T325-Technologies for digital media

T325: TMA, Fall 2016-2017 Cut-off date: December 3rd, 2016

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Total Marks: 100

Contents
Part 1 (60 marks) ………………………………………………………………………………
Part 2 (40 marks) ………………………………………………………………………………











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Part I (60 marks)
You have studied video compression in Block 1. In Part I of the TMA, you are required to perform some research related to multiple description coding of video sequences. You are required to write a report describing this method. Your report should contain the following information:

1- Why is video compression important? (5 marks)
2- Error correction techniques:
a. Forward Error Correction (FEC) (5 marks)
b. Automatic Repeat Request (ARQ) (5 marks)
3- Explanation of multiple description coding (MDC) (10 marks)
4- Advantages over traditional single description coding (SDC) (5 marks)

After learning about MDC, you are required to investigate its importance over a simple example. Consider the scenario of Figure 1, where the mobile user is connected to the cellular base station (BS) BS1 (you do not need to know the details of cellular communications in order to answer the questions in this TMA).

Fig.1. Video transmission over a single channel.

BS1 is transmitting the video sequence V to the mobile user. Since the user is located close to the cell edge, performance is poor and the achievable data rate is 100 kbits per second. We will consider a single group of pictures (GOP) with one I-frame and 14 P-frames. The size, in bits, of each of these frames is shown in Table 1.

Table 1. Size in bits of each frame in the considered GOP of the video sequence V.
I1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15
57648 11488 5208 3760 2608 1984 1320 840 1144 648 448 800 932 856 880

5- Answer the following questions:
a. Calculate the total number of bits in this GOP. (3 marks)
b. Knowing that the 15 frames in Table 1 should be transmitted in 0.5s, what is the required bit rate (in bits/s) to successfully transmit the GOP of Table 1 within the required time limit? (3 marks)
c. What can you conclude concerning the transmission of this sequence over the link between BS1 and the mobile user in Fig. 1 (be reminded that the data rate over that link is 100 kbits/s)? (4 marks)


Using a technology in 4G cellular systems called coordinated multipoint (CoMP), two BSs can cooperate to send data simultaneously to a user close to the cell edge, as shown in Fig.2. (Note that you do not need to know the details of CoMP in order to answer the questions in this TMA). Hence, the sequence of Table 1 is split into two independent descriptions: Description 1 (D1), shown in Table 2, and Description 2 (D2), shown in Table 3. BS1 sends D1 to the mobile user, whereas BS2 simultaneously sends D2 to that user.



Fig.2. Transmission of two video descriptions over two wireless channels.


Table 2. Size in bits of each frame in the considered GOP of Description 1 (D1).
I1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15
28704 8024 3400 2352 1720 1344 720 592 640 352 128 392 608 480 424

Table 3. Size in bits of each frame in the considered GOP of Description 2 (D2).
I1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15
28944 3464 1808 1408 888 640 600 248 504 296 320 408 324 376 456


6- Answer the following questions:
a. Calculate the total number of bits in the GOP of D1 (Table 2). (3 marks)
b. Knowing that the 15 frames in Table 2 should be transmitted in 0.5s, what is the required bit rate (in bits/s) to successfully transmit the GOP of Table 2 within the required time limit? (3 marks)
c. Calculate the total number of bits in the GOP of D2 (Table 3). (3 marks)
d. Knowing that the 15 frames in Table 3 should be transmitted in 0.5s, what is the required bit rate (in bits/s) to successfully transmit the GOP of Table 3 within the required time limit? (3 marks)
e. What can you conclude concerning the transmission of the video sequence, after splitting it into two descriptions, knowing that the achievable data rate over the link between BS1 and the mobile user is 100 kbits/s, and the achievable data rate over the link between BS2 and the mobile user is 95 kbits/s? (3 marks)

7- Additional requirements concerning your report:
a. The report length should be between 800-1000 words (2 marks)
b. References used should be included at the end of the report and correctly cited in the report (3 marks)



Part II (40 marks)

You have studied Block Encryption in Block 2 of the course. In this part of the TMA, you are required to:

1. Provide an explanation of the two different approaches for doing Block encryption:
a. Electronic Codebook (ECB), shown in Fig. 3, (5 marks)
b. and Cipher Block Chaining (CBC), shown in Fig. 4. (5 marks)
2. You are also required to describe the advantages and drawbacks of each approach. (10 marks)


Fig.3. Electronic Codebook.


Fig.4. Cipher Block Chaining.

Afterwards, you are required to implement a practical example for block encryption. Consider the following key and plaintext blocks (to be used with both EBC and CBC).

Key 1 0 1 0 1 0 1 0

Plaintext Block1 1 1 1 0 0 1 1 0
Plaintext Block2 1 0 0 0 1 0 0 1
Plaintext Block3 1 1 1 0 0 1 1 0

Also, consider the following initialization vector (to be used with CBC).

Initialization Vector 1 0 0 0 0 0 0 1

3. Assuming a simple form of encryption that consists of performing a XOR operation with the Key:
a. Perform ECB encryption for plaintext Blocks 1, 2, and 3. (6 marks)
b. What can you remark, especially concerning Blocks 1 and 3? (4 marks)
c. Perform CBC encryption for plaintext Blocks 1, 2, and 3. (6 marks)
d. What can you conclude, especially in comparison with the ECB results? (4 marks)



End of TMA Questions