ExOR: Opportunistic Multi- hop routing for Wireless Networks by; Sanjit Biswas and Robert Morris, MIT Presented by; Mahanth K Gowda Some pictures/graphs adopted from authors’ slides 1

O VERVIEW Traditional Routing ExOR: key intuitions and ideas ExOR: Realization Evaluation 2

T RADITIONAL R OUTING Links are abstracted as wires. A B C D E DestinationPath EA D E CA D C 3

L INK TRANSMISSION IS A B ROADCAST Probability of reception decreases with distance However, there is always a chance that data travels longer C E 90% 60% 10% 4 A D B

E X OR E XPLOITS B ROADCAST Best traditional route over 50% hops: 3( 1 / 0.5 ) = 6 tx Throughput  1 / # transmissions ExOR exploits lucky long receptions: 4 transmissions Assumes probability falls off gradually with distance srcdstN1N2N3N4 75% 50% N5 25% 5

E X OR E XPLOITS B ROADCAST Traditional routing: 1 / = 5 tx ExOR: 1 /(1 – (1 – 0.25) 4 ) + 1 = 2.5 transmissions Assumes independent losses N1 srcdst N2 N3 N4 25% 100% 6

E X OR R EALIZATION : B ATCHING Packets are queued and sent in Batches A list of forwarders prioritized by their ETX values is included In the below topology ---> Source: A, Destination: E Priority order : E C D B A In other words, if E C D B A receive packets, they should forward in that order Other nodes listen They forward packets only if a higher priority node has failed to do so 7

A N EXAMPLE A has transmitted a batch of 10 packets 1-10 E receives packets 1, 2 C receives D receives B receives E received 1, Now C forwards 3, 4, D forwards 5, B forwards 6, 7,

B ATCHING A batch map indicates highest priority node that received each packet in the batch The map is updated and sent over along with data Gossip mechanism: updated batch map propagates from high priority nodes to low priority nodes and ultimately to source When the source receives the updated batch map, it restarts transmission if all packets haven’t got through 9

E VALUATION Comparison between traditional is done with ExOR Throughput between 65 randomly selected node pairs evaluated 1 mega-byte file exchanged Batch size is 100 Data rate 1 megabit/second 10

E X OR: 2 X OVERALL IMPROVEMENT Median throughputs: 240 Kbits/sec for ExOR, 121 Kbits/sec for Traditional Throughput (Kbits/sec) Cumulative Fraction of Node Pairs ExOR Traditional 11

25 H IGHEST THROUGHPUT PAIRS Node Pair Throughput (Kbits/sec) ExOR Traditional Routing 1 Traditional Hop 1.14x 2 Traditional Hops 1.7x 3 Traditional Hops 2.3x 12

25 L OWEST THROUGHPUT PAIRS Node Pair 4 Traditional Hops 3.3x Longer Routes Throughput (Kbits/sec) ExOR Traditional Routing 13

E X OR MOVES PACKETS FARTHER ExOR average: 422 meters/transmission Traditional Routing average: 205 meters/tx Fraction of Transmissions ExOR Traditional Routing Distance (meters) 25% of ExOR transmissions 58% of Traditional Routing transmissions 14

S UMMARY ExOR opportunistically exploits wireless broadcast long distance transmission Avoids retransmission by allowing a low priority node to forward 15

I SSUES Periodic link state flooding Queuing for batching causes delay for interactive applications Uses constant data rate for evaluation 16

T HANK Y OU Questions ? 17