Opportunistic Multi-Hop Routing for Wireless NetworksSanjit Biswas and Robert Morris M.I.T. Computer Science and Artificial Intelligence Lab

Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion

An integrated routing and MAC protocol Aims to increase the throughput of large

unicast transfers in multi hop wireless networks.

Based on cooperative diversity routing

Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion

Why the need for a new routing protocol ?

Why the need for a new routing protocol ?

Traditional routing protocols were designed for wired networks.

These protocols don’t take into account underlying wireless dynamics at MAC and PHY layer.

Reference : 802.11 Wireless Networks: The Definitive Guide O'Reilly Publications . Mathew Gast. April 2005

RF Link Quality – As compared to wired Ethernet, 802.11 has to deal with interference, noise and multipath fading in unlicensed bands which results in loss of packets.

Solution – Positive Acknowledgments When ACK doesn’t arrive , packet is

retransmitted.

Reference : 802.11 Wireless Networks: The Definitive Guide O'Reilly Publications . Mathew Gast. April 2005

ExOR tries to avoid these retransmissions by relying on cooperative diversity scheme.

Reference : 802.11 Wireless Networks: The Definitive Guide O'Reilly Publications . Mathew Gast. April 2005

Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion

Traditional Routing Protocols

Choose Best Sequence of nodes Between source and Destination

Cooperative Diversity Protocols

Uses broadcast transmission to Send info through multiple relays

Routing Protocols

Classic network architectures employ point to point transmissions.

Cooperative transmission takes advantage of broadcast transmission to send information through multiple relays concurrently.

Originally meant to avoid multipath fading.

Introduction Motivation Background ExOR Why ExOR works ? Design Challenges Results and Evaluation Conclusion

ExOR broadcasts each packet, choosing a receiver to forward only after learning the set of nodes that actually received the packet.

By delaying this decision until after reception, multiple long but radio lossy links are given a trial.

Traditional Routing ExOR Routing

Source

Destination

Source

Destination

Traditional Routing ExOR Routing

Source

Destination

Source

Destination

Traditional Routing ExOR Routing

Source

Destination

Source

Destination

Traditional Routing ExOR Routing

Source

Destination

Source

Destination

Traditional Routing ExOR Routing

Source

Destination

Source

Destination

Traditional Routing ExOR Routing

Source

Destination

Source

Destination

Introduction Motivation Background ExOR Why ExOR works ? Design Challenges Results and Evaluation Conclusion

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 dstN1 N2 N3 N4

75%50%

N5

25%

Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt

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

25%

25%

25%

25%

100%

100%

100%

100%

Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt

Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion

Agreement amongst the nodes which received each packet.

Need for a metric which decides the node which is closest to the destination.

Not too many nodes should be potential forwarders.

Minimize collisions.

Agreement amongst the nodes which received each packet.

Agreement protocol should have low overhead.

Solution -- Batch Forwarding and Node State

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: 0tx: 0

tx: 9

N1

N2

Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt

Each node maintains this for each batch of packets its operating on.

Packet Buffer Local Forwarder List Forwarding timer Transmission Tracker Batch Map

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

src

N1

N2

N3

dst

N4

Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt

Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion

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

Traditional Routing ExOR802.11 unicast with link-level retransmissionsHop-by-hop batchingUDP, sending as MAC allows

802.11 broadcasts100 packet batch size

Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt

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

00 200 400 600 800C

um

ula

tive F

ract

ion o

f N

ode P

air

s

ExORTraditional

Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt

Node Pair

4 Traditional Hops3.3x

Longer Routes

Thro

ughput

(Kbit

s/se

c)

0

200

400

600

800

1000 ExORTraditional Routing

Reference : pdos.csail.mit.edu/~biswas/exor-sigcomm.ppt

Introduction Motivation Background Basic Idea of how ExOR works Why ExOR works ? Design Challenges Results and Evaluation Conclusion

Positives Works well on long range links Works well on one hop links

Weaknesses Doesn’t integrate well with TCP Working with multiple radios