Section 4.8. Exercises


4.8. Exercises

1.

In order to transmit a 500-page book with an average of 1,000 characters per page between places 5,000 km apart, we assume that each character uses 8 bits, that all signals travel at the speed of light, and that no link-control protocol is used.

  1. How much time is required if a digital voice circuit operating at the speed of 64 kb/s is used?

  2. How much time is required if a 620 Mb/s fiber- optic transmission system is used?

  3. Repeat parts (a) and (b) for a library with 2 million volumes of books.

2.

Assume that a wireless system with 200 terminals uses TDMA for its channel access. The packet lengths are T in average and are considered short compared with the TDMA long channel length. Compare the efficiency of two strategies: polling and CSMA.

3.

Design a CRC process unit for the following two standard generators of computer networking:

  1. CRC-12

  2. CRC-16

4.

For the example presented in the CRC section, we had 1010111 as a block of data ( D ), and the common value of generator, G , 10010, as the divisor.

  1. Show the dividend and the divisor in polynomial forms.

  2. Divide the dividend and the divisor in polynomial forms.

  3. Compare the results of part (b) to its binary form obtained in example.

5.

For data D , CRC = 1010111,1000, presented in the example of the CRC section, and the common value of generator, G is 10010 as the divisor. Sketch a picture of Figure 4.9 as many as needed and every time you shift in a bit, show the content of each register. Prove that the final contents of the registers show the value of CRC.

6.

Assume that 101011010101111 is a block of data ( D ) to be transmitted using the CRC error-checking method. Suppose that the common value of generator, G , is 111010. Using modulo-2 arithmetic.

  1. Produce the final value that the transmitter sends on the link ( D , CRC).

  2. Show the detail of the error-detection process at the receiver.

7.

Consider a coaxial transmission link that uses the stop-and-wait protocol requiring a propagation time to transmission time ratio of 10. Data is transmitted at rate 10 Mb/s, using 80-bit frames .

  1. Calculate the efficiency of this link.

  2. Find the length of this link.

  3. Find the propagation time.

  4. Sketch a plot of link efficiency when the ratio of propagation time to transmission time is reduced to 8, 6, 4, and 2.

8.

Consider a 2 Mb/s satellite transmission link through which 800-bit frames are transmitted. The propagation time is 200 ms.

  1. Find the link efficiency, using the stop-and-wait protocol.

  2. Find the link efficiency, using the sliding-window protocol if the window size is = 6.

9.

Consider the bidirectional control of links with the sliding-window method applied between two routers R2 and R3 (window size = 5) and stop-and-wait control between routers R3 and R4. Assume that the distance between R2 and R3 is 1,800 km and is 800 km between R3 and R4. Data frames with average size of 5,000 bits flow from R2 to R3 at 1 Gb/s rate. Acknowledgment frames are small enough to be ignored in the calculations. All links generate 1 µ s/km propagation delay.

  1. Determine a condition on the data rate at the output port of R3 toward R4 so that R3 remains congestion-free.

  2. Find the link efficiency for the R2R3 link.

  3. Find the link efficiency for the R3R4 link.



Computer and Communication Networks
Computer and Communication Networks (paperback)
ISBN: 0131389106
EAN: 2147483647
Year: 2007
Pages: 211
Authors: Nader F. Mir

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