Routing in Wireless Mesh Networks

1. Routing in Wireless Mesh Networks CS 598 HL, Fall 2006

2. Environment: Mesh Networks • Static nodes • Limited bandwidth • Multi-radio/multi-channel • Shared wireless medium

3. CUWireless Project … a free wireless networking system that any municipality, company, or group of neighbors could easily set up themselves … http://cuwireless.net/

4. CUWireless Project http://cuwireless.net/

5. Environment: Mesh Networks • Static nodes • Limited bandwidth • Multi-radio/multi-channel • Shared wireless medium • Intra-flow interference Time 97.074236

6. Environment: Mesh Networks • Static nodes • Limited bandwidth • Multi-radio/multi-channel • Shared wireless medium • Intra-flow interference • Inter-flow interference

7. Multi-hop Wireless Networks Improving Network Capacity

8. Goals • Routing or load balancing? • Maintain connectivity • Find a “good” route over “good” links • What does “good” usually mean? • Design effective routing metrics • Design requirements for routing metrics

9. A High-Throughput Path Metric for Multi-Hop Wireless Routing Douglas S. J. De Couto Daniel Aguayo, John Bicket, and Robert Morris ACM MobiCom 2003

10. Indoor wireless network 29 PCs with 802.11b radios (fixed transmit power) in ‘ad hoc’ mode 2nd floor 3rd floor 5th floor 4th floor 6th floor

11. Better Testbed UDP throughput better

12. What throughput is possible? ‘Best’ for each pair is highest measured throughput of 10 promising static routes. Routing protocol (DSDV) ‘Best’

13. Challenge: more hops, less throughput • Links in route share radio spectrum • Extra hops reduce throughput Throughput = 1 Throughput = 1/2 Throughput = 1/3

14. ‘Good’ ‘Bad’ Challenge: many links are lossy One-hop broadcast delivery ratios Smooth link distribution complicates link classification.

15. Challenge: many links are asymmetric Broadcast delivery ratios in both link directions. Very asymmetric link. Many links are good in one direction, but lossy in the other.

16. A straw-man route metric Maximize bottleneck throughput B Delivery ratio = 100% 50% C A 51% 51% D A-B-C = 50% A-D-C = 51% Bottleneck throughput: A-B-C : ABBABBABB = 33% A-D-C : AADDAADD = 25% Actual throughput:

17. Another straw-man metric Maximize end-to-end delivery ratio B 100% 51% C A 50% A-B-C = 51% A-C = 50% End-to-end delivery ratio: A-B-C : ABBABBABB = 33% A-C : AAAAAAAA = 50% Actual throughput:

18. New metric: ETX Minimize total transmissions per packet (ETX, ‘Expected Transmission Count’) Link throughput  1/ Link ETX Delivery Ratio Link ETX Throughput 100% 100% 1 50% 50% 2 33% 33% 3

19. Calculating link ETX Assuming 802.11 link-layer acknowledgments (ACKs) and retransmissions: P(TX success) = P(Data success)  P(ACK success) Link ETX = 1 / P(TX success) = 1 / [ P(Data success)  P(ACK success) ] Estimating link ETX: P(Data success) measured fwd delivery ratio rfwd P(ACK success) measured rev delivery ratio rrev Link ETX  1 / (rfwd  rrev)

20. 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)

21. Route ETX Route ETX = Sum of link ETXs Route ETX Throughput 1 100% 2 50% 2 50% 3 33% 5 20%

22. ETX Properties • ETX predicts throughput for short routes (1, 2, and 3 hops) • ETX quantifies loss • ETX quantifies asymmetry • ETX quantifies throughput reduction of longer routes

23. ETX caveats • ETX link probes are susceptible to MAC unfairness and hidden terminals • Route ETX measurements change under load • ETX estimates are based on measurements of a single link probe size (134 bytes) • Under-estimates data loss ratios, over-estimates ACK loss ratios • ETX assumes all links run at one bit-rate

24. Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks Richard Draves, Jitendra Padhye, and Brian Zill ACM MOBICOM 2004

25. Multi-Hop Networks with Single Radio With a single radio, a node can not transmit and receive simultaneously.

26. Multi-Hop Networks with Multiple Radios With two radios tuned to non-interfering channels, a node can transmit and receive simultaneously.

