High Throughput Route High Throughput Route Selection in Multi-RateSelection in Multi-Rate

Ad Hoc Wireless NetworksAd Hoc Wireless Networks

Dr. Baruch Awerbuch, David Holmer, Dr. Baruch Awerbuch, David Holmer, and Herbert Rubensand Herbert Rubens

Johns Hopkins University

Department of Computer Science

www.cnds.jhu.edu/archipelago

OverviewOverview

Problem:Problem:Route selection in multi-rate ad hoc networkRoute selection in multi-rate ad hoc network

Traditional Technique:Traditional Technique:Minimum Hop PathMinimum Hop Path

New Technique:New Technique:Medium Time Metric (MTM)Medium Time Metric (MTM)

Goal:Goal:Maximize network throughputMaximize network throughput

What is Multi-Rate?What is Multi-Rate?

Ability of a wireless card to automatically Ability of a wireless card to automatically operate at several different bit-ratesoperate at several different bit-rates

(e.g. 1, 2, 5.5, and 11 Mbps)(e.g. 1, 2, 5.5, and 11 Mbps)

Part of many existing wireless standardsPart of many existing wireless standards(802.11b, 802.11a, 802.11g, HiperLAN2…)(802.11b, 802.11a, 802.11g, HiperLAN2…)

Virtually every wireless card in use today Virtually every wireless card in use today employs multi-rateemploys multi-rate

Advantage of Multi-Rate?Advantage of Multi-Rate?

Direct relationship between Direct relationship between communication rate and communication rate and the channel quality required the channel quality required for that ratefor that rate

As distance increases, As distance increases, channel quality decreaseschannel quality decreases

Therefore: tradeoff Therefore: tradeoff between communication between communication range and link speedrange and link speed

Multi-rate provides flexibilityMulti-rate provides flexibility

1 Mbps

2 Mbps

5.5 Mbps

11 Mbps

Lucent Orinoco 802.11b card ranges usingNS2 two-ray ground propagation model

Ad hoc Network Single Rate Ad hoc Network Single Rate ExampleExample

Which route to Which route to select?select?

Source

Destination

Ad hoc Network Single Rate Ad hoc Network Single Rate ExampleExample

Which route to Which route to select?select?

Source and Source and Destination are Destination are neighbors! Just route neighbors! Just route directly.directly.Source

Destination

Multi-rate Network ExampleMulti-rate Network Example

Varied Link RatesVaried Link Rates

Source

Destination

11 Mbps5.5 Mbps2 Mbps1 Mbps

Multi-rate Network ExampleMulti-rate Network Example

Varied Link RatesVaried Link Rates

Source

Destination

11 Mbps5.5 Mbps2 Mbps1 Mbps

Throughput = 1.04 Mbps

Multi-rate Network ExampleMulti-rate Network Example

Varied Link RatesVaried Link Rates

Source

Destination

11 Mbps5.5 Mbps2 Mbps1 Mbps

Throughput = 1.15 Mbps

Multi-rate Network ExampleMulti-rate Network Example

Varied Link RatesVaried Link Rates

Min Hop Selects Min Hop Selects Direct LinkDirect Link 0.85 Mbps0.85 Mbps

Source

Destination

11 Mbps5.5 Mbps2 Mbps1 Mbps

Multi-rate Network ExampleMulti-rate Network Example

Varied Link RatesVaried Link Rates

Min Hop Selects Min Hop Selects Direct LinkDirect Link 0.85 Mbps effective0.85 Mbps effective

Highest Throughput Highest Throughput PathPath 2.38 Mbps effective2.38 Mbps effective

Source

Destination

11 Mbps5.5 Mbps2 Mbps1 Mbps

Multi-rate Network ExampleMulti-rate Network Example

Under MobilityUnder Mobility

Min HopMin Hop Path BreaksPath Breaks

High Throughput PathHigh Throughput Path Reduced Link SpeedReduced Link Speed Reliability MaintainedReliability Maintained More “elastic” pathMore “elastic” path

Source

Destination

11 Mbps5.5 Mbps2 Mbps1 Mbps

X

Challenge to the Routing ProtocolChallenge to the Routing Protocol

Must select a path from Source to Must select a path from Source to DestinationDestination

Links operate at different speedsLinks operate at different speeds

Fundamental TradeoffFundamental Tradeoff Fast/Short links = low range = many Fast/Short links = low range = many

hops/transmissions to get to destinationhops/transmissions to get to destination Slow/Long links = long range = few Slow/Long links = long range = few

hops/transmissionshops/transmissions

Minimum Hop PathMinimum Hop Path(Traditional Technique)(Traditional Technique)

A small number of long slow hops provide A small number of long slow hops provide the minimum hop paththe minimum hop path

These slow transmissions occupy the These slow transmissions occupy the medium for long times, blocking adjacent medium for long times, blocking adjacent senderssenders

Selecting nodes on the fringe of the Selecting nodes on the fringe of the communication range results in reduced communication range results in reduced reliabilityreliability

How can we achieve high How can we achieve high throughput?throughput?

