A High-Throughput Path Metric for Multi-Hop Wireless Routing

1. A High-Throughput Path Metric for Multi-Hop Wireless Routing Douglas S. J. De Couto, Daniel Aguayo, John Bicket, Robert Morris MIT Computer Science and Artificial Intelligence Laborotory Presented by Tray Cooper, Feb 27, 2007

2. Background • The most commonly used metric is minimum hop-count. Why? • Why do we need a new metric? Throughput = 1 Throughput = 1/2 Throughput = 1/3

3. Problems with Min Hop-Count: • A min hop-count protocol weighs each link the same. • Links have 2 values: 0 or 1 • Routing packets sent, but data lost • Lower hop count ~ lower SINR due to distance • Same hop count links • How does the protocol pick the best path?

4. Possible Solutions • Resend Lost Packets • Bad, only leads to lower bandwidth • Use a minimum SINR threshold value • Could lead to node/network partitioning

5. ETX? • Find paths with the fewest expected number of total transmissions • “predict” # of transmissions (including retransmissions) • Lowest # = highest throughput = best route

6. A test • A test was setup to see how the minimum hop count metric REALLY works. Note this was an experimental test, not a simulation. • During the test each packet sent contained 193 bytes (134 of data) • A “best” route was determined by trying 10 different routes and seeing which was best.

7. Results of the Test • 2 Regions • Above 250 PPS: 1 hop links • Below 250 PPS: Multihop links • Note the 0 values for 1/5 of the packets, even though a route exists

8. Other Link Problems • Poor links • Asymmetric links

9. A New Metric? • SINR threshold? • Node partitioning • Multiply link ratios? 1 1 1 = 1 .95 .95 = .90

10. ETX: Minimize the Expected Transmission Count Link throughput  1/ Link ETX Delivery Ratio Link ETX Throughput 100% 100% 1 50% 50% 2 33% 33% 3

11. Calculating ETX • Assuming 802.11 link-layer acknowledgments (ACKs) and retransmissions: • P(TX success) = P(Data success)  P(ACK success) measured fwd delivery ratio rfwd measured rev delivery ratio rrev • Link ETX = 1 / P(TX success) = 1 / [ P(Data success)  P(ACK success) ] • Link ETX  1 / (rfwd  rrev) • Why measure both ACK and Data Success?

12. Measuring Delivery Ratios • Each node broadcasts small link probes (134 bytes), once per second • Nodes remember probes received over past 10 seconds • Reverse delivery ratios estimated as rrev pkts received / pkts sent • Forward delivery ratios obtained from neighbors (piggybacked on probes)

13. 1 100% 2 50% 2 50% 3 33% 5 20% A little practice Route ETX Throughput

14. ETX Good (and Bad) • The Good: • ETX predicts throughput for short routes (<3) • ETX accounts for • asymmetric links • lossy links • long links • The Bad: • ETX probes are susceptible to loads • hidden terminals, heavy congestion, etc. • ETX always uses the same size packets (134), this causes loss estimates for data packets to be low and ack packets to be high • ETX does not take into account a variable bit rate

15. Test Specifics: • Indoor network, 802.11b, ‘ad hoc’ mode • 1 Mbps, 1 mW, small packets (134 bytes), RTS/CTS disabled • DSDV + modifications to respect metrics • Packets are routed using route table snapshot to avoid route instability under load. • DSR + modifications to respect metrics

16. DSDV overhead ETX and DSDV DSDV+hop-count DSDV+ETX ‘Best’

17. ETX and DSR DSR+hop-count DSR+ETX ‘Best’

18. A Quick Summary • ETX aims to increase throughput by determining how many times it will take to transmit a message. • This seems to work well, especially over multi-hop links.

19. Discussion: • ETX is traffic independent…Why? • Oscillations • Traffic could be included in cases of consistent traffic (access point networks?) • Size of packets? • Probing Period?