1 Paper: IMPORTANT by Bai Sadagopan et al. Michalis Faloutsos.

1

Paper: IMPORTANTby Bai Sadagopan et al.

Michalis Faloutsos

2

The Problem: How important is mobility in ad hoc performance?

Mobility has been ignored or downplayed so farQuestions:• What is the effect of mobility on performance?• How does mobility affect performance?

3

Contribution: Mobility is critical

Mobility affects the performanceDefine a set of parameters (framework):• Mobility• Network connectivity• Protocol structure (building blocks)

Relative performance of protocols varies with mobilityMobility affects: connectivity (link/path stability)

4

Motivation and Significance

W need a systematic approach to simulationsWarn us: varying scenarios gives diff. ResultsProvide a framework to characterize mobility

5

Roadmap

Previous workDefinition of metrics for mobility and connectivityExperimental resultsConclusionsPaper criticism

6

Mobility models and studies

Way- point modelGroup Mobility (RPGM)• Groups of nodes that follow a group leader• Each member has speed similar to leader:

Magnitude, angle (from x-axis)

Freeway: predetermined lanes two wayManhattan: square lattice two-way traffic• Prob. 0.5 go straght, 0.25 turn left, 0.25 turn

right

7

Previous studies

Mobility does matter Pei et al. [11]:Proactive and reactive protocols are similar performance johansson et al. [10]Proactive protocols better throughput, and e2e delay, but reactive protocols less overhead Das et al. [8] (spot any contradiction?)• [8] introduced a mobility model limiting

“change”

8

Metrics: Capturing mobility

9

Metrics: Spatial and Temporal Dependence

Spatial Dependence: movement similarity of nodes close together

Temporal Dependence: similarity of node movement with itself in the near past

Relative Speed: as in physics: difference of vectors

10

Metrics for graph connectivity

Number of link changes: how many times there was a direct link between two nodesLink duration: how long a link between nodes lastsPath availability: fraction of time that two nodes are connected

11

Simulation set up

Area: 1000m x 1000m40 nodes900 secRPGM groups of 1x40, and 4x10Speed: Vmax = 1,5,10,20,30,40,50,60 m/secRange: 250m

12

Which metrics can distinguish mobility?

Average relative speed and spatial dependence are good metrics

13

Metrics for connectivity

Average link duration distinguished mobility models

Why this dip?

14

Mobility affects protocol performance

AODV and DSDV thruput comparison variesProactive (DSDV) protocols sometimes better than reactive

Transmission range 250m

15

Mobility models

What do nodes do when they reach the end of the road?

16

Conclusions

Mobility affects performance of protocolsDSR is a well “designed” protocol with optimized parameters• Other protocols are non-optimized then?

DSR: aggressive caching of routes• Good for low mobility

17

Criticism

Mobility was examined, but what about other parameters:• Average path length of connection• Average duration of connection• Speed with respect to connection and

simulation duration

Parameters of different protocols, were they optimized?

18

Quick calculations

Relative distance in hops? 1000^2/40 = 25000 m^2Area of transmission: pi r^2 = 3.14 250^2 = 196,250Average neighborhood= 7.85 nodesNode density!Max straight path length: 1000 sqrt(2) / 250 = 5.65 hopsAverage path length: approx 3 hops • Is that “enough”?

19

Mean criticism

We knew that mobility is importantWe knew that DSR seems to perform best• In one case AODV does better slightly

Do their results suggest what is the “right” simulation? No.They provide some intuitive obseervationsBuilding blocks: do they actually explain something?

20

Fair criticism

Provide a framework, first organized attemptProvide metricsDo thorough experimentsExplain they results they see (most of them)Intersting work that makes you think

21

How could I do things better

Model topology from a graph theoretic poin of viewMore metrics: • Average path length• Average neighborhood• Node density• Path length inflation• Choice of source-destination pairs (skewed

distr.)

Measure disconnectivity: scope, duration

22

More how to do things better

Examine the effect of other parameters than just mobilityShow that subtle parameters in protocols can make significant changesArgue that protocol parameters must be dynamic!