Oct 21, 2008IMC n Under the Microscope Vivek Shrivastava Shravan Rayanchu Jongwon Yoon Suman Banerjee Department Of Computer Sciences University of Wisconsin-Madison University of Wisconsin-Madison

What is n ?  A proposed amendment to standard Oct 21, 2008IMC 2008

What is n ?  A proposed amendment to standard  Significantly improved wireless speeds Oct 21, 2008IMC 2008

What is n ?  A proposed amendment to standard  Significantly improved wireless speeds  Raw physical layer data rate up to 600 Mbps Oct 21, 2008IMC 2008

What is n ?  A proposed amendment to standard  Significantly improved wireless speeds  Raw physical layer data rate up to 600 Mbps  Increased wireless range (especially indoors) Oct 21, 2008IMC 2008

What is n ?  A proposed amendment to standard  Significantly improve wireless speeds  Raw physical layer data rate up to 600 Mbps  Increased wireless range (especially indoors) Oct 21, 2008IMC 2008 Overall, claims to make the wireless connection much more faster and robust

So what is the secret of n ?  Smarter, faster PHY and MAC layers  Physical layer diversity (MIMO)  Frame Aggregation  Wider Channel Width Oct 21, 2008IMC 2008

Physical layer diversity (MIMO) Oct 21, 2008IMC 2008 Multiple antennas at the transmitter/receiver allows multiple data streams to be sent/received simultaneously. Tx Rx

Frame Aggregation Oct 21, 2008IMC 2008 A-MSDU: Sending back to back packets A-MPDU: Combining all packet payloads with single MAC header

Wider Channel Widths Oct 21, 2008IMC 2008 Spectrum Mask for 40, 20 MHz channels

Outline  Introducing n  Our goals and takeaways  Experimental evaluation of n mechanisms  Insight into the use of wider channel widths Oct 21, 2008IMC 2008

Agenda and takeaways  Q. What is n throughput in practice and what is the contribution of each mechanism ?  A. Average throughput of an isolated n link is ~80 Mbps in our experiments. Oct 21, 2008IMC 2008

Agenda and takeaways  Q. What is n throughput in practice and what is the contribution of each mechanism ?  Q. What is n throughput when coexisting with g devices ?  A n throughput can reduce by 84% in the presence of g devices. Oct 21, 2008IMC 2008

Agenda and takeaways  Q. What is n throughput in practice and what is the contribution of each mechanism ?  Q. What is n throughput when coexisting with bg devices ?  Q. What are the tradeoffs of using 40 MHz vs. 20MHz channels ?  A. Depending on the distance between two n links, 20 or 40 MHz channels can be more useful Oct 21, 2008IMC 2008

Agenda and takeaways  Q. What is n throughput in practice and what is the contribution of each mechanism ?  Q. What is n throughput when coexisting with bg devices ?  Q. What are the tradeoffs of using 40 MHz vs. 20MHz channels ?  Q. Is MAC diversity useful in n ?  A. MAC diversity can still provide good gains on top of PHY diversity Oct 21, 2008IMC 2008

Agenda and takeaways  Q. What is n throughput in practice and what is the contribution of each mechanism ?  Q. What is n throughput when coexisting with bg devices ?  Q. What are the tradeoffs of using 40 MHz vs. 20MHz channels ?  Q. Is MAC diversity useful in n ? Oct 21, 2008IMC 2008

Outline  Introducing n  Our goals and takeaways  Experimental evaluation of n mechanisms  Insight into the use of wider channel widths Oct 21, 2008IMC 2008

Experimental Setup Oct 21, 2008IMC n 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 n (Draft 2.0) PCI wireless cards. Based on Ralink chipset, support 3X3 MIMO operation n 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 n (Draft 2.0) PCI wireless cards. Based on Ralink chipset, support 3X3 MIMO operation.

