Wireless

  1. Kurose & Ross, Chapter 7, 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.

    a. 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.

    b. Now suppose that one AP operates over channel 1 and the other over channel 11. How do your answers change?

  2. Kurose & Ross, Chapter 7, Problem P7.

    Suppose an 802.11b station is configured to always reserve the channel with the RTS/CTS sequence. Supose 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 reuqired to transmit the frame and receive the acknowledgment.

  3. Kurose & Ross, Chapter 7, Problem P8.

    Consider the scenario shown in the figure below.

    narrow

    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, witha message transmission 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, ifa 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 ar eno bit-level errors, and thus if exactly one message is sent, it will be recevied correctly by those within the transmission radius of the sender.

    a. Suppose now that an omniscient controller (i.e., a controlle 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 omnisicient 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.

    b. 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?

    c. 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?

    d. Suppose now that the wireless links are replaced by wired links. Repeat questions (a) through (c) again in this wired scenario.

    e. Now suppose we are again in the wireless scenario, and that for every data messages 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 takes up one slot. Repeat questions (a)--(c) above for this scenario.

  4. Kurose & Ross, Chapter 7, Problem P11.

    In Section 7.5, one proposed solution that allowed mobile users to maintain their IP addreses as they moved among foreign networks was to ahve 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).

    a. Will other routers be able to route datagrams immediately to the new foreign network as soon as the foreign network begins advertising its route?

    b. Is it possible for different routers to believe that different foreign networks contain the mobile user?

    c. Discuss the timescale over which other routers in the network will eventually learn the path to the mobile user.