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Comparison of Routing Metrics for Static Multihop Wireless Networks

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Richard Draves, Jitendra Padhye, Brian Zill. Microsoft Research. Outline. Introduction ... Primary focus in ad hoc environments is to provide scalable routing in the ... – PowerPoint PPT presentation

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Title: Comparison of Routing Metrics for Static Multihop Wireless Networks


1
Comparison of Routing Metrics for Static
Multi-hop Wireless Networks
  • Richard Draves, Jitendra Padhye, Brian Zill
  • Microsoft Research

2
Outline
  • Introduction
  • Static wireless networks
  • Routing background
  • Description of four routing metrics
  • Evaluation of routing metrics
  • Evaluation on a mobile scenario
  • Conclusion

3
Introduction
  • Primary focus in ad hoc environments is to
    provide scalable routing in the presence of
    mobile nodes
  • Increased appearance of applications like
    community wireless networks where most of the
    nodes are stationary
  • In static ad hoc networks focus of routing
    changes to improving network capacity or
    performance of individual transfers

4
Characteristics of Static Ad hoc Networks
  • Stationary or minimally mobile nodes
  • Network topology and links rarely change
  • Nodes have unlimited power supply
  • Link quality and transmission rates vary

5
Routing basics
  • Routing protocols use efficient algorithms for
    minimum cost path selection (Dijkstra,
    Bellman-Ford)
  • Costs need to be assigned to links between nodes
  • A routing metric is required to calculate said
    costs
  • In order for minimum cost path to be computed by
    efficient algorithms the routing metric must be
    isotonic !

6
Isotonicity
  • Isotonicity is a sufficient and necessary
    condition for both Bellman-Ford and Dijkstras
    algorithm to find minimum cost paths
  • If Dijkstras algorithm is used in hop-by-hop
    routing, strict isotonicity is a necessary
    condition for loop-free forwarding.

7
Ad hoc routing protocols
  • DSR, DSDV, AODV use hop count as a routing metric
  • Efficient for mobile nodes where topology changes
    fast and quick metric reaction is needed
  • Low performance for static mesh networks
  • Performance decreases as path length increases
  • Does not take link quality into consideration

8
Routing metrics Dos and Donts
  • Consider link loss ratio
  • Consider link rate
  • Capture contention for channel
  • Maximize throughput
  • Interfere with itself
  • Be overly sensitive to load

9
Hop count
  • Finds minimum hop paths
  • Is simple and fast to compute
  • Does not take packet loss or bandwidth of the
    path into account
  • Will prefer one-hop lossy path to a two-hop
    reliable path

10
Per-hop Round trip time (RTT)
  • Calculates round trip delay seen by probes
    between neighboring nodes
  • Send a timestamped probe to all neighbors each
    500 ms
  • Receive probe ack from neighbors echoing the
    timestamp and calculate RTT
  • Keep a weighted moving average of RTT samples
  • Increase average by 20 whenever probe or data
    packet is lost
  • Load dependent

11
RTT pros and cons
  • Captures several facets of link quality
  • If node or neighbor is busy the probe will
    experience high delay
  • If other nodes in the vicinity are busy the probe
    will be delayed due to channel contention
  • If link is lossy the probe will need to be
    retransmitted several times
  • Designed to avoid highly loaded or lossy links
  • Queuing delay distorts the RTT values on a hop
  • Overhead caused by small probe packets
  • Link data rate is not captured by small packets

12
Per-hop Packet Pair Delay (PktPair)
  • Calculates the delay between pair of back-to-back
    probes to neighboring node
  • Send 2 Packets, 1 small and one large
  • Receiving node answers with probe containing
    delay between reception of probes
  • Measures several aspects of link quality
  • If second probe requires retransmissions by
    802.11 ARQ, delay will increase
  • If link has low bandwidth the second probe will
    take more time to traverse the link
  • If there is traffic in the vicinity, contention
    will cause probes to delay

13
PktPair pros and cons
  • Not affected by queuing delay
  • More sensitive to link bandwidth due to large
    probe
  • Additional overhead
  • Probes are contending with the data packets
    causing self-interference

14
Expected Transmission Count (ETX)
  • Presented by De Couto et al.
  • Measures the loss rate of broadcast packets
    between neighboring nodes
  • Finds high throughput paths minimizing the number
    of necessary transmissions
  • where

15
ETX pros and cons
  • Broadcasting probe packets reduces overhead
  • Suffers little from self-interference because it
    does not measure delays
  • Accounts for asymmetric link loss ratios
  • Broadcast probes are small and sent at lowest
    possible rate
  • Does not directly account for link load or data
    rate
  • A heavily loaded link may have low loss rate
  • Two links with different data rates may have same
    loss rate

16
Testbed
  • 23 nodes in an office environment
  • Routing with LQSR (modified version of DSR in
    order to measure link quality)
  • Measurements taken during long lived TCP
    connections
  • y

17
Median path number and length
18
Throughput in multi-hop pathsHOP vs. ETX
19
Median throughput in multi-hop pathsRTT vs.
PktPair
20
Median Throughput of 300 TCP transfers
21
Comparison of ETX and HOP in a mobile scenario
  • Different challenges than in static scenario
  • As the sender moves around the network ETX cant
    react fast enough to track the changes in link
    quality
  • HOP uses new links as soon as it discovers them

22
Conclusions and further discussion
  • In a 23 node static ad hoc network in office
    environment
  • RTT and PktPair perform poorly due to load
    sensitivity and self-interference
  • ETX outperforms hop count
  • In a mobile scenario
  • Hop count is more effective because it reacts
    fast to topology changes
  • Issues not addressed
  • Multi-channel networks
  • Capture channel diversity
  • Minimize interference
  • Load balancing
  • Load dependence is not necessarily bad
  • Minimize the impact a new flow has on network
    resources
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