802.11n in Isolation (Setup) Oct 21, 2008IMC 2008 Transmitte r Receiver

802.11n In Isolation Oct 21, 2008IMC 2008 Packet aggregation provides up to 75% throughput gains. Wider channel widths provides up to 2X throughput gains. Packet aggregation provides up to 75% throughput gains. Wider channel widths provides up to 2X throughput gains.

802.11n in Isolation Oct 21, 2008IMC 2008 T hroughput improves with packet size. Aggregation is more effective for 600 byte packets T hroughput improves with packet size. Aggregation is more effective for 600 byte packets

Coexistence with g (Setup) Oct 21, 2008IMC 2008 Link separation distance = 10 ft Data Rate: 6M – 54M Data Rate: 300M n Link g Link

Co-existence with g Oct 21, 2008IMC n sees throughput reduction of ~ 84% when an interfering g operates at 6Mbps. Frame aggregation is very helpful, channel width is not n sees throughput reduction of ~ 84% when an interfering g operates at 6Mbps. Frame aggregation is very helpful, channel width is not. 42Mbps 62Mbps 60Mbps 80Mbps

Co-existence with g Oct 21, 2008IMC 2008 Performance improves with increase in data rate of interferer Throughput improvement is minimal Performance improves with increase in data rate of interferer Throughput improvement is minimal

Outline  Introducing n  Working of n  Our goals and takeaways  Experimental evaluation of n mechanisms  Insight into the use of wider channel widths Oct 21, 2008IMC 2008

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Spectrum Mask for 40, 20 MHz channels

Channel Width : To double or not to double ! Oct 21, 2008IMC MHz vs. 20 MHz

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Link separation distance

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Link separation distance : 15 ft

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Link separation distance : 60 ft

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Using 20/40 MHz channels has to take into account the distance between two links Link separation: 15ft Link separation: 120ft

Thank you. Questions ? Oct 21, 2008IMC 2008

Outline  Introducing n  Working of n  Our goals and takeaways  Experimental evaluation of n mechanisms  Insight into the use of wider channel widths  Exploring usefulness of MAC diversity in view of PHY diversity in n Oct 21, 2008IMC 2008

What about MAC-diversity ?  Is it still relevant on top of PHY layer diversity  What is the relevance of mechanisms like XOR, MRD with n  Does diversity gains at PHY layer preclude any MAC layer gains Oct 21, 2008IMC 2008

Setup (MAC diversity) Oct 21, 2008IMC 2008 Multiple receivers Transmitter

MAC diversity is still relevant !! Oct 21, 2008IMC 2008 P(R1ΠR2) = P(R1) * P(R2) indicates that the losses are largely independent across receiver R1 and R2.

MAC diversity is still useful Oct 21, 2008IMC 2008 Gains from MAC level diversity in g/n. MAC diversity provides better gains in g then n Gains from MAC level diversity in g/n. MAC diversity provides better gains in g then n

Oct 21, 2008IMC 2008

So what is the secret of n ?  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 Oct 21, 2008IMC 2008

Agenda and takeaways  Q. What is n throughput in practice and what is the contribution of each mechanism ?  Q. What is n throughput when coexisting with bg devices ?  A n throughput can reduce by 84% in the presence of bg devices.  Q. What are the tradeoffs of using 40 MHz vs. 20MHz channels ?  A. Depending on the distance between two n links, 20 or 40 MHz channels can be more useful  Is MAC diversity useful in n ?  A. MAC diversity can still provide good gains on top of PHY diversity Oct 21, 2008IMC 2008

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Throughput achieved when both links operate on 40MHz channels

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Link separation distance : 15 ft

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Link separation distance : 60 ft

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Link separation distance : 120 ft

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Using 20/40 MHz channels has to take into account the distance between two links Link separation: 15ft Link separation: 120ft

Co-existence with g Oct 21, 2008IMC 2008

Co-existence with g Oct 21, 2008IMC 2008

802.11n with interference Oct 21, 2008IMC n sees throughput reduction of ~ 84% when an interfering g operates at 6Mbps. Frame aggregation is very helpful, channel width is not n sees throughput reduction of ~ 84% when an interfering g operates at 6Mbps. Frame aggregation is very helpful, channel width is not.

