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802.11n Under the Microscope

Explore the proposed amendment to the 802.11 standard, with significantly improved wireless speeds and up to 600 Mbps data rate, especially indoors. Delve into MIMO, Frame Aggregation, and wider channel widths for faster and robust connections. Discover the tradeoffs and practical throughput evaluations through experimental assessments.

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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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