Exor opportunistic multi hop routing for wireless networks
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ExOR:Opportunistic Multi-Hop Routing For Wireless Networks. Sanjit Biswas and Robert Morris MIT CSAIL http://pdos.csail.mit.edu/roofnet. What is Ex Opportunistic Routing ?. A Link/Network Layer diversity routing technique that uses standard radio hardware

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ExOR:Opportunistic Multi-Hop Routing For Wireless Networks

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Exor opportunistic multi hop routing for wireless networks

ExOR:Opportunistic Multi-Hop Routing For Wireless Networks

Sanjit Biswas and Robert Morris

MIT CSAIL

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


What is ex opportunistic routing

What is Ex Opportunistic Routing ?

  • A Link/Network Layer diversity routing technique that uses standard radio hardware

  • Achieves substantial increase in throughput for large unicast transfers in mesh network.

  • Thoughts :

    • Does opportunistic mean Greedy ?

    • Do we need Dijkstra’s algorithm?

    • How is it different?


Traditional routing

Traditional Routing

Packet

Packet

B

A

Packet

D

S

C

  • Packets get forwarded on fixed path

  • Retried on failures

  • Looks like a circuit switched network


Cooperative diversity probabilistic broadcast

Cooperative Diversity / Probabilistic Broadcast

B

A

D

S

C

  • Packets are broadcasted

  • Selects each hop, after the transmission for that hop.

  • Unlike Coop Diversity only a single node forwards each packet


Why exor promises high throughput

Why ExOR promises high throughput?

N5

S

N1

N2

N3

N4

N6

N7

N8

D

Traditional Path

  • Gradual falloff of probability with distance (80%, 40%, 20%..)

  • Lucky longer path can reduce transmission count

  • Shorter path ensures some forward progress


Why exor promises high throughput1

Why ExOR promises high throughput?

S

25%

100%

S

25%

100%

S

D

25%

100%

S

100%

25%

S

  • Reception at different node is independent, no interference

  • Traditional Routing: 1/ 0.25 + 1 = 5tx

  • ExOR: 1/ (1-(1-0.25) ) + 1 = 2.5tx

4


Design challenges order in anarchy

Design Challenges – Order in Anarchy

  • The nodes must agree on which subset of them received each packet – Protocol ?

  • A metric to measure the probable cost of moving packet from any node to destination

  • Choosing most useful participants

  • Avoid simultaneous transmission to minimize collisions.


Agreement using gossip and batch

Agreement using Gossip and Batch

N7

N8

F

F

F

N1

N2

N5

S

F

Batch

N4

D

N3

1st round

N6

2nd round

F

3rd round

  • A complete schedule, undelivered packet are retried in subsequent one

  • A subset within a transmission batch is called Fragment (F)

  • After each batch destination sends packet just containing batch map

  • Okay, where is the agreement ?


Gossip protocol very similar to p2p

Gossip – Protocol – Very similar to P2P

  • Only one kind of packet/message.

    • Batch Map (Summary) - Map of each packet against the received highest priority node.

    • Forwarder List – A list of nodes ordered in priority i.e. proximity towards the destination.

  • Knowledge is shared by means of Gossip. Each data packet contains summary.

  • Summary is updated, retrofitted and transmitted by each node.


Cost metric and selection of participants

Cost Metric and Selection of Participants

  • ETX (Expected Transmission Count) metric based on Forward Delivery Probability

    • Periodically link state flooding

    • Path is weighted shortest path (Dijkstra)

  • The source runs ExOR simulation and selects only the nodes which transmit at least 10% of total transmissions in a batch.

  • Need to know about entire Mesh before hand? Is that okay?


Forwarding timer and transmission tracker

Forwarding Timer and Transmission Tracker

N7

N8

F

F

F

N1

N2

N5

S

F

Batch

N4

D

N3

1st round

N6

2nd round

F

3rd round

  • Header contains information to predict source transmission rate

  • Transmission schedule allows high priority node to send first

  • Uses EMWA to set Forwarding Timer


Batch what s going on with tcp window

Batch? What’s going on with TCP Window?


25 highest throughput pairs

25 Highest Throughput Pairs

  • ACK might get dropped even for single hop.


25 lowest throughput pairs

25 Lowest Throughput Pairs

  • Asymmetric long links affect ACK handling


Transmission range

Transmission Range

  • ExOR requires less packet transmissions to travel far.


Going forward

Going Forward

  • Leverage 802.11 bit rate for throughput improvement.

  • Cooperation between simultaneous flows.

  • Transport level protocol development ?

  • Security ?

  • More organic improvements based on cooperative diversity theory ?


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