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Mid Term Review

Mid Term Review. Mid Term. Tue, April 1 Open Book Open papers Open laptop In class exam Duration 1hour 15 minutes Exam level Not too easy, not too hard 20% of total grade. Syllabus. Topic covered till March 27 (inclusive) Basic Networking

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Mid Term Review

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  1. Mid Term Review

  2. Mid Term • Tue, April 1 • Open Book • Open papers • Open laptop • In class exam • Duration 1hour 15 minutes • Exam level • Not too easy, not too hard • 20% of total grade

  3. Syllabus • Topic covered till March 27 (inclusive) • Basic Networking • Wireless MAC (channel, rate, antenna, 802.11 …) • Misbehavior, Channel diagnosis • WLANs, Vehicular Nets, Cellular + Ad Hoc • Ad Hoc Routing, Mesh Routing • Delay Tolerant Networks • Sensor MAC, Routing • Sensor aggregation, congestion control • Basic TCP, Wireless TCP • Beneficial for you to attend wireless TCP classes

  4. Approximate Exam Format • Around 5 questions • Q1. -- Set of true/false statements • You will need to justify your answer in one sentence • Q2 to Q4 -- Topic questions with sub-parts • You may need to work out small problems • You may need to argue and/or identify flaws • You may need to give examples/counter-examples • Q5. -- Design Question • Open ended • You will need to sketch the design of a protocol

  5. Approximate Exam Format • Questions should not require you to know paper details • You will not be asked how ESRT distinguishes between HC, LC • However, need to understand event-based congestion control

  6. Some Focus Topics • Understand Basics (Undergrad review) • Transmission time, propagation delay, Q delay • CSMA/CD Vs CSMA/CA, slotted/unslotted ALOHA • Layering • Dijkstra’s, bellman ford routing on graphs • UDP/TCP, Transport schemes (stop&wait, GBN, SRQ …) • Understand 802.11 well • Carrier sensing, Backoff, Hidden/Exposed terminals • RTS/CTS, ACK, NAV, Spatial Reuse • Understand tradeoffs between CSMA/TDMA • Graph coloring as TDMA abstraction • Distributed TDMA

  7. Some Focus Topics • Directional Antennas, Multiple Channels • Beams, channels - their impact on spatial reuse • Deafness, HT, Capture, Overlapping channels • Rate Control (RC) • Channel characteristic, Tradeoffs • Thrghput/Time fairness, Problem w/ ARF, RBAR, OAR • Need for decoupling collisions and fading • Broadcast, Misbehavior, Gossip • Why misbehavior detection a challenge • Dominated sets, Why Gossip? Smart Gossip? Issues

  8. Some Focus Topics • Routing • Understand DSR, Directed Diffusion, GLS • Routing metrics - impact on performance • Reactive vs proactive routing - when good/bad • Why ETX? Why ExOR? Tradeoffs • Delay Tolerance • Understand the use of mobility, tradeoffs • Understand assumptions -- mobility model, density

  9. Some Focus Topics • Sensor Networks • MAC: Energy Efficiency • Why not 802.11, Tradeoffs with SMAC, TMAC • Pros/Cons with additional radio • Routing: • Directed Diffusion behavior, issues • Aggregation, Congestion control • Tree-based aggregation, fault-tolerance, agg. Functions • Impact of agg. Function on energy savings • Congestion control for events - why ESRT works/fails

  10. Some Example Questions

  11. Channel Utilization • (T/F): For 802.11, let X be saturation throughput when a single user transmits to an AP. With increasing number of users, the aggregate throughput will first increase and then decrease • (assume that all users transmit identical traffic and transmit at a specified data rate).

  12. Rate Adaptation • (T/F): When the channel quality is constant, ARF performs better than RBAR • (I am open to explaining to you ARF or RBAR, if necessary)

  13. Propagation and Transmission • You are trying to determine the bottleneck link on a particular route. One technique used is known as packet-pair estimation which is as follows: (i) The source sends two equal-length packets back to back on its outgoing link. (ii) When the packets are received, they are found to be separated by some duration T (time duration from the start of the first packet to the start of the second packet). (iii) Observing T, and knowing the lengths of the packets (L), it is enough to calculate the bottleneck bandwidth. Assume no other traffic in network • 1. Can packets ever reach the destination back to back? If no, explain why not. If yes, give an example. • 2. Let bottleneck bandwidth be B. Calculate B as a function of T and L. • 3. Why is it necessary to assume that there is no other traffic in the network? Answer very briefly. • 4. Modify this scheme to achieve a good estimate of bottleneck bandwidth under presence of other network traffic.

  14. IEEE 802.11 Carrier Sense • Some say that IEEE 802.11 solves hidden terminal problems (HTP) by introducing the RTS/CTS mechanism. • 1. List at least 2 counter-arguments. Use figures if necessary. • 2. Many say that adding carrier sensing to RTS/CTS can largely solve HTP if we assume that the channel does not fluctuate during communication. Calculate the carrier sensing threshold that must be used to ensure that a node that does not carrier sense an ongoing communication will also not interfere with the communication. (Assume SINR threshold as β = 1, path loss factor as α, and communication radius as R). • 3. Assume that in Jupiter, signal power degrades linearly with distance (as opposed to exponentially with factor α). Will that be better or worse for the spatial reuse of the wireless network? Discuss in 2 or 3 sentences.

  15. Location Service

  16. Some Example Questions • You are given a graph, G. Also you are told that pi > pj if i>j (I.e., p1 < p2 < p3 …). You are also told which node is the gossip originator. • (a) You will be asked to assign most efficient probabilities to each node in G • (b) You will have to kill 3 nodes. Which 3 will you kill and retain max reliability? • (c) Write an algorithm that takes as input N and Gp, and shows which N nodes should be killed. • Order the following aggregation functions in terms of energy consumption: • median, avg, min, max, mode

  17. Some Example Questions • A DTN protocol X is described. You are asked: • Order which mobility model will perform best, worst • Random walk, random waypoint, manhattan, brownian (I am open to explaining the mobility models if needed) • What is the problem when X runs over Rand Waypoint • Modify X to make it suitable for RW • You are given a scenario and application. You are asked to choose a routing protocol between DSR and AODV. • Justify your decision briefly.

  18. Some Non-Examples • What is the size of RTS/CTS packets? • When may DRAND not converge to a schedule? • What is the interference function in the paper on “partially overlapping channels …” • Describe the operations of DSR • How is the expected zone defined in LAR protocol? These type of questions will not be asked

  19. Any Questions? …

  20. Announcements • Hope you are making rapid project progress • Should be comfortable with your ealuation tool now • Final project presentation tentatively May 2 • Every group presents project • 12 to 15 minutes per presentation • 5 minutes questions • Cisco Champions Award • For best 3 projects • Judged by Duke Faculty and Cisco Researchers

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