Fall 2018 CSE30264 Homework 2


Total Points: 60 points
Scope: Chapters 2
Due: October 8, 2018, by the beginning of the class
Grouping: To be completed individually

Problems

Problem 1: [12 points: 6 points for each sub-question] Suppose we want to transmit a bit sequence 11010110 and protect it from errors using CRC polynomial x3 +1.
  1. Use polynomial long division to determine the message that should be sent.

  2. Suppose the second bit from the left of the message is inverted due to noise on the transmission link. What is the result of the receiver's CRC calculation? How does receiver know an error has occurred?

Problem 2: [6 points, 2 points for each sub-question] Consider a Stop-and-Wait algorithm running over a 30-km point-to-point fiber link.
  1. Compute the one-way propagation delay for this link. Assuming speed of light is 2×108 m/s in the fiber.

  2. Suggest a suitable timeout value for the Stop-and-Wait algorithm to use.

  3. Given the timeout value you suggest in b), is it still possible for the Stop-and-Wait algorithm to timeout and retransmit a frame? Please also justify your answer.

Problem 3: [6 points] Suppose you are designing a sliding window protocol for a 1.5 Mbps point-to-point link to the moon, which has a one-way latency of 3 seconds. Assuming each frame carries 1KB of data, what's the minimum number of bits you need for the sequence number (Assume SWS=RWS)? Note: B denotes Byte and b denotes bit.

Problem 4: [22 points: 9 points (a), 13 points for (b)] Draw a timeline diagram for the sliding window algorithm with SWS=RWS=4 frames, for the following situations. Use a timeout interval of about 2×RTT.

Note: In problem 4, assume the sequence number of the first frame is 1 and accumulative ACKs is used. Please draw Frame 1 to 7 (including the lost and retransmission ones) from the sender and all ACKs from the receiver in the timeline diagram. Please also highlight the timeout and retransmission events in the diagram. You only need to consider the timeout and re-transmission of the lost frames.
  1. Frames 3 is lost.

  2. Frames 4-5 are lost.

Problem 5: [8 points] What is the MAC protocol of Ethernet and describe how it works using your own word. Does Wireless MAC protocol use "collision detection"? If yes, explain why. If not, explain why not.

Problem 6: [6 points] What is a hidden terminal problem? How is the hidden terminal problem addressed in 802.11 networks?

Submission Instructions

Create a PDF file containing your answers. Place your name at the beginning of the file. Copy the file into dropbox/hw2/hw2.pdf.