Mid Term • Wed, April 19 • Open Book • Open papers • Open laptop • In class exam • Duration 1 hour 15 minutes • Exam level • Not too easy, not too hard • 20% of total grade
Syllabus • Basic Networking • Wireless MAC • 802.11, Multi-channel, rate, antenna … • ZigZag, AccuRate … • Routing • Ad Hoc, Mesh, Delay Tolerant • Sensor Networks • Directed Diffusion, SMAC • Mobile Computing • Social apps, localization, mobility • Location privacy • Basic TCP, Wireless TCP • Beneficial for you to attend wireless TCP classes
Approximate Exam Format • Around 5 questions -- you will need to answer any 4 • 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
Approximate Exam Format • Questions should not require you to know paper details • You will not be asked to derive interference function in “Paritally Overlapping Channels not considered harmful” paper … • However, need to understand what is the need to compute an interference function
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
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
Some Focus Topics • Understang ZigZag, constellations, AccuRate • Ideas with ZigZag. Why is it useful? • Why AccuRate claims optimality • Routing • Understand DSR, LAR, Directed Diffusion • 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
Some Focus Topics • Sensor Networks • MAC: Energy Efficiency • Why not 802.11, Tradeoffs with SMAC • Routing: • Directed Diffusion behavior, issues • Mobile Computing • Design choices in localization. Which is better when? • Impact of mobility in various aspects • Difficulty with location privacy. Issues with path cloaking
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).
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)
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. 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.
To Channel or Not to Channel • You want to schedule traffic from laptops to a single AP You have two choices : • Make 2 channels, each 10 Mbps, and divide 50 people in each channel • Make a single channel of 20 Mbps, and have all 100 in the same channel. Which will you choose? Why?
Open Q: Location Proof • You were supposed to meet John at Twinnies. You went there a bit late, and could not find John around. Later, John does not believe that you were late -- he accuses you of not showing up at all. You need to prove to John that you did go to Twinnies. Can you design a scheme that allows you to prove your location.
Open Q: 3G problems • The 3G network is severly strained and WiFi hotspots are quite underutilized in general. • Design a scheme that will reduce the load on 3G networks by exploiting WiFi hotspots better.
Open Q: SurroundSense • SurroundSense requires that all locations be fingerprinted first. This is a hurdle, and may affect quick deployment of the system. • Design a scheme to quickly create a database of ambience fingerprints for Durham. • Discuss why your scheme will work • Discuss where it may not work (identify assumptions you are making)
Some Non-Examples • What is the size of RTS/CTS packets? • When may DRAND not converge to a schedule? • What fingerpriting technique used in SurroundSense? • 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
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.
Some Example Questions • You are given a graph, G. Also you are told that p_i > p_j 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.
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.