# Opportunistic Routing in Multihop Wireless Networks Sanjit Biswas

• Slides: 26

Opportunistic Routing in Multi-hop Wireless Networks Sanjit Biswas and Robert Morris MIT CSAIL http: //pdos. csail. mit. edu/roofnet/

Ex. OR: a new approach to routing in multi-hop wireless networks 1 kilometer • Dense 802. 11 -based mesh • Goal is high-throughput and capacity

Initial approach: Traditional routing packet A packet B src dst C • Identify a route, forward over links • Abstract radio to look like a wired link

Radios aren’t wires A 1 2 33 4455 56 66 B src dst C • Every packet is broadcast • Reception is probabilistic

Ex. OR: exploiting probabilistic broadcast packet A packet B src dst C • Decide who forwards after reception • Goal: only closest receiver should forward • Challenge: agree efficiently and avoid duplicate transmissions

Outline • • • Introduction Why Ex. OR might increase throughput Ex. OR protocol Measurements Related Work

Why Ex. OR might increase throughput (1) src N 1 N 2 N 3 N 4 N 5 dst 75% 50% 25% • • Best traditional route over 50% hops: 3(1/0. 5) = 6 tx Throughput 1/# transmissions Ex. OR exploits lucky long receptions: 4 transmissions Assumes probability falls off gradually with distance

Why Ex. OR might increase throughput (2) N 1 % 5 2 src 25% 25 N 2 N 3 10 0% 100% dst % 0 0 1 % N 4 • Traditional routing: 1/0. 25 + 1 = 5 tx • Ex. OR: 1/(1 – 0. 25)4) + 1 = 2. 5 transmissions • Assumes independent losses

Outline • • • Introduction Why Ex. OR might increase throughput Ex. OR protocol Measurements Related Work

Ex. OR batching tx: 100 9 src rx: 99 88 N 1 tx: 8 rx: 85 57 N 2 tx: 57 -23 24 N 3 N 4 rx: 40 0 tx: 0 rx: 22 0 dst rx: 53 23 tx: 23 • Challenge: finding the closest node to have rx’d • Send batches of packets for efficiency • Node closest to the dst sends first – Other nodes listen, send remaining packets in turn • Repeat schedule until dst has whole batch

Reliable summaries tx: {2, 4, 10. . . 97, 98} summary: {1, 2, 6, . . . 97, 98, 99} N 2 N 4 src dst N 1 N 3 tx: {1, 6, 7. . . 91, 96, 99} summary: {1, 6, 7. . . 91, 96, 99} • Repeat summaries in every data packet • Cumulative: what all previous nodes rx’d • This is a gossip mechanism for summaries

Priority ordering N 2 N 4 src dst N 1 N 3 • Goal: nodes “closest” to the destination send first • Sort by ETX metric to dst – Nodes periodically flood ETX “link state” measurements – Path ETX is weighted shortest path (Dijkstra’s algorithm) • Source sorts, includes list in Ex. OR header • Details in the paper

Using Ex. OR with TCP Client PC Node Proxy Web Server TCP Gateway Ex. OR Batches (not TCP) Web Proxy Ex. OR • Batching requires more packets than typical TCP window

Outline • • • Introduction Why Ex. OR might increase throughput Ex. OR protocol Measurements Related Work

Ex. OR Evaluation • Does Ex. OR increase throughput? • When/why does it work well?

65 Roofnet node pairs 1 kilometer

Evaluation Details • • 65 Node pairs 1. 0 MByte file transfer 1 Mbit/s 802. 11 bit rate 1 KByte packets Traditional Routing 802. 11 unicast with linklevel retransmissions Hop-by-hop batching UDP, sending as MAC allows Ex. OR 802. 11 broadcasts 100 packet batch size

Cumulative Fraction of Node Pairs Ex. OR: 2 x overall improvement 1. 0 0. 8 0. 6 0. 4 0. 2 Ex. OR Traditional 0 0 200 400 600 Throughput (Kbits/sec) • Median throughputs: 800 240 Kbits/sec for Ex. OR, 121 Kbits/sec for Traditional

25 Highest throughput pairs Throughput (Kbits/sec) 3 Traditional Hops 2. 3 x 1000 800 2 Traditional Hops 1 Traditional Hop 1. 7 x 1. 14 x Ex. OR Traditional Routing 600 400 200 0 Node Pair

Throughput (Kbits/sec) 25 Lowest throughput pairs 1000 800 Ex. OR Traditional Routing 4 Traditional Hops 3. 3 x 600 400 200 0 Node Pair Longer Routes