CS 460 Computer Communications Midterm

The following questions are an approximate superset of the question topics that you will find on the midterm exam. The exam will be mostly short answers with a couple of essay type questions.

I will continue to add questions to the end of the list, so check back later for new questions. They will be added to the bottom of the file.

  1. How many bits will be "airborne" in a STS-48 link between two ground stations through a geosynchronous satellite (assuming a 270 msec round-trip time)?. Calculate the minimum RTT given the distance, time to send an image, and number of bits in a sliding window sequence number if each packet is 2Kbytes long.
  2. If we use x^3 + x^2 + 1 as our CRC generating polynomial and are sending "0xB6", what is our Q and R? What is transmitted?
  3. An STS-1 channel has a one-way propagation delay of 10 ms. For 1 Kbit packets, what window size is needed to achieve an efficiency of at least 50%?
  4. Explain why the ethernet protocol does not scale to cross-country distances. Be precise.
  5. Assuming DMA in and out of main memory, how many interfaces to 45-Mbps T3 links could a workstation-based switch handle if its I/O bus speed is 800 Mbps and the memory bandwidth is 2 Gbps?
  6. Know the definitions of the following acronymns: FTP, RFC, HTTP, URL, DAN, LAN, MAN, WAN, STDM, FDM, RTT, FDDI, IP, TCP, UDP, ISDN, ATM, CSMA/CD, TDM, FDM, RTT
  7. What are the advantages and disadvantages of statistical multiplexing?
  8. What are the three kinds of failures that network designers have to worry about?
  9. Why is the dealay x bandwidth product important?
  10. What is jitter? Why would it be difficult to send video over a network with a high jitter value?
  11. What are the 7 layers in the OSI network architecture? What is an example of each layer?
  12. What layer is IP, Ethernet, TCP, email.
  13. Messages are normally represented by a complex DAG. Why isn't the message just copied into consecutive memory locations? How much of a performance hit do you take for each memory copy?
  14. Contrast the Process-per-protocol and the Process-per-message models.
  15. Be able to encode a message using NRZ and NRZI encoding.
  16. Be able to use 4B/5B to encode a message given the table.
  17. Why are 4B/5B and NRZI used together in encoding data?
  18. What is the sentinal approach to framing? Given a bit stream, perform bit stuffing to generate the correct message.  Be able to take a stream and unbitstuff it.
  19. What are the advantages and disadvantages of Ethernet and FDDI technologies?
  20. How does CSMA/CD work?
  21. How does FDDI recover from a lost token?
  22. Given a source routing example, determine the destination that a message will go to. (Something like Figure 3.7)
  23. Given a virtual circuit routing example, determine the destination that a message will go to. (Something like Figure 3.5)
  24. Compare datagrams, virtual circuits and source routing. What are the advantages and disadvantages of each approach?
  25. What is the spanning tree algorithm and which links would be eliminated in a problem similar to Figure 3.11?
  26. Why does an ATM cell have 48 bytes of payload?
  27. What kind of routing does ATM use?
  28. What is packetization delay and how much packetization delay is there for an ATM cell?
  29. Given a banyan network similar to Figure 3.33, decide if there will be a contention free route for two packets.
  30. When using a crossbar switch, contention can occur within the fabric. Would you use input buffering to solve these problems?
  31. What is the hidden node problem (page 138) and how does 802.11 deal with it?
  32. Describe the process for selecting an AP with a mobile node (pages 140-141).
  33. What is an internetwork?
  34. What is the service model for IP?
  35. How is fragmentation performed with IPv4?
  36. Assume a host with an IP address of 222.123.250.3. To what class does this IP address belong? To what network is it connected?
  37. What is an ARP packet and how is it used? How would ARP be implemented in a network such as ATM that doesnt have broadcast capabilities?
  38. What does an ARP cache contain? How does the ARP protocol work?
  39. Is source routing practical? Explain.
  40. What type of IP address class would be adequate for a corporation that needs to establish up to 100 sub-networks, each of which with up to 500 hosts? Justify your choice.
  41. Assume the following routing table:
    Subnet Number Subnet Mask Next Hop
    128.96.39.0 255.255.255.128 Interface 0
    128.96.39.128 255.255.255.128 Interface 1
    128.96.40.0 255.255.255.128 R2
    128.4.153.0 255.255.255.192 R3
    <default> --- R4

    What does the router do with the following packets?

    a) 128.96.39.10 b) 128.96.40.12 c) 128.96.40.151 d) 192.4.153.17 e) 192.4.153.90

  42. What are the problems with stop and wait, and how are they solved?
  43. How do bridges learn the location of a node?
    Created Feb 11, 1998 by Mark Clement
    Updated Oct 21, 1999
    (clement@cs.byu.edu)