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802.11n Under the Microscope

802.11n Under the Microscope. Vivek Shrivastava Shravan Rayanchu Jongwon Yoon Suman Banerjee Department Of Computer Sciences University of Wisconsin-Madison. What is 802.11n ?. A proposed amendment to 802.11 standard. What is 802.11n ?.

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802.11n Under the Microscope

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  1. 802.11n Under the Microscope Vivek Shrivastava Shravan Rayanchu Jongwon Yoon Suman Banerjee Department Of Computer Sciences University of Wisconsin-Madison IMC 2008

  2. What is 802.11n ? • A proposed amendment to 802.11 standard IMC 2008

  3. What is 802.11n ? • A proposed amendment to 802.11 standard • Significantly improved wireless speeds IMC 2008

  4. What is 802.11n ? • A proposed amendment to 802.11 standard • Significantly improved wireless speeds • Raw physical layer data rate up to 600 Mbps IMC 2008

  5. What is 802.11n ? • A proposed amendment to 802.11 standard • Significantly improved wireless speeds • Raw physical layer data rate up to 600 Mbps • Increased wireless range (especially indoors) IMC 2008

  6. What is 802.11n ? • A proposed amendment to 802.11 standard • Significantly improve wireless speeds • Raw physical layer data rate up to 600 Mbps • Increased wireless range (especially indoors) • Overall, claims to make the wireless connection much more faster and robust IMC 2008

  7. So what is the secret of 802.11n ? • Smarter, faster PHY and MAC layers • Physical layer diversity (MIMO) • Frame Aggregation • Wider Channel Width IMC 2008

  8. Physical layer diversity (MIMO) Rx Tx Multiple antennas at the transmitter/receiver allows multiple data streams to be sent/received simultaneously. IMC 2008

  9. Frame Aggregation A-MPDU: Combining all packet payloads with single MAC header A-MSDU: Sending back to back packets IMC 2008

  10. Wider Channel Widths Spectrum Mask for 40, 20 MHz channels IMC 2008

  11. Outline • Introducing 802.11n • Our goals and takeaways • Experimental evaluation of 802.11n mechanisms • Insight into the use of wider channel widths IMC 2008

  12. Agenda and takeaways • Q. What is 802.11n throughput in practice and what is the contribution of each mechanism ? • A. Average throughput of an isolated 802.11n link is ~80 Mbps in our experiments. IMC 2008

  13. Agenda and takeaways • Q. What is 802.11n throughput in practice and what is the contribution of each mechanism ? • Q. What is 802.11n throughput when coexisting with 802.11g devices ? • A. 802.11n throughput can reduce by 84% in the presence of 802.11 g devices. IMC 2008

  14. Agenda and takeaways • Q. What is 802.11n throughput in practice and what is the contribution of each mechanism ? • Q. What is 802.11n throughput when coexisting with 802.11bg devices ? • Q. What are the tradeoffs of using 40 MHz vs. 20MHz channels ? • A. Depending on the distance between two 802.11n links, 20 or 40 MHz channels can be more useful IMC 2008

  15. Agenda and takeaways • Q. What is 802.11n throughput in practice and what is the contribution of each mechanism ? • Q. What is 802.11n throughput when coexisting with 802.11bg devices ? • Q. What are the tradeoffs of using 40 MHz vs. 20MHz channels ? • Q. Is MAC diversity useful in 802.11n ? • A. MAC diversity can still provide good gains on top of PHY diversity IMC 2008

  16. Agenda and takeaways • Q. What is 802.11n throughput in practice and what is the contribution of each mechanism ? • Q. What is 802.11n throughput when coexisting with 802.11bg devices ? • Q. What are the tradeoffs of using 40 MHz vs. 20MHz channels ? • Q. Is MAC diversity useful in 802.11n ? IMC 2008

  17. Outline • Introducing 802.11n • Our goals and takeaways • Experimental evaluation of 802.11n mechanisms • Insight into the use of wider channel widths IMC 2008

  18. Experimental Setup • 802.11n testbed used for experiments. Nodes are placed in location L1 – L9. • Nodes are desktop machines (512 MB RAM, 1.2 GHz). • Equipped with the Sparklan 802.11n (Draft 2.0) PCI wireless cards. • Based on Ralink chipset, support 3X3 MIMO operation. IMC 2008

  19. 802.11n in Isolation (Setup) Receiver Transmitter IMC 2008

  20. 802.11n In Isolation • Packet aggregation provides up to 75% throughput gains. • Wider channel widths provides up to 2X throughput gains. IMC 2008

