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Expected Data Rate (EDR): An

Accurate High-Throughput Path Metric

For Multi-Hop Wireless Routing

Jun Cheol Park (jcpark@cs.utah.edu)

Sneha Kumar Kasera (kasera@cs.utah.edu)

School of Computing

University of Utah

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labtop

PDA

labtop

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The Internet

Multi-hop wireless networks

Flexible solution regardless of existence of fixed wired infrastructure

Efficient ad hoc routing necessary to achieve high throughput

Path metric crucial in selecting ad hoc paths

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Related Work

ETX (Expected Transmission Count) [MobiCom’03] considers packet loss, but does not accurately model

transmission interference

Existing transmission interference models do not consider packet loss

None of existing work has comprehensively addressed packet loss, transmission interference

together

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ETX

Average # transmissions (including retransmissions) needed for successful packet delivery on wireless link with loss rate p

ETX sum of ad hoc path sum of ETX of individual links used as path metric for selecting best ad hoc path

Achievable Data Rate of a link:

Maximal data rate / ETX

Maximal data rate delivery ratio

ETX =1 - p

1

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Limitations of ETX sum

PathA:ETX=1.5 ETX=1.5

ETX=1.0 ETX=2.0PathB:

ETX=1.7 ETX=1.7PathC:

UDP packet size: 1500 bytes

Source node always backlogged (11 Mbps)

ETX sum cannot accurately differentiate ad hoc paths

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Goal

Develop an accurate high-throughput path metric for multi-hop wireless

networks

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Outline

Problem Setting

EDR (Expected Data Rate)Transmission Contention DegreeBack-off procedure

Performance Evaluation

Summary

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Problem Setting

IEEE 802.11 networks Distributed Coordination Function (DCF) all links use single data rate

Load-insensitive path metric, routing does not consider “dynamic interference” due to other

flows considers “unavoidable” transmission interference within

single flow1 2 3 4

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Basic Ideas of EDR

Every link relies on supplying rate from previous link

EDR : achievable data rate of whole ad hoc path = achievable data rate of bottleneck link

B: Bottleneck linkD: Maximal Data rate on

link BETX(B)

DEDR =

ETX(B) for wired links

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Basic Ideas of EDR

Every link relies on supplying rate from previous link

EDR : achievable data rate of whole ad hoc path = achievable data rate of bottleneck link

B: Bottleneck linkD: Maximal Data rate on

link B

I: Total transmission interference factorETX(B)

D

ETX(B) I EDR = for wireless

links

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Total Transmission Interference Factor

Depends upon TCD: Transmission Contention Degree RTCD: Relatively Increased TCD

I = Sum of all TCD and RTCD on links that interfere with bottleneck link B

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Transmission Contention Degree for Link k

Represents how busy link k transmitting, retransmitting packets range [0.0, 1.0], normalized value compared

maximal data rate of link k when node always backlogged, TCD = 1.0

Considers load due to original transmission, retransmissions

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How to calculate TCD?

TCD(k)

ETX(k)Supplying rate at link k+1 =

TCD(k+1) ?TCD(k)

ETX(k) ETX(k+1)

ETX(k)

TCD(k)TCD(k+1) = Min { 1, ETX(k+1) }

Assume ETX values of links are given TCD(k+1) in terms of TCD(k)?

TCD(1) = 1.0

Original load

Increased load due to lost packets

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Effect of 802.11 Back-off

No mechanism to differentiate packet loss due to collisions, channel noise

Upon packet loss exponential back-off used for occupying shared medium

Different loss rates between adjacent links different average contention window sizes different medium occupancy probabilities relatively increased TCD (RTCD) on higher

loss rate link

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How to calculate RTCD?

Assume W(1) = 5, W(2) = 10

Node 1 twice more likely to occupy shared medium than Node 2

Thus, higher loss rate node (Node 2) experiences relative increase in TCD due to different window sizes

RTCD(k+1) = W(k+1)/W(k) -1

1 2 3

105Window size W(k)

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EDR

ETX(B) IEDR =

D

D: Maximum data rate on bottleneck link B

ETX(B): ETX of link B

I: Sum of (TCD+ RTCD) over all links that interfere with link B

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Performance Evaluation NS-2 simulations

Independent, temporally correlated loss models

Randomly generate 270 ad hoc paths hop lengths: 2 - 5 link loss rates: 0.0 - 0.5 (ETX: 1.0 - 2.0)

Construct groups of 4 ad hoc paths between source, destination for given group as input set, find how well each metric

selects best ad hoc path

Use 1500-byte UDP packets, send rate at source node = 11 Mbps

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Independent loss

EDR performs much better than ETX sum

EDR: for 90% of input cases, throughput more than 90 % of best

00.10.20.30.40.50.60.70.80.9

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1 27 53 79 105 131 157 183 209 235 261

Input sets, each having four different ad hoc paths

Thp

ut r

atio

(ch

osen

/ be

st)

ETX sum EDR

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Temporally correlated loss

Packet burst loss modeled using two-state continuous time Markov chain

Burst length borrowed from experimental results [Divert, MobiSys ’04]

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 27 53 79 105 131 157 183 209 235 261

Input sets, each having four different ad hoc paths

Thpu

t rat

io (c

hose

n/be

st)ETX sum EDR

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Summary

Proposed a new metric, EDR

Showed that EDR can accurately determine achievable data rates of ad hoc paths

Future work investigate TCP over EDR routing apply EDR in multi-radio wireless networks

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Backup

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EDR for TCP on multi-rate paths

IEDR =

R

Bottleneck link B such that R = Min { D(k) / ETX(k) }

I = TCD(k)/TCDmax, k over interference range of link B, Normalized total transmission contention degree in terms of B

For TCP flows, EDR does not include RTCD in I because TCP window mechanism is able to avoid unnecessary overhead of RTCD by adjusting send rate at source node

ETX(k+1)ETX(k)

TCD(k+1) = TCD(k) D(k)D(k+1)

, TCD(1) = 1.0