eric rozner - etx.ppt1 a high-throughput path metric for multi-hop wireless routing douglas s.j....
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Eric Rozner - ETX.ppt 1
A High-Throughput Path Metric for Multi-Hop Wireless Routing
Douglas S.J. CoutoDaniel AguayoJohn BicketRobert Morris
Presented by: Eric Rozner
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Eric Rozner - ETX.ppt 2
Overview
Hop count metric Describe ETX Implementation Experiments and Comparisons Related Work Conclusions Discussion
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Eric Rozner - ETX.ppt 3
Hop Count Metric
Paths decided upon by shortest route Maximizes the distance traveled by
each hop Minimizes signal strength -> Maximizes
the loss ratio Uses a higher TxPower -> Interference
Possibly many shortest routes Avoid lossy links?
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Eric Rozner - ETX.ppt 4
Hop Count vs. “Optimal”
x axis:
throughput
y axis:
fraction ofpairs withlessthroughput
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Eric Rozner - ETX.ppt 5
Hop Count Route Selection
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Eric Rozner - ETX.ppt 6
Motivation for a Better Routing Metric
Bidirectional loss rates Fine-grained choices
Intermediate loss rates
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Eric Rozner - ETX.ppt 7
Potential Ideas (and their cons) Product of per-link delivery ratios
A perfect 2-hop route is viewed as better than a 1-hop route with 10% loss ratio
Throughput of a path’s bottleneck link Same as above
End-to-End delay Changes with network load as interference
queue lengths vary… can cause oscillations Everyone convinced?
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Eric Rozner - ETX.ppt 8
ETX The predicted number of data
transmissions required to send a packet over a link
The ETX of a path is the sum of the ETX values of the links over that path
Examples: ETX of a 3-hop route with perfect links is
3 ETX of a 1-hop route with 50% loss is 2
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Eric Rozner - ETX.ppt 9
ETX continued…
Expected probability that a transmission is successfully received and acknowledged is df x dr
df is forward delivery ratio dr is reverse delivery ratio
Each attempt to transmit a packet is a Bernoulli trial, so…
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Eric Rozner - ETX.ppt 10
Hooray for ETX! Based on delivery ratios, which affect throughput Detects and handles asymmetry by incorporating loss
ratios in each direction Uses precise link loss ratios measurements to make
fine-grained decisions between routes Assumes you can measure these ratios precisely
Penalizes routes with more hops, which have lower throughput due to inter-hop interference Assumes loss rates are generally equal over links
Tends to minimize spectrum use, which should maximize overall system capacity (reduce power too) Each node spends less time retransmitting data
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Eric Rozner - ETX.ppt 11
ETX always the best?
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Eric Rozner - ETX.ppt 12
Acquiring ETX values Measured by broadcasting dedicated link
probe packets with an average period τ (jittered by ±0.1τ)
Delivery ratio:
count(t-w,t) is the # of probes received during window w
w/τ is the # of probes that should have been received
Each probe contains this information
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Eric Rozner - ETX.ppt 13
Implementation and such… Authors modified DSDV and DSR
See paper for details τ = 1 packet per second, w = 10 sec Multiple queues (different priorities)
Loss-ratio probes, protocol packets, data packets Are these experiments unfair or unrealistic?
In DSDV w/ ETX, route table is a snapshot taken at end of 90 second warm-up period
In DSR w/ ETX, source waits additional 15 sec before initiating the route request
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Eric Rozner - ETX.ppt 14
DSDV Performance
One hop
Asymmetric
ETX inaccurate
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Eric Rozner - ETX.ppt 15
DSDV and High Transmit Power
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Eric Rozner - ETX.ppt 16
Packet Size Problems
Less throughputadvantagethan whendata packetsare smaller(134 bytes)
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Eric Rozner - ETX.ppt 17
Packet sizes continuedACK’s smaller than probe packets
ETX underestimates ACK delivery ratios -> overestimates total number of transmissions per packet
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Eric Rozner - ETX.ppt 18
DSR Performance
Link-layer transmission feedback disabled
Link-layer transmission feedback enabled
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Eric Rozner - ETX.ppt 19
Related Work Yarvis et al. [33] propose a path metric which
approximates the product of the per-link delivery ratios.
Link hand-shaking to avoid “gray areas” [8], [22]
SNR ratio is also a possible path metric [14] SNR threshold value to filter links discovered by DSR
Route Discovery. See Related Works section of “
Comparison of Routing Metrics for Static Multi-Hop Wireless Networks “
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Eric Rozner - ETX.ppt 20
Related Work Continued… WCETT proposed in MR-LQSR
Weighted Cumulative Expected Transmission Time
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Eric Rozner - ETX.ppt 21
Conclusions Pros
ETX performs better or comparable to Hop Count Metric Accounts for bi-directional loss rates
Can easily be incorporated into routing protocols A lot of other work built off this paper
Cons May not be best metric for all networks
Mobility, Power-limited, Adaptive Rate (multi-rate), Multi-radio
Predications of loss ratios not always accurate Experiments (30 sec transfer of small packets)
may not complement real-world scenarios
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Eric Rozner - ETX.ppt 22
Questions?