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


PPT – Comparison of Routing Metrics for Static Multihop Wireless Networks PowerPoint presentation | free to view - id: 12774e-MmM5N


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation

Comparison of Routing Metrics for Static Multihop Wireless Networks


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

Number of Views:68
Avg rating:3.0/5.0
Slides: 23
Provided by: NIKH


Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Comparison of Routing Metrics for Static Multihop Wireless Networks

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

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

  • 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

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

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

  • 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.

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

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

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

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

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

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

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

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

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
  • A heavily loaded link may have low loss rate
  • Two links with different data rates may have same
    loss rate

  • 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
  • y

Median path number and length
Throughput in multi-hop pathsHOP vs. ETX
Median throughput in multi-hop pathsRTT vs.
Median Throughput of 300 TCP transfers
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
  • HOP uses new links as soon as it discovers them

Conclusions and further discussion
  • In a 23 node static ad hoc network in office
  • 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