CPSC 441: Computer Communications

Professor Carey Williamson

Winter 2012

Assignment 5: Network Performance Analysis (25 marks)

Due: TUESDAY, April 10, 2012 (11:59pm)

The purpose of this assignment is to learn how different communication protocols affect the user-perceived performance for network applications. Pen and paper (plus a calculator) should suffice for this assignment. There is no programming required. (Most of the material for this assignment will be covered sporadically during the semester, so there is no need to wait until April to start on this one. Really!)

  1. Nutty Networks (6 marks)
    An entrepreneurial CPSC 441 student has launched a new wireless network service so that messages can be sent to Caveman Carey, who is the last person on earth without a cell phone. The new network uses a hybrid of cellular and primitive wireless technologies. In particular, the student is encamped on top of McKimmie Library with a cellphone, a pencil, and a stack of paper. The student receives incoming voice messages destined for Carey, transcribes them onto a piece of paper, and then folds the paper into a paper airplane. From this vantage point, a message can be directed to Caveman Carey anywhere on campus. Assume that the writing and folding can be done in negligible time, and that paper airplanes fly at a constant rate of 10 meters per second, with zero message loss, regardless of the wind direction. Answer the following questions: (2 marks each)
    • (a) What is the bandwidth (in bits per second) achieved when sending a single 1024-byte message to Caveman Carey's office in ICT, which is exactly 300 meters from the top of McKimmie Library?
    • (b) How large a message is needed to achieve a data rate of 1 Mbps between McKimmie Library and Caveman Carey in ICT?
    • (c) How fast would a paper airplane need to fly to achieve 10 Mbps to Caveman Carey at his ICT office, if it was carrying a 4-kilobyte message?
  2. TCP (6 marks)
    Slide 101 of Chapter 3 gives a simple formula for the steady-state TCP throughput achieved over a network path, while slide 102 shows a more complicated formula for lossy networks. Use your knowledge of TCP to answer the following questions: (2 marks each)
    • (a) What end-to-end steady-state throughput (in bits per second) is achievable with a 1460-byte MSS over a network path with an RTT of 250 milliseconds, assuming that a congestion window size of 24 segments saturates the path?
    • (b) What end-to-end throughput (in bits per second) is achievable with a 1460-byte MSS over a lossy network path with an RTT of 250 milliseconds, if the average packet loss rate is 1%?
    • (c) What end-to-end throughput (in bits per second) is achievable with a 1460-byte MSS over a lossy network path with an RTT of 250 milliseconds, if the average packet loss rate is 4%?
  3. Ethernet (6 marks)
    In the lecture material on Ethernet, we learned that the maximum efficiency achievable with CSMA/CD in a multiple-access Ethernet LAN environment depends upon the one-way propagation delay in the LAN as well as the frame size used. Assuming that transmitted signals propagate at two-thirds of the speed of light (i.e., 200,000 kilometers per second), use your knowledge of Ethernet efficiency to answer the following questions: (2 marks each)
    • (a) What is the maximum efficiency for a classic 10 Mbps Ethernet LAN segment that is 2.0 km in length, if all frame transmissions are 64 bytes in size?
    • (b) What is the maximum efficiency for a classic 10 Mbps Ethernet LAN segment that is 2.0 km in length, if all frame transmissions are 1500 bytes in size?
    • (c) What is the maximum efficiency for a 100 Mbps Fast Ethernet LAN segment that is 2.0 km in length, if all frame transmissions are 1500 bytes in size?
  4. WiFi (6 marks)
    An IEEE 802.11b Wireless LAN supports a maximum physical-layer data transmission rate of 11 Mbps. Use your knowledge of IEEE 802.11b WLANs to answer the following questions: (2 marks each)
    • (a) What is the maximum user-level throughput (in bits per second) achievable for a large FTP download from a wireless AP to your laptop, assuming a 1500-byte MTU for TCP/IP packets, and per-segment acknowledgements at the transport layer? You can assume standard 40-byte TCP/IP headers, and a total of 34 bytes of headers at the MAC, LLC, and PLCP layers. Don't forget to consider the time consumed for DIFS, SIFS, PLCP headers, and MAC-layer ACKs. Drawing a diagram for the successful exchange of a single TCP data packet (and its corresponding TCP ACK) between the AP and the laptop AP might be very instructive.
    • (b) What is the maximum user-level throughput (in bits per second) achievable for the same file download if you use the maximum IEEE 802.11 payload size (i.e., 2312-byte MTU) for the transfer?
    • (c) Assuming 1500-byte MTUs, which would be "faster" in terms of user-perceived throughput for this transfer: a 10 Mbps Ethernet LAN, or an 11 Mbps IEEE 802.11b WLAN? By how much?

When you are finished, submit your solution to your TA in either email or hardcopy form. One free mark for having your name and student ID on it.