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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 ?. 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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  1. ExOR:Opportunistic Multi-Hop Routing For Wireless Networks Sanjit Biswas and Robert Morris MIT CSAIL http://pdos.csail.mit.edu/roofnet

  2. 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?

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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.

  8. 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 ?

  9. 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.

  10. 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?

  11. 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

  12. Batch? What’s going on with TCP Window?

  13. 25 Highest Throughput Pairs • ACK might get dropped even for single hop.

  14. 25 Lowest Throughput Pairs • Asymmetric long links affect ACK handling

  15. Transmission Range • ExOR requires less packet transmissions to travel far.

  16. 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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