Expected Data Rate (EDR): An Accurate High-Throughput Path Metric For Multi-Hop Wireless Routing

1. Expected Data Rate (EDR): An Accurate High-Throughput Path Metric For Multi-Hop Wireless Routing Jun Cheol Park (jcpark@cs.utah.edu) Sneha Kumar Kasera (kasera@cs.utah.edu) School of Computing University of Utah

2. Multi-hop wireless networks • Flexible solution regardless of existence of fixed wired infrastructure • Efficient ad hoc routing necessary to achieve high throughput • Path metric crucial in selecting ad hoc paths

3. Related Work • ETX (Expected Transmission Count) [MobiCom’03] • considers packet loss, but does not accurately model transmission interference • Existing transmission interference models do not consider packet loss None of existing work has comprehensively addressed packet loss, transmission interference together

4. ETX • Average # transmissions (including retransmissions) needed for successful packet delivery on wireless link with loss rate p • ETX sum of ad hoc path • sum of ETX of individual links • used as path metric for selecting best ad hoc path Achievable Data Rate of a link: Maximal data rate / ETX  Maximal data rate  delivery ratio 1 ETX = 1 - p

5. Limitations of ETX sum • UDP packet size: 1500 bytes • Source node always backlogged (11 Mbps) ETX sum cannot accurately differentiate ad hoc paths

6. Goal Develop an accurate high-throughput path metric for multi-hop wireless networks

7. Outline • Problem Setting • EDR (Expected Data Rate) • Transmission Contention Degree • Back-off procedure • Performance Evaluation • Summary

8. Problem Setting • IEEE 802.11 networks • Distributed Coordination Function (DCF) • all links use single data rate • Load-insensitive path metric, routing • does not consider “dynamic interference” due to other flows • considers “unavoidable” transmission interference within single flow 1 2 3 4

9. D EDR = for wired links ETX(B) Basic Ideas of EDR • Every link relies on supplying rate from previous link • EDR : achievable data rate of whole ad hoc path = achievable data rate of bottleneck link B: Bottleneck link D: Maximal Data rate on link B ETX(B)

10. D EDR = ETX(B)  I for wireless links Basic Ideas of EDR • Every link relies on supplying rate from previous link • EDR : achievable data rate of whole ad hoc path = achievable data rate of bottleneck link B: Bottleneck link D: Maximal Data rate on link B ETX(B) I: Total transmission interference factor

11. Total Transmission Interference Factor • Depends upon • TCD: Transmission Contention Degree • RTCD: Relatively Increased TCD I = Sum of all TCD and RTCD on links that interfere with bottleneck link B

12. Transmission Contention Degree for Link k • Represents how busy link k transmitting, retransmitting packets • range [0.0, 1.0], normalized value compared maximal data rate of link k • when node always backlogged, TCD = 1.0 • Considers load due to original transmission, retransmissions

13. TCD(k) TCD(k) Supplying rate at link k+1 = ETX(k+1) ETX(k) ETX(k) TCD(k) TCD(k+1) = Min { 1,  ETX(k+1) } Increased load due to lost packets Original load How to calculate TCD? • Assume • ETX values of links are given • TCD(k+1) in terms of TCD(k)? • TCD(1) = 1.0 TCD(k+1) ? ETX(k)

14. Effect of 802.11 Back-off • No mechanism to differentiate packet loss due to collisions, channel noise • Upon packet loss exponential back-off used for occupying shared medium • Different loss rates between adjacent links  different average contention window sizes  different medium occupancy probabilities  relatively increased TCD (RTCD) on higher loss rate link

15. How to calculate RTCD? • Assume W(1) = 5, W(2) = 10 • Node 1 twice more likely to occupy shared medium than Node 2 • Thus, higher loss rate node (Node 2) experiences relative increase in TCD due to different window sizes RTCD(k+1) = W(k+1)/W(k) -1 Window size W(k) 5 10 1 2 3

16. D EDR = ETX(B)  I EDR • D: Maximum data rate on bottleneck link B • ETX(B): ETX of link B • I: Sum of (TCD+ RTCD) over all links that interfere with link B

17. Performance Evaluation • NS-2 simulations • Independent, temporally correlated loss models • Randomly generate 270 ad hoc paths • hop lengths: 2 - 5 • link loss rates: 0.0 - 0.5 (ETX: 1.0 - 2.0) • Construct groups of 4 ad hoc paths between source, destination • for given group as input set, find how well each metric selects best ad hoc path • Use 1500-byte UDP packets, send rate at source node = 11 Mbps

18. Independent loss • EDR performs much better than ETX sum • EDR: for 90% of input cases, throughput more than 90 % of best

19. Temporally correlated loss • Packet burst loss modeled using two-state continuous time Markov chain • Burst length borrowed from experimental results [Divert, MobiSys ’04]

20. Summary • Proposed a new metric, EDR • Showed that EDR can accurately determine achievable data rates of ad hoc paths • Future work • investigate TCP over EDR routing • apply EDR in multi-radio wireless networks

21. Backup

22. R EDR = I D(k) ETX(k+1) TCD(k)  TCD(k+1) =  D(k+1) ETX(k) EDR for TCP on multi-rate paths • Bottleneck link B such that R = Min { D(k) / ETX(k) } • I = TCD(k)/TCDmax, k over interference range of link B, • Normalized total transmission contention degree in terms of B • For TCP flows, EDR does not include RTCD in I because TCP window mechanism is able to avoid unnecessary overhead of RTCD by adjusting send rate at source node , TCD(1) = 1.0