ExOR: Opportunistic Multi-Hop routing for Wireless Networks

Presentation based on paper & Slides from Sanjit Biswas & Robert Morris (MIT) on ExOR at SIGCOMM’05, USA.

- Presented by : Yatindra Shashi (Matriculation No.:181396) Wireless Network Course 1

Proof of concept was carried out as Roofnet project of MIT with 65 , 802.11 based mesh nodes

Goal was to maximize the through put and minimize the number of transmission.

ROOFNET

ExOR: Routing approach to Increase throughput in Wireless Network

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Introduction How ExOR increasing the throughput ExOR Protocol Realization Evaluation & Measurement Summary

Outline

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Packet forwarding

- Identify the route, forward over link - Radio looks like wired link

Traditional routing

SRC

A C

Dst

B

Packet

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How radios actually works..

Every packet is broadcast Not as Wired link

123456123 63 51 42345612 456 src

A B

dst

C

5

packet

packetpacketpacketpacketpacket

ExOR: Exploiting the Broadcast

src

A B

dst

C

packetpacketpacket

Multiple nodes receive the packet Node closest to destination forwards

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Introduction How ExOR increases the throughput ExOR Protocol Realization Evaluation and Measurement Summary

Outline

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How ExOR increases the throughput..

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 N475%

50%

N5

25%

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Continue…

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

N1

src dstN2

N3

N4

25%

25%25%25%

100%

100%

100%

100%

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Introduction How ExOR increases the throughput ExOR Protocol Realization Evaluation & Measurement Summary

Outline

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ExOR Protocol Realization Packets are queued and sent in Batches A list of forwarders prioritized based on ETX metric In the below topology ---> Source: A, Destination: E

Priority order : E C D B A (Lowe ETX) Other nodes listen They forward packets only if a higher priority node has

failed to do so

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An example A has transmitted a batch of 10 packets 1-10 E receives packets 1, 2 C receives 1 3 4 10 D receives 1 2 5 9 10 B receives 1 2 3 4 5 6 7 8 9 10

E received 1,2 1 2 3 4 5 6 7 8 9 10 Now C forwards 3, 4,10 1 2 3 4 5 6 7 8 9 10 D forwards 5,9 1 2 3 4 5 6 7 8 9 10 B forwards 6, 7, 8 1 2 3 4 5 6 7 8 9 10

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Reliable summaries

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

A B

CE

D

tx: {3, 4, 10}

tx: {5, 9}summary: { 1,2,3,4,5,9,10}

summary: {1, 2, 3,4,10}

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ExOR: Packet Format

-HdrLen & PayloadLen indicate size of ExOR header and payload respectively-PktNum is current packet’s offset in the batch, corresponding to the current batch-map entry-FragSz is size of currently sending node’s fragment (in packets)-FragNum is current packet’s offset within the fragment-FwdListSise is is number of forwarders in list-ForwarderNum is current sender’s offset within the list-Forwarder List is copy of sender’s local forwarder list-Batch Map is copy of sending node’s batch map, where each entry is an index into Forwarder List

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Transmission Timeline for an ExOR transfer

N24 not able to listen to N5.

N8 does not send

N17 might have missed some batch-maps

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Introduction How ExOR increases the throughput ExOR Protocol Realization Evaluation & Measurement Summary

Outline

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1 kilometer

65 Roofnet node pairs

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Evaluation Comparison between traditional routing done with ExOR Throughput between 65 randomly selected node pairs

evaluated 1 mega-byte file exchanged Batch size is 100 packet Data rate 1 megabit/second 1 Kbyte of payload

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Throughput (Kbits/sec)

1.0

0.8

0.6

0.4

0.2

00 200 400 600 800Cu

mul

ativ

e Fr

actio

n of

Nod

e Pa

irs

ExORTraditional

ExOR: 2x Improvement in throughput

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

Figure 8: The distribution of throughputs of ExOR and traditional routing between the 65 node pairs. The plots shows the median throughput achieved for each pair over nine experimental runs.

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25 Highest throughput pairs

Node Pair

Thro

ughp

ut (K

bits

/sec

)

0200

400

600

8001000 ExOR

Traditional Routing

1 Traditional Hop

1.14x

2 Traditional Hops1.7x

3 Traditional Hops2.3x

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Figure 9: The 25 highest throughput pairs, sorted by traditional routing throughput. The bars show each pair's median throughput, and the error bars show the lowest and highest of the nine experiments.

25 Lowest throughput pairs

Node Pair

4 Traditional Hops3.3x

Longer Routes

Thro

ughp

ut (K

bits

/sec

)

0200

400

600

8001000 ExOR

Traditional Routing

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Figure 10: The 25 lowest throughput pairs. The bars show each pair's median throughput, and the error bars show the lowest and the highest of the nine experiments. ExOR outperforms traditional routing by a factor of two or more.

Retransmissions affected by selection of hops

Traditional routing has to select the ‘shortest’ path which results in compromise on selecting drop probability, thus increasing the number of transmissions

ExOR has no limitations on number of nodes, from the forwarder list, that can forward the packet. Hence it uses both nodes closer to source and nodes closer to destination, irrespective of their drop probability

Figure 11: The number of transmissions made by each node during a 1000-packet transfer from N5 to N24. The X axis indicates the sender's ETX metric to N24. The Y axis indicates the number of packet transmissions that node performs. Bars higher than 1000 indicate nodes that had to re-send packets due to losses.

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ExOR moves packets farther

Figure 12: Distance traveled towards N24 in ETX space by each transmission. The X axis indicates the di®erence in ETX metric between the sending and receiving nodes; the receiver is the next hop for traditional routing, and the highest-priority receiving node for ExOR. The Y axis indicates the number of transmissions that travel the corresponding distance. Packets with zero progress are not received by the next hop (for traditional routing) or by any higher-priority node (for ExOR).

Max. distance traveled by hops in traditional routingDistance traveled by transmissions in ExOR

Big chunk of transmission, in traditional routing, takes place over shorter distances

Number of packets carried over individual long distance links is small

But cumulative transmission is substantial

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Introduction How ExOR increases the throughput ExOR Protocol Realization Evaluation & Measurement Summary

Outline

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Summary ExOR opportunistically exploits wireless broadcast

◦ long distance transmission◦ Avoids retransmission by allowing a low priority

node to forward◦ Increasing the throughput

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Questions ??????

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