Introduction to Wireless Networks: Ad Hoc Communication Overview

1. Introduction to Wireless Networks Michalis Faloutsos

2. What is an ad hoc network • A collection of nodes that can communicate with each other without the use of existing infrastructure • Each node is a sender, a receiver, and a relay • There are no “special nodes” (in principal) • No specialized routers, no DNS servers • Nodes can be static or mobile • Can be thought of us: peer-to-peer communication

3. Example: Ad hoc network • Nodes have power range • Communication happens between nodes within range

4. Some Introductory Things • The MAC layer 802.11 • Typical Simulations • The routing protocols • TCP and ad hoc networks

5. What Is Different Here? • Broadcasts of nodes can “overlap” -> collision • How do we handle this? • A MAC layer protocol could be the answer • If one node broadcasts, neighbors keeps quite • Thus, nearby nodes compete for air time • This is called contention

6. Contention in ad hoc networks • A major difference with wireline networks • Air-time is the critical resource • Fact 1: connections that cross vertically interfere • Fact 2: connections that do not share nodes interfere • Fact 3: a single connection with itself interferes!

7. B A C D Example of contention • Yellow connection bothers pink connection • Yellow bothers itself • When A-E is active • E-F is silent • F-G is silent (is it?) F E G H

8. The 802.11 MAC protocol RTS RTS • Introduced to reduce collisions • Sender sends Request To Send (RTS): ask permission • Case A: Receiver gives permission Clear To Send (CTS) • Sender sends Data • Receiver sends ACK, if received correctly • Case B: Receiver does not respond • Sender waits, times out, exponential back-off, and tries again A B D CTS C CTS

9. RTS A B D CTS C Why is this necessary? • A: RTS, and B replies with a CTS • C hears RTS and avoids sending anything • C could have been near B (not shown here) • D hears CTS so it does not send anything to B

10. Some numbers for 802.11 • Typical radius of power-range: 250m • Interference range: 500m • At 500m one can not hear, but they are bothered! • RTS packet 40 bytes • CTS and ACK 39 bytes • MAC header is 47 bytes

11. Typical Simulation Environment • A 2-dimensional rectangle • Fixed number of nodes • Static: uniformly distributed • Dynamic: way-point model • Pick location, move with speed v, pause • Power range: fixed or variable • Sender-receivers uniformly distributed

12. Various Communication Paradigms • Broadcasting: • one nodes reaches everybody • Multicasting: • One node reaches some nodes • Anycasting: • One node reaches a subset of some target nodes (one) • Application Layer protocols and overlays • Applications like peer-to-peer

13. Layered and Cross Layer Protocols • Layering: • Modular • Isolates details of each layer • Cross Layer: • Information of other layers is used in decisions • Pros: efficiency • Cons: deployability and compatibility application transport Network Link physical application transport Network Link physical

14. Example: application layer multicast • Source unicasts data to some destinations • Destinations unicast data to others • Pros: easy to deploy, no need to change network layer • Cons: not as efficient

15. Example: application layer multicast II • Members need to make multiple copies • It would happened anyway • Link A B gets two packets • Similarly in wireline multicast • Node B sends and receives packet 4 times s A B

16. Some major assumptions • The way-point model is a good model for mobility • Homogeneity is a good assumption • Links are bidirectional: I hear U, U hear me • Uniform distribution of location is good • 802.11 will be used at the MAC layer • Space is two dimensional

17. Some “proven” claims • The smallest the range, the better the throughput • Mobility increases the capacity of a network • A node should aim for 6-7 neighbors • We can challenge these claims

18. End of Introduction • Resources: • Google: • Citeseer: http://citeseer.nj.nec.com/cs • C. Perkins book: Ad Hoc Networking

19. Modeling Contention(based on Nandagopal et al MOBICOM 200) Seminar 260 Michalis Faloutsos

20. Problem: Find Hotspot in a graph • Given a graph and source-destinations • Where is the bottleneck? • Or how much bandwidth can each connection have?

21. Solution: Find areas of contention • Intuition • Step 1: create graph “range connectivity” • Step 2: create graph of flows (route flows on graph) • Step 3: find which flows contend for airtime (find areas where only one flow can be active)

22. Clarification: interference • When C->D • A-B, B-C, D-E, E-F can not be active!

23. Clarification: Dual graph • Each edge becomes a node in G’ • An edge exists between two nodes in G’ iff the edges have a common node edge Interference

24. In more detail • Find topological graph • Find dual graph: edges -> nodes • Consider “interference” between non adjacent edges • Find Maximal cliques

25. Contention Modeling: conclusion • Elegant approaches and tools are available • The realism of the modeling must be considered • Do not over-generalize results when heavy assumptions have been made

26. Considering Connections • If we know which pairs want to communicate, we consider only these flows as contenders • Routing could be independent of contention of an area • If routing is contention aware, then we have a closed loop system: • Routing -> contention -> routing -> ….

27. Question: what is optimal routing? • Given a graph, source-destination pairs • How do I route the flows to minimize contention? • What happens if I do not know the connections ahead of time (online version of problem)?

28. Modeling the Physical Channel • There are several ways depending on degree of accuracy • Binary, simplified: • in one prange you communication • In two prange you interfere but do not communicate

29. Considering the power: path loss • P_R: received power • P_t: transmission power • d: distance • alpha: constant

30. The physical model • Node Y hears node i, iff received power of i is above a threshold beta • Needs to rise above noise and other transmissions Pi = Noise + SUM_k Pk

31. A more optimistic channel model • Node Y hears i, if i is the “loudest” • Interference from other nodes: per pair comparison • Delta>0 is a protocol specified “guard zone”

32. Channel Modeling: Conclusion • Several different models • You need to find and justify the model you use

33. Topology Control • We cannot always control the mobility • We can control the network topology • Power control • Deciding to ignore particular neighbors • From a given graph G of possible connections we keep a subset G’ of these connections • What is good topology? …

34. Topology Control Metrics • What is good topology? • Energy efficiency, • Robustness to mobility, • Throughput - capacity

36. Topics Of Interest - Wireless • Characterizing the ad hoc topology • A snapshot • Its evolution • Mobility • Realistic mobility models • Effect of topology/mobility in • Routing • Multicasting in ad hoc networks

37. Topics of Interest - Wireline • Generating a realistic directed graph • Reducing a real (directed) graph to a small realistic • Survey on graph generation models • Measuring the Internet topology • Router level • AS level

38. We need to model contention • First the obvious • Adjacent edges • Second, one edge away, considering RTS CTS • Third, interference (500m instead of 250m) • Modeling issue

39. Typical “Errors” • Mobility: • too slow or too fast • Mobility speed may not be the expected • Homogeneity may “hide” issues • Few nodes are responsible for most traffic • Some spots are more popular than others • Power range is too large for the area • Ie radius 250m, a grid of 1Km -> one broadcast covers “half” the area

40. What’s the problem? • There is no systematic way to model and simulate such networks • No clue what are the right assumptions • Not sure how the assumptions affect the results

41. Consequences • Simulation results are • Meaningless • Unrepeatable • Incomparable between different analysis • Prone to manipulation • Claim: give me any statement, I can create simulations to prove it

42. What Will We Do Here? • Identify assumptions • Some of them are subtle • Characterize the scenarios • Study their effect on the performance results