ExOR:Opportunistic Multi-Hop Routing For Wireless NetworksSanjit Biswas and Robert MorrisMIT CSAILhttp://pdos.csail.mit.edu/roofnet

What is Ex Opportunistic Routing ?A Link/Network Layer diversity routing technique that uses standard radio hardwareAchieves substantial increase in throughput for large unicast transfers in mesh network.Thoughts :Does opportunistic mean Greedy ?Do we need Dijkstras algorithm?How is it different?

Traditional RoutingSABCDPackets get forwarded on fixed pathRetried on failuresLooks like a circuit switched network

PacketPacketPacket

Cooperative Diversity / Probabilistic BroadcastSABCDPackets are broadcastedSelects each hop, after the transmission for that hop.Unlike Coop Diversity only a single node forwards each packet

Why ExOR promises high throughput?N1N3N5N7N6N2N4N8SDGradual falloff of probability with distance (80%, 40%, 20%..)Lucky longer path can reduce transmission countShorter path ensures some forward progress

Traditional Path

Why ExOR promises high throughput?SSSDSS25%25%25%25%100%100%100%100%Reception at different node is independent, no interferenceTraditional Routing: 1/ 0.25 + 1 = 5txExOR: 1/ (1-(1-0.25) ) + 1 = 2.5tx

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Design Challenges Order in AnarchyThe 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 destinationChoosing most useful participantsAvoid simultaneous transmission to minimize collisions.

Agreement using Gossip and BatchSN2N3N4N1N5N6DN7N8BatchFFFFFA complete schedule, undelivered packet are retried in subsequent oneA subset within a transmission batch is called Fragment (F)After each batch destination sends packet just containing batch mapOkay, where is the agreement ?

1st round2nd round3rd round

Gossip Protocol Very similar to P2POnly 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 ParticipantsETX (Expected Transmission Count) metric based on Forward Delivery ProbabilityPeriodically link state floodingPath 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 TrackerSN2N3N4N1N5N6DN7N8BatchFFFFF1st round2nd round3rd roundHeader contains information to predict source transmission rateTransmission schedule allows high priority node to send firstUses EMWA to set Forwarding Timer

Batch? Whats going on with TCP Window?

25 Highest Throughput PairsACK might get dropped even for single hop.

25 Lowest Throughput PairsAsymmetric long links affect ACK handling

Transmission RangeExOR requires less packet transmissions to travel far.

Going ForwardLeverage 802.11 bit rate for throughput improvement.Cooperation between simultaneous flows.Transport level protocol development ?Security ?More organic improvements based on cooperative diversity theory ?