High Throughput Route Selection in Multi-Rate Ad Hoc Wireless Networks

1
High Throughput Route Selection in Multi-RateAd
Hoc Wireless Networks

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

Johns Hopkins University
Department of Computer Science
www.cnds.jhu.edu/archipelago
2
Overview

• Problem
• Route selection in multi-rate ad hoc network
• Traditional Technique
• Minimum Hop Path
• New Technique
• Medium Time Metric (MTM)
• Goal
• Maximize network throughput

3
What is Multi-Rate?

• Ability of a wireless card to automatically
  operate at several different bit-rates
• (e.g. 1, 2, 5.5, and 11 Mbps)
• Part of many existing wireless standards
• (802.11b, 802.11a, 802.11g, HiperLAN2)
• Virtually every wireless card in use today
  employs multi-rate

4
Advantage of Multi-Rate?

• Direct relationship between communication rate
  and the channel quality required for that rate
• As distance increases, channel quality decreases
• Therefore tradeoff between communication range
  and link speed
• Multi-rate provides flexibility

1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
Lucent Orinoco 802.11b card ranges using NS2
two-ray ground propagation model
5
Ad hoc Network Single Rate Example

• Which route to select?

Destination
Source
6
Ad hoc Network Single Rate Example

• Which route to select?
• Source and Destination are neighbors! Just route
  directly.

Destination
Source
7
Multi-rate Network Example

• Varied Link Rates

Destination
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
8
Multi-rate Network Example

• Varied Link Rates

Destination
Throughput 1.04 Mbps
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
9
Multi-rate Network Example

• Varied Link Rates

Destination
Throughput 1.15 Mbps
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
10
Multi-rate Network Example

• Varied Link Rates
• Min Hop Selects Direct Link
• 0.85 Mbps

Destination
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
11
Multi-rate Network Example

• Varied Link Rates
• Min Hop Selects Direct Link
• 0.85 Mbps effective
• Highest Throughput Path
• 2.38 Mbps effective

Destination
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
12
Multi-rate Network Example

• Under Mobility
• Min Hop
• Path Breaks
• High Throughput Path
• Reduced Link Speed
• Reliability Maintained
• More elastic path

Destination
X
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
13
Challenge to the Routing Protocol

• Must select a path from Source to Destination
• Links operate at different speeds
• Fundamental Tradeoff
• Fast/Short links low range many
  hops/transmissions to get to destination
• Slow/Long links long range few
  hops/transmissions

14
Minimum Hop Path(Traditional Technique)

• A small number of long slow hops provide the
  minimum hop path
• These slow transmissions occupy the medium for
  long times, blocking adjacent senders
• Selecting nodes on the fringe of the
  communication range results in reduced reliability

15
How can we achieve high throughput?

• Throughput depends on several factors
• Physical configuration of the nodes
• Fundamental properties of wireless communication
• MAC protocol

16
Wireless Shared Medium

• Transmission blocks all nearby activity to avoid
  collisions
• MAC protocol provides channel arbitration

Carrier Sense Range
Carrier Sense Range
1
2
17
Transmission Duration
4.55 Mbps
3.17 Mbps
1.54 Mbps
0.85 Mbps
Medium Time consumed to transmit 1500 byte packet
18
Hops vs. Throughput

• Since the medium is shared, adjacent
  transmissions compete for medium time.
• Throughput decreases as number of hops increase.

1
2
3
19
Effect of Transmission
Source
Destination
X
X
X
X
X
X
X
1
2
3
4
5
6
7
8
Request to Send (RTS)
Clear to Send (CTS)
DATA
ACK
20
Multi-Hop Throughput Loss (TCP)
21
Analysis

• General Model of ad hoc network throughput
• Multi-rate transmission graph
• Interference graph
• Flow constraints
• General Throughput Maximization Solution is NP
  Complete
• Derived an optimal solution under a full
  interference assumption

22
New Approach Medium Time Metric (MTM)

• Assigns a weight to each link proportional to the
  amount of medium time consumed by transmitting a
  packet on the link
• Existing shortest path protocols will then
  discover the path that minimizes total
  transmission time

