BYUBRIGHAM YOUNG UNIVERSITY
Computer Science
CS 460 Computer Communications and Networking
Archive

Homework #6

Review Questions (not graded, do not turn in)

  1. Kurose & Ross, Chapter 6, Review Question R3.

    What are the differences between the following types of wireless channel impairments: path loss, multipath propagation, interference from other sources?

  2. Kurose & Ross, Chapter 6, Review Question R4.

    As a mobile node gets farther and farther away from a base station, what are two actions that a base station could take to ensure that the loss probability of a transmitted frame does not increase?

  3. Kurose & Ross, Chapter 6, Review Question R7.

    Why are acknowledgments used in 802.11 but not in wired Ethernet?

  4. Kurose & Ross, Chapter 6, Review Question R10.

    Suppose the IEEE 802.11 RTS and CTS frames were as long as the standard DATA and ACK frames. Would there be any advantage to using the CTS and RTS frames? Why or why not?

  5. Kurose & Ross, Chapter 6, Review Question R14.

    What is meant by "opportunistic scheduling" in WiMAX?

  6. Kurose & Ross, Chapter 6, Review Question R21.

    What are three approaches that can be taken to avoid having a single wireless link degrade the performance of an end-to-end transport layer TCP connection?

  7. What is the difference between 2G and 3G wireless network standards? Why are some standards considered 2.5G?
  8. How does indirect routing work in mobile IP? Direct routing?
  9. Recall the on-demand, unicast ad hoc routing protocols we discussed. Why is broadcast necessary in order to find a path to the destination? How can a node in the middle of the network decide whether a particular route is broken?

Problems (10 points each)

  1. Kurose & Ross, Chapter 6, Problem P5.

    Suppose there are two ISPs providing WiFi access in a particular cafe, with each ISP operating its own AP and having its own IP address block.

    1. Further suppose that by accident, each ISP has configured its AP to operate over channel 11. Will the 802.11 protocol completely break down in this situation? Discuss what happens when two stations, each associated with a different ISP, attempt to transmit at the same time.
    2. Now suppose that one AP operates over channel 1 and the other over channel 11. How do your answers change?
  2. Kurose & Ross, Chapter 6, Problem P7.

    Suppose an 802.11b station is configured to always reserve the channel with the RTS/ CTS sequence. Suppose this station suddenly wants to transmit 1,000 bytes of data, and all other stations are idle at this time. As a function of SIFS and DIFS, and ignoring propagation delay and assuming no bit errors, calculate the time required to transmit the frame and receive the acknowledgment.

  3. Kurose & Ross, Chapter 6, Problem P8.

    Consider the scenario shown in the figure below, in which there are four wireless nodes, A, B, C, and D. The radio coverage of the four nodes is shown via the shaded ovals; all nodes share the same frequency. When A transmits, it can only be heard/received by B; when B transmits, both A and C can hear/receive from B; when C transmits, both B and D can hear/receive from C; when D transmits, only C can hear/receive from D. Suppose now that each node has an infinite supply of messages that it wants to send to each of the other nodes. If a message's destination is not an immediate neighbor, then the message must be relayed. For example, if A wants to send to D, a message from A must first be sent to B, which then sends the message to C, which then sends the message to D. Time is slotted, with a message transmission time taking exactly one time slot, e. g., as in slotted Aloha. During a slot, a node can do one of the following: (i) send a message; (ii) receive a message (if exactly one message is being sent to it), (iii) remain silent. As always, if a node hears two or more simultaneous transmissions, a collision occurs and none of the transmitted messages are received successfully. You can assume here that there are no bit-level errors, and thus if exactly one message is sent, it will be received correctly by those within the transmission radius of the sender.

    wireless network
    1. Suppose now that an omniscient controller (i. e., a controller that knows the state of every node in the network) can command each node to do whatever it (the omniscient controller) wishes, i. e., to send a message, to receive a message, or to remain silent. Given this omniscient controller, what is the maximum rate at which a data message can be transferred from C to A, given that there are no other messages between any other source/ destination pairs? Give your answer in terms of the number of messages per time slot.
    2. Suppose now that A sends messages to B, and D sends messages to C. What is the combined maximum rate at which data messages can flow from A to B and from D to C?
    3. Suppose now that A sends messages to B, and C sends messages to D. What is the combined maximum rate at which data messages can flow from A to B and from C to D?
    4. Suppose now that the wireless links are replaced by wired links. Repeat questions (a) through (c) again in this wired scenario.
    5. Now suppose we are again in the wireless scenario, and that for every data message sent from source to destination, the destination will send an ACK message back to the source (e. g., as in TCP). Also suppose that each ACK message take up one slot. Repeat questions (a) - (c) above for this scenario.
  4. Kurose & Ross, Chapter 6, Problem P10. Consider the following idealized WiMAX scenario. The downstream sub-frame (see the figure below) is slotted in time, with N downstream slots per sub-frame, with all time slots of equal length in time. There are four nodes, A, B, C, and D, reachable from the base station at rates of 10 Mbps, 5 Mbps, 2.5 Mbps, and 1 Mbps, respectively, on the downstream channel. The base station has an infinite amount of data to send to each of the nodes, and can send to any one of these four nodes during any time slot in the downstream sub-frame. WiMAX
    1. What is the maximum rate at which the base station can send to the nodes, assuming it can send to any node it chooses during each time slot? Is your solution fair? Explain and define what you mean by "fair."
    2. If there is a fairness requirement that each node must receive an equal amount of data during each downstream sub-frame, what is the average transmission rate by the base station (to all nodes) during the downstream sub-frame? Explain how you arrived at your answer.
    3. Suppose that the fairness criterion is that any node can receive at most twice as much data as any other node during the sub-frame. What is the average transmission rate by the base station (to all nodes) during the sub-frame? Explain how you arrived at your answer.
  5. Kurose & Ross, Chapter 6, Problem P11.

    In Section 6.5, one proposed solution that allowed mobile users to maintain their IP addresses as they moved among foreign networks was to have a foreign network advertise a highly specific route to the mobile user and use the existing routing infrastructure to propagate this information throughout the network. We identified scalability as one concern. Suppose that when a mobile user moves from one network to another, the new foreign network advertises a specific route to the mobile user, and the old foreign network withdraws its route. Consider how routing information propagates in a distance-vector algorithm (particularly for the case of interdomain routing among networks that span the globe).

    1. Will other routers be able to route datagrams immediately to the new foreign network as soon as the foreign network begins advertising its route?
    2. Is it possible for different routers to believe that different foreign networks contain the mobile user?
    3. Discuss the timescale over which other routers in the network will eventually learn the path to the mobile users.