Comparison of Routing Metrics for Static Multi-Hop Wireless Networks - - PowerPoint PPT Presentation

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Comparison of Routing Metrics for Static Multi-Hop Wireless Networks - - PowerPoint PPT Presentation

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1 Comparison of Routing Metrics for Static Multi-Hop Wireless Networks Richard Draves, Jitendra Padhye and Brian Zill Microsoft Research 2 Multi-hop Wireless Networks Static Mobile Community wireless Motivating networks (Mesh

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Comparison of Routing Metrics for Static Multi-Hop Wireless Networks

Richard Draves, Jitendra Padhye and Brian Zill Microsoft Research

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Multi-hop Wireless Networks

Handling mobility, node failures, limited power. Improving network capacity Key challenge Battlefield networks Community wireless networks (“Mesh Networks”) Motivating scenario Mobile Static

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Routing in Multi-hop Wireless Networks

  • Mobile networks:

– Minimum-hop routing (“shortest path”) – DSR, AODV, TORA ….

  • Static networks:

– Minimum-hop routing tends to choose long, lossy wireless links – Taking more hops on better-quality links can improve throughput

[De Couto et. al., HOTNETS 2003]

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Link-quality Based Routing

  • Metrics to measure wireless link quality:

– Signal-to-Noise ratio – Packet loss rate – Round trip time – Bandwidth – … Our paper: experimental comparison of performance of three metrics in a 23 node, indoor testbed.

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Contributions of our paper

  • Design and implementation of a routing protocol

that incorporates notion of link quality

– Link Quality Source Routing (LQSR) – Operates at layer “2.5”

  • Detailed, “side-by-side” experimental

comparison of three link quality metrics:

– Per-hop Round Tip Time (RTT) [Adya et al 2004] – Per-hop Packet Pair (PktPair) – Expected Transmissions (ETX) [De Couto et al 2003]

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Summary of Results

  • ETX provides best performance
  • Performance of RTT and PktPair suffers due to

self-interference

  • PktPair suffers from self-interference only on

multi-hop paths

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Outline of the rest of the talk

  • LQSR architecture (brief)
  • Description of three link quality metrics
  • Experimental results
  • Conclusion
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LQSR Architecture

  • Source-routed, link-state protocol

– Derived from DSR

  • Each node measures the quality of links to its

neighbors

  • This information propagates throughout the

mesh

  • Source selects route with best cumulative metric
  • Packets are source-routed using this route
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Link Quality Metrics

  • Per-hop Round Trip Time (RTT)

– Per-hop Packet-Pair (PktPair) – Expected transmissions (ETX) – Minimum-hop routing (HOP)

  • Binary link quality
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Metric 1: Per-hop RTT

  • Node periodically pings each of its neighbors

– Unicast probe/probe-reply pair

  • RTT samples are averaged using TCP-like low-

pass filter

  • Path with least sum of RTTs is selected
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Metric 1: Per-hop RTT

  • Advantages

– Easy to implement – Accounts for link load and bandwidth – Also accounts for link loss rate

  • 802.11 retransmits lost packets up to 7 times
  • Lossy links will have higher RTT
  • Disadvantages

– Expensive – Self-interference due to queuing

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Metric 2: Per-hop Packet-Pair

  • Node periodically sends two back-to-back

probes to each neighbor

– First probe is small, second is large

  • Neighbor measures delay between the arrival of

the two probes; reports back to the sender

  • Sender averages delay samples using low-pass

filter

  • Path with least sum of delays is selected
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Metric 2: Per-hop Packet-Pair

  • Advantages

– Self-interference due to queuing is not a problem – Implicitly takes load, bandwidth and loss rate into account

  • Disadvantages

– More expensive than RTT

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Metric 3: Expected Transmissions

  • Estimate number of times a packet has to be

retransmitted on each hop

  • Each node periodically broadcasts a probe

– 802.11 does not retransmit broadcast packets

  • Probe carries information about probes received from

neighbors

  • Node can calculate loss rate on forward (Pf) and reverse

(Pr) link to each neighbor

  • Select the path with least total ETX

) P 1 ( * ) P 1 ( 1

r f

− − = ETX

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Metric 3: Expected Transmissions

  • Advantages

– Low overhead – Explicitly takes loss rate into account

  • Disadvantages

– Loss rate of broadcast probe packets is not the same as loss rate of data packets

  • Probe packets are smaller than data packets
  • Broadcast packets are sent at lower data rate

– Does not take data rate or link load into account

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Mesh Testbed

  • Approx. 61 m
  • Approx. 32 m

23 Laptops running Windows XP. 802.11a cards: mix of Proxim and Netgear. Diameter: 6-7 hops.

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Link bandwidths in the testbed

5 10 15 20 25 30 5 10 15 20 25 30 Higher Bandwidth (Mbps) Lower Bandwdith (Mbps)

  • Cards use Autorate
  • Total node pairs:

23x22/2 = 253

  • 90 pairs have non-zero

bandwidth in both directions. Bandwidths vary significantly; lot of asymmetry.

