CENTAUR: Realizing the Full Potential of Centralized WLANs Through a Hybrid Data Path

1. CENTAUR: Realizing the Full Potential of Centralized WLANs Through a Hybrid Data Path • Vivek Shrivastava* ,Shravan Rayanchu, Suman Banerjee • University of Wisconsin-Madison Nabeel Ahmed, Srinivasan Keshav University of Waterloo, Ontario Konstantina Papagiannaki Intel Labs, Pittsburgh Arunesh Mishra Google Inc. * viveks@cs.wisc.edu 1

2. Centralization of Enterprise WLANs Clients Internet Access Point Wireless controller Vivek Shrivastava 2

3. Centralization of Enterprise WLANs Common control plane functions Power control Channel assignment Vivek Shrivastava 3

4. Centralization of Enterprise WLANs What about data plane functions? Power control Channel assignment Data scheduling ? Vivek Shrivastava 4

5. Can centralized scheduling help? Vivek Shrivastava 5

6. Can centralized scheduling help? Hidden terminals Vivek Shrivastava 6

7. Can centralized scheduling help? Hidden terminals Vivek Shrivastava 7

8. Can centralized scheduling help? Hidden terminals Vivek Shrivastava 8

9. Can centralized scheduling help? Hidden terminals Carrier sense Vivek Shrivastava 9

10. Can centralized scheduling help? Hidden terminals 2. Channel free, transmit Vivek Shrivastava 10

11. Can centralized scheduling help? • Collision! • Backoffs • Low throughputs 3. Collision ! Vivek Shrivastava 11

12. How bad is it ? • Experiments on production building-wide WLANs • W1: 5 floors • 9 APs, 45 clients • W2: 1 floor • 21 APs, 51 clients 10% links suffer more than 70% throughput reduction Vivek Shrivastava 12

13. A lost opportunity ? Suppose infrastructure can gather conflict data Vivek Shrivastava 13

14. A lost opportunity ? And when packets arrive … Vivek Shrivastava 14

15. A lost opportunity ? … realize interference will happen … Vivek Shrivastava 15

16. A lost opportunity ? Transmit first packet Vivek Shrivastava 16

17. A lost opportunity ? Transmit first packet Transmit second packet with delay Vivek Shrivastava 17

18. Use an in-band scheduler Simple FIFO schedule with interference avoidance Scheduling functionality Vivek Shrivastava 18

19. What about exposed terminals ? • In our experiments, about 41%of link pairs suffer from exposed terminal interference • Disabling carrier sense to solve the problem can be dangerous for uplink, non-enterprise traffic • We will show how centralization can help even here Vivek Shrivastava 19

20. Challenge • Centralization has obvious overheads • How to make this feasible and useful under • Real applications • Common large-scale wireless environments • Presence of uplink and non-enterprise traffic • Requirements of no client modifications Vivek Shrivastava 20

21. Contributions • Design CENTAUR, a hybrid (partly centralized, partly distributed) scheduling approach • Resolves hidden and exposed terminals • Requires no client modifications • Requires no carrier sense disabling • Evaluate CENTAUR on two WLAN testbeds with real-world traffic traces • 1.48x greater throughput for bulk data traffic • 1.38x reduction in web-transaction times Vivek Shrivastava 21

22. Outline • A naïve attempt at centralized scheduling (DET) • Our hybrid centralized scheduler (CENTAUR) • Evaluation • Related Work • Summary Vivek Shrivastava 22

23. Outline • A naïve attempt at centralized scheduling (DET) • Our hybrid centralized scheduler (CENTAUR) • Evaluation • Related Work • Summary Vivek Shrivastava 23

24. DET: A Simple Deterministic Scheduler Vivek Shrivastava 24

25. DET: A Simple Deterministic Scheduler Input: Conflict graph Vivek Shrivastava 25

26. DET: A Simple Deterministic Scheduler Goal: Schedule each incoming downlink packet Input: Conflict graph Vivek Shrivastava 26

27. DET: A Simple Deterministic Scheduler Goal: Schedule each incoming downlink packet Input: Conflict graph Transmission slots 1 2 3 4 Vivek Shrivastava 27

28. DET: A Simple Deterministic Scheduler • New packet arrives Transmission slots 1 2 3 4 Vivek Shrivastava 28

29. DET: A Simple Deterministic Scheduler • New packet arrives • Find the earliest conflict free slot Transmission slots 1 2 3 4 Vivek Shrivastava 29

30. DET: A Simple Deterministic Scheduler • New packet arrives • Find the earliest conflict free slot Transmission slots 1 2 3 4 Vivek Shrivastava 30

31. DET: A Simple Deterministic Scheduler • New packet arrives • Find the earliest conflict free slot • Schedule the packet in that slot Transmission slots 1 2 3 4 5 Vivek Shrivastava 31

32. Performance of DET No gains for exposed terminals; Non-conflicting links perform worse under load 4x Vivek Shrivastava

33. Outline • A naïve attempt at centralized scheduling (DET) • Our hybrid centralized scheduler (CENTAUR) • Evaluation • Related Work • Summary Vivek Shrivastava 33

34. Overview of CENTAUR • Incorporate basic DET scheduler • Tackle DET’s shortcomings: • Amortize scheduling overhead • Improve performance for exposedlinks • Avoid degrading normal links • Coexist with non-enterprise and uplink traffic Vivek Shrivastava 34

35. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Vivek Shrivastava 35

36. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Vivek Shrivastava 36

37. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Wired ack Vivek Shrivastava 37

38. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Vivek Shrivastava 38

39. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Vivek Shrivastava 39

40. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Vivek Shrivastava 40

41. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Vivek Shrivastava 41

42. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Wired ack Vivek Shrivastava 42

43. (1) Avoid Scheduling Overheads • Problem: Per-packet scheduling performs poorly under high network loads • Solution: • Schedule packets in batches (or epochs) Vivek Shrivastava 43

44. Problem: Exposed links can operate in parallel but don’t due to carrier-sensing • Solution: • Schedule batch of packets, fix backoff periods (2) Improve Exposed Terminals Vivek Shrivastava 44

45. Problem: Exposed links can operate in parallel but don’t due to carrier-sensing • Solution: • Schedule batch of packets, fix backoff periods (2) Improve Exposed Terminals Vivek Shrivastava 45

46. Problem: Exposed links can operate in parallel but don’t due to carrier-sensing • Solution: • Schedule batch of packets, fix backoff periods (2) Improve Exposed Terminals Variable wired delay Vivek Shrivastava 46

47. Problem: Exposed links can operate in parallel but don’t due to carrier-sensing • Solution: • Schedule batch of packets, fix backoff periods (2) Improve Exposed Terminals Vivek Shrivastava 47

48. Problem: Exposed links can operate in parallel but don’t due to carrier-sensing • Solution: • Schedule batch of packets, fix backoff periods (2) Improve Exposed Terminals Carrier sense, deferral Vivek Shrivastava 48

49. Problem: Exposed links can operate in parallel but don’t due to carrier-sensing • Solution: • Schedule batch of packets, fix backoff periods (2) Improve Exposed Terminals Vivek Shrivastava 49

50. Problem: Exposed links can operate in parallel but don’t due to carrier-sensing • Solution: • Schedule batch of packets, fix backoff periods (2) Improve Exposed Terminals Vivek Shrivastava 50