Opportunistic Routing in Multi-hop Wireless Networks

1
Opportunistic Routing in Multi-hop Wireless
Networks

• Sanjit Biswas and Robert Morris
• MIT CSAIL
• http//pdos.csail.mit.edu/roofnet/

2
ExOR a new approach to routing in multi-hop
wireless networks
1 kilometer

• Dense 802.11-based mesh
• Goal is high-throughput and capacity

3
Initial approach Traditional routing
packet
packet
A
B
src
dst
packet
C

• Identify a route, forward over links
• Abstract radio to look like a wired link

4
Radios arent wires
A
B
src
dst
1
2
3
4
5
6
1
2
3
6
3
5
1
4
2
3
4
5
6
1
2
4
5
6
C

• Every packet is broadcast
• Reception is probabilistic

5
ExOR exploiting probabilistic broadcast
packet
packet
packet
packet
A
B
src
dst
packet
packet
packet
packet
packet
C

• Decide who forwards after reception
• Goal only closest receiver should forward
• Challenge agree efficiently and avoid duplicate
  transmissions

6
Outline

• Introduction
• Why ExOR might increase throughput
• ExOR protocol
• Measurements
• Related Work

7
Why ExOR might increase throughput (1)
src
dst
N1
N2
N3
N4
N5
75
50
25

• Best traditional route over 50 hops 3(1/0.5)
  6 tx
• Throughput ? 1/ transmissions
• ExOR exploits lucky long receptions 4
  transmissions
• Assumes probability falls off gradually with
  distance

8
Why ExOR might increase throughput (2)
N1
25
100
N2
25
100
src
dst
100
25
N3
100
25
N4

• Traditional routing 1/0.25 1 5 tx
• ExOR 1/(1 (1 0.25)4) 1 2.5 transmissions
• Assumes independent losses

9
Outline

• Introduction
• Why ExOR might increase throughput
• ExOR protocol
• Measurements
• Related Work

10
ExOR batching
tx 0
N4
N2
tx 57 -23 ? 24
tx 100
tx ? 9
src
dst
N3
N1
tx ? 8
tx 23

• Challenge finding the closest node to have rxd
• Send batches of packets for efficiency
• Node closest to the dst sends first
• Other nodes listen, send remaining packets in
  turn
• Repeat schedule until dst has whole batch

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Reliable summaries
tx 2, 4, 10 ... 97, 98 summary 1,2,6, ...
97, 98, 99
N2
N4
src
dst
N1
N3
tx 1, 6, 7 ... 91, 96, 99
summary 1, 6, 7 ... 91, 96, 99

• Repeat summaries in every data packet
• Cumulative what all previous nodes rxd
• This is a gossip mechanism for summaries

12
Priority ordering
N2
N4
src
dst
N1
N3

• Goal nodes closest to the destination send
  first
• Sort by ETX metric to dst
• Nodes periodically flood ETX link state
  measurements
• Path ETX is weighted shortest path (Dijkstras
  algorithm)
• Source sorts, includes list in ExOR header
• Details in the paper

13
Using ExOR with TCP
Web Server
Client PC
Node
Gateway
Proxy
Web Proxy
ExOR

• Batching requires more packets than typical TCP
  window

14
Outline

• Introduction
• Why ExOR might increase throughput
• ExOR protocol
• Measurements
• Related Work

15
ExOR Evaluation

• Does ExOR increase throughput?
• When/why does it work well?

16
65 Roofnet node pairs
1 kilometer
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Evaluation Details

• 65 Node pairs
• 1.0MByte file transfer
• 1 Mbit/s 802.11 bit rate
• 1 KByte packets

Traditional Routing ExOR
802.11 unicast with link-level retransmissions Hop-by-hop batching UDP, sending as MAC allows 802.11 broadcasts 100 packet batch size
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ExOR 2x overall improvement
1.0
0.8
0.6
Cumulative Fraction of Node Pairs
0.4
0.2
ExOR
Traditional
0
0
200
400
600
800
Throughput (Kbits/sec)

• Median throughputs 240 Kbits/sec for ExOR,
• 121 Kbits/sec for Traditional

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25 Highest throughput pairs
1 Traditional Hop 1.14x
2 Traditional Hops 1.7x
3 Traditional Hops 2.3x
1000
ExOR
Traditional Routing
800
600
Throughput (Kbits/sec)
400
200
0
Node Pair
20
25 Lowest throughput pairs
1000
ExOR
4 Traditional Hops 3.3x
Traditional Routing
800
600
Throughput (Kbits/sec)
400
200
0
Node Pair
Longer Routes
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ExOR uses links in parallel

• Traditional Routing
• 3 forwarders
• 4 links

• ExOR
• 7 forwarders
• 18 links

22
ExOR moves packets farther
0.6
ExOR
Traditional Routing
Fraction of Transmissions
0.2
0.1
0
0
100
200
300
400
500
600
700
800
900
1000
Distance (meters)

• ExOR average 422 meters/transmission
• Traditional Routing average 205 meters/tx

23
Future Work

• Choosing the best 802.11 bit-rate
• Cooperation between simultaneous flows
• Coding/combining

24
Related work

• Relay channels
• Van der MeulenLanemanWornell
• Flooding in meshes / sensor nets
• PengLevis
• Multi-path routing
• GanesanHaas
• Selection Diversity
• MiuRoy ChowdhuryKnightlyZorzi

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Summary

• ExOR achieves 2x throughput improvement
• ExOR implemented on Roofnet
• Exploits radio properties, instead of hiding them

26
Thanks!

• For more information and source code
• http//pdos.csail.mit.edu/roofnet/