Opportunistic Routing in Multi-hop Wireless Networks Sanjit Biswas - - PowerPoint PPT Presentation

Opportunistic Routing in Multi-hop Wireless Networks

Sanjit Biswas and Robert Morris MIT CSAIL

http://pdos.csail.mit.edu/roofnet/

ExOR: a new approach to routing in multi-hop wireless networks

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

1 kilometer

packet packet packet

Initial approach: Traditional routing

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

src src src src A A A A B B B B dst dst dst dst C C C C

Radios aren’t wires

• Every packet is broadcast
• Reception is probabilistic

1 2 3 4 5 6 1 2 3 6 3 5 1 4 2 3 4 56 1 2 4 5 6

src src src src A A A A B B B B dst dst dst dst C C C C

packet packet packet packet packet packet

ExOR: exploiting probabilistic broadcast

src src src src A A A A B B B B dst dst dst dst C C C C

packet packet packet

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

Outline

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

Why ExOR might increase throughput (1)

• 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

src dst N1 N2 N3 N4 75% 50% N5 25%

Why ExOR might increase throughput (2)

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

N1 src dst N2 N3 N4 2 5 % 2 5 % 25% 25% 100% 100% 100% 100%

Outline

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

ExOR batching

• Challenge: finding the closest node to have rx’d
• 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

src N3 dst N4 tx: 23 tx: 57 -23

≅ 24

tx: ≅ 8 tx: 100 rx: 23 rx: 57 rx: 88 rx: 0 rx: 0 tx: 0 tx: ≅ 9 rx: 53 rx: 85 rx: 99 rx: 40 rx: 22 N1 N2

Reliable summaries

• Repeat summaries in every data packet
• Cumulative: what all previous nodes rx’d
• This is a gossip mechanism for summaries

src N1 N2 N3 dst N4

tx: {1, 6, 7 ... 91, 96, 99} tx: {2, 4, 10 ... 97, 98} summary: {1,2,6, ... 97, 98, 99} summary: {1, 6, 7 ... 91, 96, 99}

Priority ordering

• 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 (Dijkstra’s algorithm)

• Source sorts, includes list in ExOR header
• Details in the paper

src N1 N2 N3 dst N4

Using ExOR with TCP

Node Proxy ExOR ExOR ExOR ExOR Gateway Web Proxy Client PC Web Server

TCP TCP TCP TCP TCP TCP TCP TCP ExOR ExOR ExOR ExOR Batches (not TCP) Batches (not TCP) Batches (not TCP) Batches (not TCP)

• Batching requires more packets than typical

TCP window

Outline

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

ExOR Evaluation

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

65 Roofnet node pairs

1 kilometer

Evaluation Details

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

802.11 broadcasts 100 packet batch size 802.11 unicast with link-level retransmissions Hop-by-hop batching UDP, sending as MAC allows

ExOR Traditional Routing

ExOR: 2x overall improvement

• Median throughputs:

240 Kbits/sec for ExOR, 121 Kbits/sec for Traditional

Throughput (Kbits/sec) 1.0 0.8 0.6 0.4 0.2 200 400 600 800 Cumulative Fraction of Node Pairs

ExOR Traditional

25 Highest throughput pairs

Node Pair Throughput (Kbits/sec) 200 400 600 800 1000 ExOR Traditional Routing

1 Traditional Hop 1 Traditional Hop 1 Traditional Hop 1 Traditional Hop 1.14x 1.14x 1.14x 1.14x 2 Traditional Hops 2 Traditional Hops 2 Traditional Hops 2 Traditional Hops 1.7x 1.7x 1.7x 1.7x 3 Traditional Hops 3 Traditional Hops 3 Traditional Hops 3 Traditional Hops 2.3x 2.3x 2.3x 2.3x

25 Lowest throughput pairs

Node Pair

4 Traditional Hops 4 Traditional Hops 4 Traditional Hops 4 Traditional Hops 3.3x 3.3x 3.3x 3.3x Longer Routes Longer Routes Longer Routes Longer Routes

Throughput (Kbits/sec) 200 400 600 800 1000 ExOR Traditional Routing

ExOR uses links in parallel

Traditional Routing 3 forwarders 4 links ExOR 7 forwarders 18 links

ExOR moves packets farther

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

Fraction of Transmissions

0.1 0.2 0.6 ExOR Traditional Routing 100 200 300 400 500 600 700 800 900 1000

Distance (meters) 25% of 25% of 25% of 25% of ExOR ExOR ExOR ExOR transmissions transmissions transmissions transmissions 58% of Traditional Routing transmissions 58% of Traditional Routing transmissions 58% of Traditional Routing transmissions 58% of Traditional Routing transmissions

Future Work

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

Related work

• Relay channels

[Van der Meulen][Laneman+Wornell]

• Flooding in meshes / sensor nets

[Peng][Levis]

• Multi-path routing

[Ganesan][Haas]

• Selection Diversity

[Miu][Roy Chowdhury][Knightly][Zorzi]

Summary

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

Thanks!

For more information and source code:

http://pdos.csail.mit.edu/roofnet/