Throughput depends on several factorsThroughput depends on several factors Physical configuration of the nodesPhysical configuration of the nodes Fundamental properties of wireless Fundamental properties of wireless

communicationcommunication MAC protocolMAC protocol

Wireless Shared MediumWireless Shared Medium

Transmission blocks Transmission blocks all nearby activity to all nearby activity to avoid collisionsavoid collisions

MAC protocol MAC protocol provides channel provides channel arbitrationarbitration

Carrier Sense Range Carrier Sense Range

1 2

Transmission DurationTransmission Duration

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

1.0

2.0

5.5

11.0

Ra

te (

Mb

ps

)

Medium Time (milliseconds)

MAC Overhead Data4.55 Mbps

3.17 Mbps

1.54 Mbps0.85 Mbps

Medium Time consumed to transmit 1500 byte packet

Hops vs. ThroughputHops vs. Throughput

Since the medium is Since the medium is shared, adjacent shared, adjacent transmissions transmissions compete for medium compete for medium time.time.

Throughput Throughput decreases as number decreases as number of hops increase.of hops increase.

1 2 3

Effect of TransmissionEffect of Transmission

Source Destination

Request to Send (RTS)Clear to Send (CTS)DATAACK

1 2 3 4 5 6 7 8X X X X X X X

Multi-Hop Throughput Loss (TCP)Multi-Hop Throughput Loss (TCP)

1 2 3 4 5 6 7 8 9

1.0 Mbps2.0 Mbps

5.5 Mbps11.0 Mbps0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Th

rou

gh

pu

t (M

bp

s)

Hops

1.0 Mbps

2.0 Mbps

5.5 Mbps

11.0 Mbps

AnalysisAnalysis

General Model of ad hoc network General Model of ad hoc network throughputthroughput Multi-rate transmission graphMulti-rate transmission graph Interference graphInterference graph Flow constraintsFlow constraints

General Throughput Maximization Solution General Throughput Maximization Solution is NP Completeis NP CompleteDerived an optimal solution under a full Derived an optimal solution under a full interference assumptioninterference assumption

New Approach:New Approach: Medium Time Metric (MTM) Medium Time Metric (MTM)

Assigns a weight to each link proportional Assigns a weight to each link proportional to the amount of medium time consumed to the amount of medium time consumed by transmitting a packet on the linkby transmitting a packet on the link

Existing shortest path protocols will then Existing shortest path protocols will then discover the path that minimizes total discover the path that minimizes total transmission timetransmission time

MTM ExampleMTM Example

11

1

4.55 Mbps

0.85 Mbps

2.5ms

13.9ms

= 2.5

Path ThroughputPath Medium Time Metric (MTM)

= 13.9

Link Rate

1 MbpsSource Destination

11 Mbps

MTM ExampleMTM Example

11 + 11

1

2.36 Mbps

0.85 Mbps

2.5ms

13.9ms

= 5.0

Path ThroughputPath Medium Time Metric (MTM)

= 13.9

Link Rate

1 MbpsSource Destination

11 Mbps

2.5ms

11 Mbps

MTM ExampleMTM Example

11 + 11 + 11

1

1.57 Mbps

0.85 Mbps

2.5ms

13.9ms

= 7.5

Path ThroughputPath Medium Time Metric (MTM)

= 13.9

Link Rate

1 MbpsSource Destination

11 Mbps

2.5ms

11 Mbps

11 Mbps

2.5ms

MTM ExampleMTM Example

11 + 11 + 11 + 11

1

1.18 Mbps

0.85 Mbps

2.5ms

13.9ms

= 10.0

Path ThroughputPath Medium Time Metric (MTM)

= 13.9

Link Rate

1 MbpsSource Destination

11 Mbps

2.5ms 2.5ms 2.5ms

MTM ExampleMTM Example

11 + 11 + 11 + 11 + 11

1

0.94 Mbps

0.85 Mbps

2.5ms

13.9ms

= 12.5

Path ThroughputPath Medium Time Metric (MTM)

= 13.9

Link Rate

1 MbpsSource Destination

11 Mbps

2.5ms 2.5ms 2.5ms 2.5ms

MTM ExampleMTM Example

11 + 11 + 11 + 11 + 11 + 11

1

0.78 Mbps

0.85 Mbps

2.5ms

13.9ms

= 15

Path ThroughputPath Medium Time Metric (MTM)

= 13.9

Link Rate

1 MbpsSource Destination

11 Mbps

2.5ms 2.5ms 2.5ms 2.5ms 2.5ms

MTM ExampleMTM Example

Source

Destination

11 Mbps

5.5 Mbps

2 Mbps

1 Mbps

1 0.85 Mbps

2.5ms

3.7ms

7.6ms

13.9ms

11 Mbps

5.5 Mbps

2 Mbps

1 Mbps

13.9ms

Medium Time Usage

4.55 Mbps

3.17 Mbps

1.54 Mbps

0.85 Mbps

Path ThroughputPath Medium Time Metric (MTM)