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Experiments

  • 1. Bulk-transfer TCP Flows
  • 4. Impact of mobility
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Experiment 1

  • 3-Minute TCP transfer between each node pair

– 23 x 22 = 506 pairs – 1 transfer at a time – Long transfers essential for consistent results

  • For each transfer, record:

– Throughput – Number of paths

  • Path may change during transfer

– Average path length

  • Weighted by fraction of packets along each path
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Median Throughput

200 400 600 800 1000 1200 1400 1600 HOP ETX RTT PktPair Median Throughput (Kbps)

ETX performs best. RTT performs worst.

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Why does ETX perform well?

0.2 0.4 0.6 0.8 1 2000 4000 6000 8000 10000 Throughput (Kbps) Cumulative Fraction

ETX HOP

ETX performs better by avoiding low-throughput paths.

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1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 Path Length with ETX

Path Length with HOP

Impact on Path Lengths

Path length is generally higher under ETX.

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Why does RTT perform so poorly?

RTT suffers heavily from self-interference

Median Number of Paths

5 10 15 20 25 HOP ETX RTT PktPair Number of Paths

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What ails PktPair?

PktPair

2000 4000 6000 8000 10000 12000 1 2 3 4 5 6 7 8 Average Pathlength (Hops) Throughput (Kbps)

ETX

2000 4000 6000 8000 10000 12000 1 2 3 4 5 6 7 8 Average Path Length (Hops) Throughput (Kbps)

RTT

2000 4000 6000 8000 10000 12000 1 2 3 4 5 6 7 8 Average Path Length (Hops) Throughput (Kbps)

PktPair suffers from self-interference only on multi-hop paths.

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Summary

  • ETX performs well despite ignoring link

bandwidth

  • Self-interference is the main reason behind poor

performance of RTT and PktPair. Similar results for multiple simultaneous flows.

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Experiment 2

  • Walk slowly around network periphery for 15

minutes with a laptop

  • Mobile laptop is the sender, a corner node is

receiver

  • Repeated 1-minute TCP transfers
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Testbed Layout

  • Approx. 61 m
  • Approx. 32 m
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28 100 200 300 400 500 600 HOP ETX Metric Median TCP Throughput (Kbps)

Shortest path routing is best in mobile scenarios?

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Conclusions

  • ETX metric performs best in static scenarios
  • RTT performs worst
  • PacketPair suffers from self-interference on

multi-hop paths

  • Shortest path routing seems to perform best in

mobile scenarios

– Metric-based routing does not converge quickly?

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Ongoing/Future work

  • Explicitly take link bandwidth into account
  • Support for multiple heterogeneous radios per

node

– To appear in MOBICOM 2004

  • Detailed study of TCP performance in multi-hop

networks

  • Repeat study in other testbeds
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For more information http://research.microsoft.com/mesh/

Source code, binaries, tech reports, …

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Backup slides

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LQSR Architecture

  • Implemented in a shim layer

between Layer 2 and 3.

  • The shim layer acts as a virtual

Ethernet adapter

– Virtual Ethernet addresses – Multiplexes heterogeneous physical links

  • Advantages:

– Supports multiple link technologies – Supports IPv4, IPv6 etc unmodified – Preserves the link abstraction – Can support any routing protocol

  • Architecture:
  • Header Format:

Ethernet 802.11 802.16 Mesh connectivity Layer with LQSR IPv4 IPv6 IPX Ethernet MCL Payload: TCP/IP, ARP, IPv6…

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Web transfers

  • Simulated Web transfer using Surge
  • One node serves as web server
  • Six nodes along periphery act as clients
  • Results: ETX reduces latency by 20% for hosts

that are more than one hop away from server.

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Static Multi-hop Wireless Networks

  • Motivating scenario:

– Community wireless networks (“Mesh Networks”)

  • Very little node mobility
  • Energy not a concern
  • Main Challenge:

– Improve Network capacity

  • Minimum-hop count routing is inadequate

– Tends to choose long, lossy wireless links [De Couto et.

al., HOTNETS 2003]

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“Traditional” Multi-hop Wireless Networks

  • Envisioned for mobility-intensive scenarios
  • Main concerns:

– Reduce Power consumption – Robustness in presence of mobility, link failures

  • Routing:

– Minimum-hop routing (“shortest path”) with various modifications to address power and mobility concerns – DSR, AODV, TORA ….