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 [email protected]