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 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 Typical Simulations The routing protocols TCP and ad hoc networks

12 The MAC protocol 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 D C B RTS CTS

13 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 A D C B RTS CTS

14 Some numbers for 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 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?) A B C D E G F 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

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