TDDD36 – Mobile systems (TEN1)
Final Examination: 8:00-12:00, Wednesday, October 17th, 2012
Time: 240 minutes
Total Marks: 40
Grade Requirements: three (20/40); four (28/40); and five (36/40).
Assistance: None (closed book, closed notes, and no electronics)
Instructor: Niklas Carlsson
Instructions:
• Read all instructions carefully (including these)!!!!
• The total possible marks granted for each question are given in parentheses. The
entire test will be graded out of 40. This gives you 10 marks per hour, or six
minutes per mark, plan your time accordingly.
• This examination consists of a total of 11 questions (including one bonus
question). Check to ensure that this exam is complete.
• When applicable, please explain how you derived your answers. Your final
answers should be clearly stated.
• Write answers legibly; no marks will be given for answers that cannot be read
easily.
• Where a discourse or discussion is called for, be concise and precise.
• If necessary, state any assumptions you made in answering a question. However,
remember to read the instructions for each question carefully and answer the
questions as precisely as possible. Solving the wrong question may result in
deductions! It is better to solve the right question incorrectly, than the wrong
question correctly.
• Please write your AID number, exam code, page numbers (even if the questions
indicate numbers as well), etc. at the top/header of each page. (This ensures that
marks always can be accredited to the correct individual, while ensuring that the
exam is anonymous.)
• Answers can be provided in either English or Swedish. (If needed, feel free to
bring a dictionary from an official publisher. Hardcopy, not electronic!! Also,
your dictionary is not allowed to contain any notes; only the printed text by the
publisher.)
• Good luck with the exam.
TDDD36/TEN1 – Mobile systems
Final Exam: 8:00-12:00, Wednesday, October. 17th, 2012
1) Question: Encapsulation (6)
Show the link-layer frame for a small HTTP response (that fits in a single frame) when it
arrive at the link layer at a mobile client using 802.11. You do not have to show all the
details of the different headers; however, you should (i) explain what protocols the
different headers are associated with, and (ii) provide the address information (e.g.,
source and destination) associated with the fields for each of the different headers. You
can assume that the client machine uses 802.11, has a MAC address
AA:AA:AA:BB:BB:BB and an IP address 111.222.111.222. The closest gateway router
has three interfaces: the interface closest to the client has MAC and IP addresses
BB:BB:BB:DD:DD:DD and 111.222.111.111, and the other interface on the path to/from
the server has MAC and IP addresses BB:BB:BB:CC:CC:CC and 111.222.111.133. The
MAC addresses of the access point that the client is associated is
AA:AA:AA:CC:CC:CC. Finally, the MAC and IP addresses of the HTTP server are
CC:CC:CC:AA:AA:AA and 222.222.222.222.
2) Question: 802.11 (3)
The 802.11 protocol can handle some hidden-terminal problems using the RTS-CTS
mechanism. Please explain the following: (a) What is the hidden-terminal problem?
When and how does it occur? (b) How does the RTS-CTS mechanism help towards
solving the hidden-terminal problem? Please illustrate with the communication sequence
when two nodes A and C both want to communicate with an intermediate node B.
3) Question: 802.11 (3)
Please derive an expression for the channel efficiency of 802.11 with and without the
RTS-CTS part. For simplicity you can assume a simple scenario with two nodes. Can
you say something about the overhead and channel efficiency as a function of packet
size? (You may also want to say something about when RTS-CTS is beneficial and not
here.) Please clearly state any necessary assumptions.
4) Question: Routing (4)
List the most significant advantages and disadvantages with table-driven and on-demandbased routing approaches, respectively, as well as discuss the tradeoffs between these two
routing approaches.
5) Question: GPSR routing (4)
Illustrate and discuss a scenario in which greedy geographic routing reaches a local
minimum, and how some alternative routing mechanisms would need to be employed to
reach the final destination. Also, please explain/illustrate why and how can distributed
graph planarization play a role here?
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TDDD36/TEN1 – Mobile systems
Final Exam: 8:00-12:00, Wednesday, October. 17th, 2012
6) Question: Routing (4)
Explain and illustrate how indirect routing is done in the context of mobile IP.
7) Question: Steady-state throughput (4)
Explain why TCP is not well equipped for mobile environments. For this question, you
may want to illustrate and discuss what the expected steady-state throughput may be
under different packet loss rates and delay conditions. Also, please give an example how
the throughput can be improved using some technique discussed in class.
8) Question: Packet losses (4)
Please consider a mobile client in Sweden accessing a Website in the US. Assume that
the last link (closest to the user) is wireless and has a high loss rate. Now, compare and
discuss the efficiency of a MAC protocol with and without retransmissions, as well as the
impact it may have on end-to-end throughput. A figure and assumptions regarding
delays and loss rates may help here.
9) Question: Packet losses (4)
For each of (i) forward error correction and (ii) piggy backing, please give an example
how these techniques can be used to handle packet losses in video streaming?
10) Question: HTTP-based adaptive streaming (4)
Wireless and mobile users can have highly variable bandwidth availability. Please
explain how HTTP-based adaptive streaming can be useful here. (A figure illustrating
the operation of the player and the available bandwidth may help here.) Also, please
explain the main difference between range-request-based solutions and chunk-based
solutions of HTTP-based adaptive streaming?
11) Bonus question: Power save mode (4)
Illustrate and explain how the power save mode (in 802.11, for example) can be used to
save energy of the mobile nodes. In the context of 802.11, what is the role of the access
point? Also, plot a figure that illustrates the tradeoff between the potential energy
savings and storage requirements on the APs, as a function of the beacon period duration,
for example.
Good luck!
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