CS 4514 Computer Networks WPI, C Term 1995
Craig E. Wills Midterm Exam (100 pts)
Given: Friday, Feb 3, 1995

NAME: BOX:

1.5em 1.5em

This is a closed book (and notes) examination. Answer all questions on the exam itself. Take the number of points assigned to each problem and the amount of space provided for your answer as a measure of the length and difficulty of the expected solution. The exam totals 100 points.

Potentially useful formulas:

• Nyquist rule for a noiseless channel:
  max data rate = bps where H is the bandwidth and V is the number of states encoded.

• Shannon's theorem for channels with noise:
  max data rate = where noise is measured in decibels (db) and
  db =

SCORE:

1. (4 points) Which type of transport service did the original USENET network use: circuit switching, message switching or packet switching?

2. (6 points) What baud rate is required to realize a 10 Mbps data rate using Manchester encoding? Using differential Manchester encoding?

3. (10 points) What is the Hamming distance for the following set of four 6-bit codewords? 000000 000111 111000 111111

   1. How many single-bit errors can be guaranteed to be detected for this set?

   2. How many single-bit errors can be guaranteed to be corrected for this set?

4. (18 points) This question concerns the use of CRC checksums, which you implemented in the first assignment.

   1. Assume that a new CRC polynomial ``CEW-3'' polynomial is defined. How many bits will the resulting checksum contain?

   2. Calculate the checksum if the data being sent is the character `a' (hexadecimal 61).

   3. What will be the bit string that is actually transmitted?

   4. What is the remainder when this bit string is divided by the CEW-3 polynomial?

   5. Are there any other bit strings of this length that yield the same remainder when divided by CEW-3? If so, indicate one. If not, briefly explain why no others exist.

5. (10 points) What is the maximum data rate that can be achieved over a 3 kHz voice grade line with a signal-to-noise ratio of 30 dB? What is the maximum data rate if 3 bits of data are encoded for each baud on this line?

6. (10 points) Station A has two frames in a queue ready to send. Station B has one frame ready to send at the same time. On an 802.3 CSMA/CD network, is it guaranteed that Station B's frame will be transmitted before the second frame of Station A is transmitted? Why? What is the answer for the same scenario on an 802.5 token ring network? Assume all frames have the same priority.

7. (10 points) The following diagram shows the bottom three layers of the OSI reference model on two separate machines. Name these three layers.

   What is the path of communication when sending data from layer 3 on machine A to layer 3 on machine B? As part of your answer indicate any headers/trailers that are added to/removed from the data.

8. (10 points) Define in-band signaling and out-of-band signaling in the context of data communication and give an example where each is used.

9. (15 points) Suppose two stations are using CSMA/CD with a modified version of the binary exponential backoff algorithm. In the modified algorithm, each station will always wait 0 or 1 time slots regardless of how many collisions have occurred. Assume two stations have just sent transmissions that have collided.

   1. What is the probability that contention ends on the first round of retransmissions?

   2. What is the probability that contention ends on the second round of retransmissions?

   3. What is the probability that contention ends on the third round of retransmissions?

   4. In general how does this algorithm compare with the normal binary exponential backoff algorithm in terms of performance under different types of load?

10. (7 points) If the minimum packet size for the 802.3 LAN standard is 64 bytes and the data rate of the network is 10 Mbps then what does this indicate about , the propagation delay, for such a network?