ExOR: Opportunistic Multi-Hop Routing for Wireless Networks

Sigcomm 2005

Sanjit Biswas and Robert MorrisMIT Computer Science and Artificial Intelligence Laboratory

Presented by Sungwon Yang 2009.05.12

What is ExOR? Extremely Opportunistic Routing

Routing in multi-hop wireless networks

Cross-Layer Protocol: Routing + MAC

Aims to increase the throughput of large unicast transfers

Based on cooperative diversity routing

Motivation Traditional routing protocols were designed for

wired networks Identify a route, forward over links

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

packet

packet

packet

src

A B

dst

C

Motivation Radio is not wired

Every packet is broadcast Reception is probabilistic

123456123 63 51 42345612 456 src

A B

dst

C

Basic concept of ExOR exploiting probabilistic broadcast

Decide who forwards after reception Goal: only closest receiver should forward

packet

packetpacketpacketpacketpacket src

AB

dst

C

packetpacketpacket

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 Assumes probability falls off gradually with

distance

src dstN1 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

25%

25%

25%

25%

100%

100%

100%

100%

ExOR Design Challenges How to determine which nodes have received

a packet? Agreement amongst the nodes which received each

packet

What node (of the receivers) should forward a packet? Need for a metric which decides the node which is closest

to the destination

Minimize communication cost of coordination Not too many nodes should be potential forwarders

Minimize collisions

ExOR Mechanism: Source’s Behavior Collects enough packets of the same

destination to form a batch ExOR operates on batches of packets for efficiency Source gathers batch of packets to same

destination

Selects a set of nodes to be candidate forwarders, and includes the prioritized list in the header of every packet Potential forwarders are prioritized by

estimated cost to destination (by sender) ETX (Expected Transmission Count)

Forwarding in order of priority

What is ETX (1) Expected Transmission Count Proposed by the MIT AI Lab in MobiCom 2003 Predict the number of transmission(including

retransmission) Designed for finding the high-throughput path

in DSDV & DSR routing protocols Using periodical probe packets

What is ETX (2)

Forward list: ECDBA Broadcast in this order

ExOR Mechanism: Intermediate nodes’ Behavior (1) Q: How can a node know whether it is one of

the forwarders or not?

A: Check the forwarder list in the header of the received packet If the node finds itself in the list, buffer the packet

and keep state of this batch If no, discard the packet

ExOR Mechanism: Intermediate nodes’ Behavior (2) Q: How can a node know whether the packet it

receives has also been received by a node with higher priority or not?

A: ExOR uses “Batch Map” Acts as a gossip mechanism to carry

reception information-- from high priority nodes to low

Included in every transmission so that node’s local batch maps will converge

Low priority node unlikely to forward a packet received by high-priority node

ExOR Mechanism: Intermediate nodes’ Behavior (3) Q: How can a node know when it should send

packets?

A: ExOR uses “Forwarding Timer” Initially set long-enough A node adjusts the timer when it hear other nodes’

packets “Transmission Tracker” keeps track of the

remaining number of packets needed to be sent

ExOR Mechanism: Destination’s Behavior Actually destination is the last intermediate

node and has the highest priority.

After the finish of src’s transmission. Destination sends out packets only including the batch map, to inform other nodes about the packets it has received

Upon >90% of batch reception in batch map, packet is not forwarded further -- finish using traditional mechanisms

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

1 kilometer

•Roofnet: 38 nodes•ExOR implemented on Linux with 802.11b•65 node pairs randomly chosen•1.0MByte file transfer•1 Mbit/s 802.11 bit rate•1 KByte packets•9 iterations

Traditional Routing ExOR

802.11 unicast with link-level retransmissions

802.11 broadcasts100 packet batch size

Results (1) Median Throughput

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

Results (2) 25 Highest throughput pairs

Node Pair

Thro

ughput

(Kbit

s/se

c)

0

200

400

600

800

1000 ExORTraditional Routing

1 Traditional Hop

1.14x

2 Traditional Hops1.7x

3 Traditional Hops2.3x

Results (3) 25 Lowest throughput pairs

Node Pair

4 Traditional Hops3.3x

Longer Routes

Thro

ughput

(Kbit

s/se

c)

0

200

400

600

800

1000 ExORTraditional Routing

Results (4) ExOR moves packets farther

Fract

ion o

f Tra

nsm

issi

ons

0

0.1

0.2

0.6 ExORTraditional Routing

0 100 200 300 400 500 600 700 800 900 1000

Distance (meters)

25% of ExOR transmissions

58% of Traditional Routing transmissions

Conclusion & Secret Sauce Exploits radio properties, instead of hiding them

Benefit from long and lossy link Also work well on one-hop link

ExOR achieves 2x throughput improvement

Real implementation and experiments

Clearly-defined primary goal Achieve high throughput in large unicast transfer

Thank you