Homework 3 - Chapter 3

Due: Friday February 3, 2012 at 5:30pm

Answer the following questions. Each question is worth the points indicated.

  1. (4 pts) If a packet from the network layer is broken into 10 frames and each frame has a 20% chance of being corrupted in transit, how many frames on average will be transmitted before the whole packet is received without error? Answer this question for the following types of data link protocols:
    1. A data link protocol that does no frame retransmission or error detection. So if a frame is corrupted in transit, the data link layer will not detect it. The responsibility for error detection and recovery is on the higher layers (ie the network or transport layer will retransmit the whole packet).
    2. A data link protocol with error detection that will retransmit a frame that is corrupted in transit using selective repeat.
    Hint: Use the Binomial Distribution calculations for determining the probability of success for part (a).
  2. (4 pts) The message 0110 0111 1110 0000 0111 1110 1111 1001 is to be transmitted. Show the bit sequence for the message if it is encoded using the following framing methods:
    1. Flag bytes with byte stuffing. FLAG = 0111 1110, ESC = 1110 0000
    2. Starting and ending flag bits with bit stuffing, FLAG = 0111 1110
  3. (4 pts) The character 'A' is represented in 8-bit ASCII as the bit string 01000001. You are transmitting 'A' with an even-parity Hamming code.
    1. What is the bit string that is transmitted? Show all work in computing each check bit, not just the resulting message.
    2. Suppose that while your answer to part (a) is in transit, bit 7 is corrupted (it flips to the opposite value). Show how the receiver would detect and correct this error.
  4. (2 pts) Compute the CRC checksum for 0110 0010 1111 1110 divided by the generator polynomial 1001. Show all work (similar to Figure 3.8 in the book).
  5. (2 pts) Under what conditions would error correcting codes be preferred over error detection with retransmission?
  6. (2 pts) What features of the physical line should be considered when choosing a sending window size for a sliding window protocol? Remember that one purpose of sliding window is to keep the line in use as much as possible.
  7. (2 pts) Why do most data link protocols put the frame checksum in the footer instead of in the header?