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MTM Example
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11
2.5
4.55 Mbps
2.5ms
1
0.85 Mbps
13.9ms
13.9
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MTM Example
11 Mbps
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 11
5.0
2.36 Mbps
2.5ms
2.5ms
1
0.85 Mbps
13.9ms
13.9
25
MTM Example
11 Mbps
11 Mbps
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 11 11
7.5
1.57 Mbps
2.5ms
2.5ms
2.5ms
1
0.85 Mbps
13.9ms
13.9
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MTM Example
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 11 11 11
10.0
1.18 Mbps
2.5ms
2.5ms
2.5ms
2.5ms
1
0.85 Mbps
13.9ms
13.9
27
MTM Example
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 11 11 11 11
12.5
0.94 Mbps
2.5ms
2.5ms
2.5ms
2.5ms
2.5ms
1
0.85 Mbps
13.9ms
13.9
28
MTM Example
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 11 11 11 11 11
15
0.78 Mbps
2.5ms
2.5ms
2.5ms
2.5ms
2.5ms
2.5ms
1
0.85 Mbps
13.9ms
13.9
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MTM Example
Medium Time Usage
Link Throughput
Destination
4.55 Mbps
11 Mbps
2.5ms
3.17 Mbps
5.5 Mbps
3.7ms
1.54 Mbps
2 Mbps
7.6ms
0.85 Mbps
1 Mbps
13.9ms
Source
Path Medium Time Metric (MTM)
Path Throughput
11 Mbps
5.5 Mbps
1
0.85 Mbps
13.9ms
13.9 ms
2 Mbps
1 Mbps
30
MTM Example
Medium Time Usage
Link Throughput
Destination
4.55 Mbps
11 Mbps
2.5ms
3.17 Mbps
5.5 Mbps
3.7ms
1.54 Mbps
2 Mbps
7.6ms
0.85 Mbps
1 Mbps
13.9ms
Source
Path Medium Time Metric (MTM)
Path Throughput
5.5 2
11 Mbps
1.04 Mbps
11.3 ms
7.6ms
3.7ms
5.5 Mbps
1
0.85 Mbps
13.9ms
13.9 ms
2 Mbps
1 Mbps
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MTM Example
Medium Time Usage
Link Throughput
Destination
4.55 Mbps
11 Mbps
2.5ms
3.17 Mbps
5.5 Mbps
3.7ms
1.54 Mbps
2 Mbps
7.6ms
0.85 Mbps
1 Mbps
13.9ms
Source
Path Medium Time Metric (MTM)
Path Throughput
11 2
1.15 Mbps
2.5ms
7.6ms
10.1 ms
5.5 2
11 Mbps
1.04 Mbps
11.3 ms
7.6ms
3.7ms
5.5 Mbps
1
0.85 Mbps
13.9ms
13.9 ms
2 Mbps
1 Mbps
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MTM Example
Medium Time Usage
Link Throughput
Destination
4.55 Mbps
11 Mbps
2.5ms
3.17 Mbps
5.5 Mbps
3.7ms
1.54 Mbps
2 Mbps
7.6ms
0.85 Mbps
1 Mbps
13.9ms
Source
Path Medium Time Metric (MTM)
Path Throughput
11 11
5.0 ms
2.38 Mbps
2.5ms
2.5ms
11 2
1.15 Mbps
2.5ms
7.6ms
10.1 ms
5.5 2
11 Mbps
1.04 Mbps
11.3 ms
7.6ms
3.7ms
5.5 Mbps
1
0.85 Mbps
13.9ms
13.9 ms
2 Mbps
1 Mbps
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Advantages

• Its an additive shortest path metric
• Paths which minimize network utilization,
  maximize network capacity
• Global optimum under complete interference
• Single flow optimum up to pipeline distance (7-11
  hops)
• Excellent heuristic in even larger networks
• Avoiding low speed links inherently provides
  increased route stability

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Disadvantages

• MTM paths require more hops
• More transmitting nodes
• Increased contention for medium
• Results in more load on MAC protocol
• Only a few percent reduction under the simulated
  conditions
• Increase in buffering along path
• However, higher throughput paths have lower
  propagation delay

35
Sounds great but

• Do faster paths actually exist?
• There needs to be enough nodes between the source
  and the destination to provide a faster path
• Therefore performance could vary as a function of
  node density
• When density is low MTM Min Hop

36
Performance Increase vs. Node Density in Static
Random Line
37
MTM Throughput IncreaseUnder 802.11MAC
-NS2 Network Simulations -20 TCP Senders and
receivers
-Random Waypoint mobility (0-20m/s) -DSDV
Protocol modified to find MTM path
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MTM OAR Throughput Increaseover Min Hop
802.11
-NS2 Network Simulations -20 TCP Senders and
receivers
-Random Waypoint mobility (0-20m/s) -DSDV
Protocol modified to find MTM path
39
Thank You!
Questions??
Herb Rubens herb_at_cs.jhu.edu
More Information
http//www.cnds.jhu.edu/networks/archipelago/