= 13.9 ms

Link Throughput

MTM ExampleMTM Example

Source

Destination

11 Mbps

5.5 Mbps

2 Mbps

1 Mbps

5.5 + 2

1

1.04 Mbps

0.85 Mbps

2.5ms

3.7ms

7.6ms

13.9ms

11 Mbps

5.5 Mbps

2 Mbps

1 Mbps

7.6ms3.7ms

13.9ms

= 11.3 ms

Medium Time Usage

4.55 Mbps

3.17 Mbps

1.54 Mbps

0.85 Mbps

Path ThroughputPath Medium Time Metric (MTM)

= 13.9 ms

Link Throughput

MTM ExampleMTM Example

Source

Destination

11 Mbps

5.5 Mbps

2 Mbps

1 Mbps

11 + 2

5.5 + 2

1

1.15 Mbps

1.04 Mbps

0.85 Mbps

2.5ms

3.7ms

7.6ms

13.9ms

11 Mbps

5.5 Mbps

2 Mbps

1 Mbps

2.5ms 7.6ms

7.6ms3.7ms

13.9ms

= 10.1 ms

= 11.3 ms

Medium Time Usage

4.55 Mbps

3.17 Mbps

1.54 Mbps

0.85 Mbps

Path ThroughputPath Medium Time Metric (MTM)

= 13.9 ms

Link Throughput

MTM ExampleMTM Example

Source

Destination

11 Mbps

5.5 Mbps

2 Mbps

1 Mbps

11 + 11

11 + 2

5.5 + 2

1

2.38 Mbps

1.15 Mbps

1.04 Mbps

0.85 Mbps

2.5ms

3.7ms

7.6ms

13.9ms

11 Mbps

5.5 Mbps

2 Mbps

1 Mbps

2.5ms 2.5ms

2.5ms 7.6ms

7.6ms3.7ms

13.9ms

= 5.0 ms

= 10.1 ms

= 11.3 ms

Medium Time Usage

4.55 Mbps

3.17 Mbps

1.54 Mbps

0.85 Mbps

Path ThroughputPath Medium Time Metric (MTM)

= 13.9 ms

Link Throughput

AdvantagesAdvantages

It’s an additive shortest path metricIt’s an additive shortest path metricPaths which minimize network utilization, Paths which minimize network utilization, maximize network capacitymaximize network capacity Global optimum under complete interferenceGlobal optimum under complete interference Single flow optimum up to pipeline distance Single flow optimum up to pipeline distance

(7-11 hops)(7-11 hops) Excellent heuristic in even larger networksExcellent heuristic in even larger networks

Avoiding low speed links inherently Avoiding low speed links inherently provides increased route stabilityprovides increased route stability

DisadvantagesDisadvantages

MTM paths require more hopsMTM paths require more hops More transmitting nodesMore transmitting nodes

Increased contention for mediumIncreased contention for medium

Results in more load on MAC protocolResults in more load on MAC protocol

Only a few percent reduction under the simulated Only a few percent reduction under the simulated conditionsconditions

Increase in buffering along pathIncrease in buffering along pathHowever, higher throughput paths have lower However, higher throughput paths have lower propagation delaypropagation delay

Sounds great but…Sounds great but…

Do faster paths actually exist?Do faster paths actually exist? There needs to be enough nodes between the There needs to be enough nodes between the

source and the destination to provide a faster source and the destination to provide a faster pathpath

Therefore performance could vary as a Therefore performance could vary as a function of node densityfunction of node density

When density is low: MTM = Min HopWhen density is low: MTM = Min Hop

Performance Increase vs. Node Performance Increase vs. Node Density in Static Random LineDensity in Static Random Line

0%

50%

100%

150%

200%

250%

0 5 10 15 20 25 30 35

Density (nodes per radius)

Av

era

ge

Th

rou

gh

pu

t In

cre

as

e(v

s. M

in H

op

)

1600 m 3200 m 4800 m 6400 m

MTM Throughput IncreaseMTM Throughput IncreaseUnder 802.11MACUnder 802.11MAC

0%

10%

20%

30%

40%

50%

60%

20 30 40 50 60 70

Average Density (nodes per radius)

Av

era

ge

Th

rou

gh

pu

t In

cre

as

e

-NS2 Network Simulations-20 TCP Senders and receivers

-Random Waypoint mobility (0-20m/s)-DSDV Protocol modified to find MTM path

MTM + OAR Throughput IncreaseMTM + OAR Throughput Increaseover Min Hop + 802.11over Min Hop + 802.11

0%

20%

40%

60%

80%

100%

120%

140%

160%

180%

200%

20 30 40 50 60 70

Average Density (nodes per radius)

Av

era

ge

Th

rou

gh

pu

t In

cre

as

e

-NS2 Network Simulations-20 TCP Senders and receivers

-Random Waypoint mobility (0-20m/s)-DSDV Protocol modified to find MTM path

Thank You!

Questions??

More Information:

http://www.cnds.jhu.edu/networks/archipelago/

Herb Rubensherb@cs.jhu.edu