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

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
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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