27. Other Advantages of Multiple Radios • Increased robustness due to frequency diversity • e.g. 2.4GHz (802.11b) and 5GHz (802.11a) have different fading characteristics • Possible tradeoff between range and data rate • Can be helpful during early deployment

28. Existing Routing Metrics are Inadequate 2 Mbps 18 Mbps 18 Mbps 11 Mbps 11 Mbps Shortest path: 2 Mbps Path with fastest links: 9 Mbps Best path: 11 Mbps

29. Contributions • New routing metric for multi-radio mesh networks • Weighted Cumulative Expected Transmission Time (WCETT) • Implementation of the metric in a link-state routing protocol • Multi-Radio Link-Quality source routing (MR-LQSR) • Experimental evaluation of WCETT: • 24-node, multi-radio mesh testbed • 2 radios per node, 11a and 11g • Side-by-side comparison with: • Shortest path (HOP) • ETX (De Couto et. al. MOBICOM 2003)

30. Design of Routing Metric: Assumptions • No power constraints • Little or no node mobility • Relatively stable links • Nodes have one or more 802.11 radios • Multiple radios on a node are tuned to non-interfering channels • Channel assignment is fixed

31. Implementation Framework • Implemented in a source-routed, link-state protocol • Multi-Radio Link Quality Source Routing (MR-LQSR) • Nodes discovers links to its neighbors; Measure quality of those links • Link information floods through the network • Each node has “full knowledge” of the topology • Sender selects “best path” • Packets are source routed using this path

32. Components of a Routing Metric Link Metric: Assign a weight to each link HOP: each link has weight 1 EXT: low loss link WCETT: high-bw, low loss link Path Metric: Combine metrics of links on path HOP: short path EXT: short, low loss path WCETT: short, channel diverse path

33. Link Metric: Expected Transmission Time (ETT) • Link loss rate = p • Expected number of transmissions • Packet size = S, Link bandwidth = B • Each transmission lasts for S/B • Lower ETT implies better link

34. ETT: Illustration 11 Mbps 5% loss 18 Mbps 10% loss 18 Mbps 50% loss 1000 Byte Packet ETT : 0.77 ms ETT : 0.40ms 1000 Byte Packet ETT : 0.77 ms ETT : 0.89 ms

35. Add ETTs of all links on the path Use the sum as path metric Combining Link Metric into Path Metric Proposal 1 SETT = Sum of ETTs of links on path (Lower SETT implies better path) Pro: Favors short paths Con: Does not favor channel diversity

36. SETT does not favor channel diversity 6 Mbps No Loss 6 Mbps No Loss 1.33ms 1.33ms 1.33ms 1.33ms 6 Mbps No Loss 6 Mbps No Loss

37. Combining Link Metric into Path Metric Proposal 2 • Group links on a path according to channel • Links on same channel interfere • Add ETTs of links in each group • Find the group with largest sum. • This is the “bottleneck” group • Too many links, or links with high ETT (“poor quality” links) • Use this largest sum as the path metric • Lower value implies better path “Bottleneck Group ETT” (BG-ETT)

38. BG-ETT Example 6 Mbps 6 Mbps 6 Mbps 6 Mbps 6 Mbps 6 Mbps 1.33 ms 1.33 ms 1.33 ms 1.33 ms 1.33 ms 1.33 ms BG-ETT favors high-throughput, channel-diverse paths.

39. 6 Mbps 6 Mbps 6 Mbps 2 Mbps 4 ms 1.33 ms 1.33 ms 1.33 ms BG-ETT does not favor short paths S D 6 Mbps 6 Mbps 6 Mbps 1.33 ms 1.33 ms 1.33 ms S D

40. SETT favors short paths BG-ETT favors channel diverse paths Path Metric: Putting it all together • Weighted Cumulative ETT (WCETT) • WCETT = (1-β) * SETT + β * BG-ETT β is a tunable parameter Higher value: more preference to channel diversity Lower value: more preference to shorter paths

41. How to measure loss rate and bandwidth? • Loss rate measured using broadcast probes • Similar to ETX • Updated every second • Bandwidth estimated using periodic packet-pairs • Updated every 5 minutes

42. Conclusions • Previously proposed routing metrics are inadequate in multi-radio scenario • WCETT improves performance by judicious use of 2nd radio • Benefits are more prominent for shorter paths • Optimal value of β depends on load

43. Routing Design Requirement 1:Maintaining Network Stability • Load-sensitive metrics • Metrics are related to traffic load • Traffic load may change frequently and irregularly • Frequently varying metrics may cause network instability • Topology-sensitive metrics • Metrics depend on network topology • Metrics are more stable • Preferred for mesh networks due to its stability

44. Requirement 2: Good Performance for Minimum Weight Paths • Routing protocols route packets along minimum weight paths • Performance of minimum weight paths impact the performance of routing protocols • Metrics must capture characteristics of path • Path length • Link packet loss ratio • Link capacity • Intra-flow interference • Inter-flow interference

45. Outstanding Problems w/ Mesh • Routing • naïve metrics, wrong wireless “link” concepts • Expressive metrics • Transport layer • Link adaptation • what can we learn from damaged packets? • Broadband Internet access • Share with your neighbor? • Security • De-centralized identity management (sybil attacks)