Challenging the Modeling Assumptions of Mobile Networks

1. Challenging the Modeling Assumptions of Mobile Networks Seminar 266 Michalis Faloutsos

2. Scope: Challenge the Assumptions • Targeted to graduate students (ugrad nets req.) • Reading papers • Presenting • Papers • Your project • Do a cool project • Publish!

3. 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

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

5. 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

6. 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

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

8. Topics Of Interest • The capacity of ad hoc networks • What is the inherent capacity of a network • Characterizing the topology • Mobility and its effect • Mobility models • Characterizing the topology of mobility pattern • The effect of power-range • Simulating TCP over ad hoc networks • Simulating multicasting in ad hoc networks

9. 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

10. 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 will challenge these claims

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

12. The 802.11 MAC protocol 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

13. 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 • D hears CTS so it does not send anything to B

14. 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

15. Typical Simulation Environment • A 2-dimensional rectangle • Fixed number of nodes • Static: uniformly distributed • Dynamic: way-point model • Power range: fixed or variable • Sender-receivers uniformly distributed

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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!

22. 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

23. 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