  21. 802.11n in Isolation • Throughput improves with packet size. • Aggregation is more effective for 600 byte packets IMC 2008

  22. Coexistence with 802.11g (Setup) 802.11n Link Data Rate: 300M Link separation distance = 10 ft 802.11g Link Data Rate: 6M – 54M IMC 2008

  23. Co-existence with 802.11g 80Mbps 62Mbps 60Mbps 42Mbps • 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps. • Frame aggregation is very helpful, channel width is not. IMC 2008

  24. Co-existence with 802.11g • Performance improves with increase in data rate of interferer • Throughput improvement is minimal IMC 2008

  25. Outline • Introducing 802.11n • Working of 802.11n • Our goals and takeaways • Experimental evaluation of 802.11n mechanisms • Insight into the use of wider channel widths IMC 2008

  26. Channel Width : To double or not to double ! Spectrum Mask for 40, 20 MHz channels IMC 2008

  27. Channel Width : To double or not to double ! 40 MHz vs. 20 MHz IMC 2008

  28. Channel Width : To double or not to double ! Link separation distance IMC 2008

  29. Channel Width : To double or not to double ! Link separation distance : 15 ft IMC 2008

  30. Channel Width : To double or not to double ! Link separation distance : 60 ft IMC 2008

  31. Channel Width : To double or not to double ! Link separation: 120ft Link separation: 15ft Using 20/40 MHz channels has to take into account the distance between two links IMC 2008

  32. Thank you. Questions ? IMC 2008

  33. Outline • Introducing 802.11n • Working of 802.11n • Our goals and takeaways • Experimental evaluation of 802.11n mechanisms • Insight into the use of wider channel widths • Exploring usefulness of MAC diversity in view of PHY diversity in 802.11n IMC 2008

  34. What about MAC-diversity ? • Is it still relevant on top of PHY layer diversity • What is the relevance of mechanisms like XOR, MRD with 802.11n • Does diversity gains at PHY layer preclude any MAC layer gains IMC 2008

  35. Setup (MAC diversity) Transmitter Multiple receivers IMC 2008

  36. MAC diversity is still relevant !! P(R1ΠR2) = P(R1) * P(R2) indicates that the losses are largely independent across receiver R1 and R2. IMC 2008

  37. MAC diversity is still useful Gains from MAC level diversity in 802.11g/n. MAC diversity provides better gains in 802.11g then 802.11n IMC 2008

  38. IMC 2008

  39. So what is the secret of 802.11n ? • Smarter, faster PHY and MAC layer • PHY layer diversity (MIMO) • Maximum Ratio Combining (MRC) • Cyclic Shift Diversity (CSD) • Space Time Block Coding (STBC) • Frame Aggregation • AMSDU • AMPDU IMC 2008

  40. Agenda and takeaways • Q. What is 802.11n throughput in practice and what is the contribution of each mechanism ? • Q. What is 802.11n throughput when coexisting with 802.11bg devices ? • A. 802.11n throughput can reduce by 84% in the presence of 802.11 bg devices. • Q. What are the tradeoffs of using 40 MHz vs. 20MHz channels ? • A. Depending on the distance between two 802.11n links, 20 or 40 MHz channels can be more useful • Is MAC diversity useful in 802.11n ? • A. MAC diversity can still provide good gains on top of PHY diversity IMC 2008

  41. Channel Width : To double or not to double ! Throughput achieved when both links operate on 40MHz channels IMC 2008

  42. Channel Width : To double or not to double ! Link separation distance : 15 ft IMC 2008

  43. Channel Width : To double or not to double ! Link separation distance : 60 ft IMC 2008

  44. Channel Width : To double or not to double ! Link separation distance : 120 ft IMC 2008

  45. Channel Width : To double or not to double ! Link separation: 120ft Link separation: 15ft Using 20/40 MHz channels has to take into account the distance between two links IMC 2008

  46. Co-existence with 802.11g IMC 2008

  47. Co-existence with 802.11g IMC 2008

  48. 802.11n with interference • 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps. • Frame aggregation is very helpful, channel width is not. IMC 2008

  49. Co-existence with 802.11g • 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps. • Frame aggregation is very helpful, channel width is not. IMC 2008

  50. Co-existence with 802.11g • Performance improves with increase in data rate of interferer • Throughput improvement is minimal IMC 2008

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