Comparison of Routing Metrics for a Static Multi-Hop Wireless Network Richard Draves, Jitendra Padhye, Brian Zill Microsoft Research Presented by: Jón

Comparison of Routing Metrics for a Static Multi-Hop

Wireless Network

Richard Draves, Jitendra Padhye, Brian Zill

Microsoft Research

Presented by: Jón T. Grétarsson

CS577: Advanced Computer Networks

Outline

• Introduction

• Setup

• Results

• Conclusions

• Discussion


Introduction


The Problem

• In recent years, ad hoc wireless networks have emerged as a hot topic

• Started with Military Applications• Commercial Applications of multi-hop

wireless networks becoming popular (Roofnet, BAWUG, Seattle Wireless)

• Quality of links aren’t taken into account in current routing algorithms


The Authors

• Richard Draves

• Jitendra Padhye

• Brian Zill


The Paper

• About Routing Metrics in Mesh Networks

• Presented in ACM SIGCOMM, 2004

• A summary for the impatient


Setup


The Metrics

• Hop Count (HOP)

• Per-hop Round Trip Time (RTT)

• Per-hop Packet Pair Delay (PktPair)

• Expected Transmission Count (ETX)


Ad Hoc Routing Architecture

• Mesh Connectivity Layer• Layer 2.5 Architecture• Link Quality Source Routing


LQSR

• Modified DSR to include Link Quality Metrics

• Link-State routing


Testbed


Testbed

• 23 Nodes

• Not Wireless-Friendly

• High Node Density

• Wide Variety of Multi-Hop Paths

• 801.11a Wireless Network

• Static Positions


Results


LQSR Overhead

• CPU Bottleneck for shorter paths

• Channel Contention for longer paths


Link Variability

• 183 of 506 Links displayed activity


Link Variability

• 90 Links with non-zero bandwidth in both directions


Long Lived TCP Flows

• Transfer duration fixed

• One active transfer at a time

• Semi-Inter Quartile Range bars

• Large variations in throughput

• UDP vs TCP

• Self-Interference


Median Throughput


Median Number of Paths


Path Length

• As path length increases, throughput decays

• Testbed diameter is 6 ~ 7 hops

• Self-Interference is still a big problem for RTT and PktPair

• ETX appears to approach a non-zero asymptote


Median Path Length


Average Path of ETX vs HOP


RTT Throughput vs Path Length


PktPair Throughput vs Path Length


HOP Throughput vs Path Length


EXT Throughput vs Path Length


Variability of Throughput

• Coefficient of Variation

• 6 periphery nodes to 5 receivers

• 1 active transfer at any time


Median Throughput


CoV of ETX vs HOP


Competing TCP Transfers

• RTT not worth demonstrating

• Multiple Median Throughput (MMT)


Competing TCP Transfers


Web Traffic

• Only one client active at any time

• 1300 files fetched

• Transfer using Surge

• File size within the range [77B, 700KB]

• Measured latency


Median Overall Latency


Median Latency <1KB


Median Latency >8KB


Web Traffic Conclusions

• In longer paths, ETX dominates

• In shorter paths, HOP sometimes wins


Mobile Scenario


Mobile Results


Mobile Results

• ETX has problems adjusting quickly enough

• HOP has no such problems


Conclusions

Paper Conclusions

• RTT and PktPair are load-sensitive and suffer from Self-Interference

• ETX significantly outperforms HOP in the stationary ad hoc network

• ETX relative performance gain increases as path length increases

• HOP responds faster to the changes of a mobile ad hoc network


Discussion

Discussion

• Experimental Flaws

• Logical Fallacies

• “Beating Up” competition

• What didn’t the authors do?


Documents

Comparison of Routing Metrics for a Static Multi-Hop Wireless Network Richard Draves, Jitendra Padhye, Brian Zill Microsoft Research Presented by: Jón