comparison of routing metrics for a static multi-hop wireless network richard draves, jitendra...
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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
CS577: Advanced Computer Networks
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
CS577: Advanced Computer Networks
The Paper
• About Routing Metrics in Mesh Networks
• Presented in ACM SIGCOMM, 2004
• A summary for the impatient
CS577: Advanced Computer Networks
The Metrics
• Hop Count (HOP)
• Per-hop Round Trip Time (RTT)
• Per-hop Packet Pair Delay (PktPair)
• Expected Transmission Count (ETX)
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Ad Hoc Routing Architecture
• Mesh Connectivity Layer• Layer 2.5 Architecture• Link Quality Source Routing
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LQSR
• Modified DSR to include Link Quality Metrics
• Link-State routing
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Testbed
• 23 Nodes
• Not Wireless-Friendly
• High Node Density
• Wide Variety of Multi-Hop Paths
• 801.11a Wireless Network
• Static Positions
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LQSR Overhead
• CPU Bottleneck for shorter paths
• Channel Contention for longer paths
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Link Variability
• 90 Links with non-zero bandwidth in both directions
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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
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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
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Variability of Throughput
• Coefficient of Variation
• 6 periphery nodes to 5 receivers
• 1 active transfer at any time
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Competing TCP Transfers
• RTT not worth demonstrating
• Multiple Median Throughput (MMT)
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Web Traffic
• Only one client active at any time
• 1300 files fetched
• Transfer using Surge
• File size within the range [77B, 700KB]
• Measured latency
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Web Traffic Conclusions
• In longer paths, ETX dominates
• In shorter paths, HOP sometimes wins
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Mobile Results
• ETX has problems adjusting quickly enough
• HOP has no such problems
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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
CS577: Advanced Computer Networks
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