capacity for a teleprinter channel with a 300-Hz bandwidth and a

signal-to-noise ratio of 3 dB?

Ans. Use Shannon capacity : C = B log2 (1+(S/N))