1. ExOR:Opportunistic Multi-Hop Routing For Wireless Networks
Sanjit Biswas and Robert Morris
MIT CSAIL

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 Packets get forwarded on fixed pathB
Packet D S C
Packets get forwarded on fixed path
Retried on failures
Looks like a circuit switched network

4. Cooperative Diversity / Probabilistic Broadcast
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?N1 N2 N3 N4 N5 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?
25%
100%
25% S
100% S D
25%
100% S
100%
25% S
Reception at different node is independent, no interference
Traditional Routing: 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 BatchF 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
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 ?