Co-existence with g Oct 21, 2008IMC n sees throughput reduction of ~ 84% when an interfering g operates at 6Mbps. Frame aggregation is very helpful, channel width is not n sees throughput reduction of ~ 84% when an interfering g operates at 6Mbps. Frame aggregation is very helpful, channel width is not.

Co-existence with g Oct 21, 2008IMC 2008 Performance improves with increase in data rate of interferer Throughput improvement is minimal Performance improves with increase in data rate of interferer Throughput improvement is minimal

802.11n In Isolation Oct 21, 2008IMC 2008 Packet aggregation provides up to 75% throughput gains, more effective for smaller packet size. Wider channel widths provides up to 2X throughput gains. Packet aggregation provides up to 75% throughput gains, more effective for smaller packet size. Wider channel widths provides up to 2X throughput gains.

Channel Width : To double or not to double !  We extend I-factor proposed earlier for partially overlapped channels to incorporate channel widths. Oct 21, 2008IMC 2008 Spectrum Mask for 40, 20 MHz channels

Gains from MRC Oct 21, 2008IMC 2008 SNR distribution at the three antennas in Non Line of Sight scenarios. MRC will benefit in above two scenarios by combining the SNR at the three antennas.

What is n ?  A new standard  Bridging the gap between WiFi and Ethernet  300 Mbps theoretical speed  High speed, Robust, Reliable and Predictable  Realizing an all wireless office Real time high definition video conferencing over wireless Oct 21, 2008IMC 2008

What is n ?  A new standard  Bridging the gap between WiFi and Ethernet  300 Mbps theoretical speed  High speed, Robust, Reliable and Predictable  Realizing an all wireless office Oct 21, 2008IMC 2008

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Theoretical I-factor for different combinations of transmitter-receiver widths

Experimental Setup Oct 21, 2008IMC n testbed used for experiments. Nodes are placed in location L1 – L9. Nodes are desktop machines (512 MB RAM, 1.2 GHz). Equipped with the Edimax (EW-7728In) n (Draft 2.0) PCI wireless cards. Based on Ralink chipset, support 3X3 MIMO operation n testbed used for experiments. Nodes are placed in location L1 – L9. Nodes are desktop machines (512 MB RAM, 1.2 GHz). Equipped with the Edimax (EW-7728In) n (Draft 2.0) PCI wireless cards. Based on Ralink chipset, support 3X3 MIMO operation.

Physical layer diversity (MIMO) Oct 21, 2008IMC 2008 Maximum ratio combining selects the best signal from all antennas at all time instants Intelligent mechanisms exploit such physical level diversity One such mechanism is Maximum Ratio Combining (MRC) Intelligent mechanisms exploit such physical level diversity One such mechanism is Maximum Ratio Combining (MRC)

Experimental Setup Oct 21, 2008IMC 2008

Channel Width : To double or not to double !  We extend I-factor proposed earlier for partially overlapped channels to incorporate channel widths. Oct 21, 2008IMC 2008 Center Frequency 1 Center Frequency 2

Channel Width : To double or not to double !  We extend I-factor proposed earlier for partially overlapped channels to incorporate channel widths. Oct 21, 2008IMC 2008 Center Frequency 1 Center Frequency 2

Physical layer diversity (MIMO) Oct 21, 2008IMC 2008 Tx Rx Intelligent mechanisms exploit such physical level diversity One such mechanism is Maximum Ratio Combining (MRC) Other examples are Space Time Block Coding (STBC), Cyclic Shift Diversity (CSD) Intelligent mechanisms exploit such physical level diversity One such mechanism is Maximum Ratio Combining (MRC) Other examples are Space Time Block Coding (STBC), Cyclic Shift Diversity (CSD)

Physical layer diversity (MIMO) Oct 21, 2008IMC 2008 Tx Rx Intelligent mechanisms exploit such physical level diversity One such mechanism is Maximum Ratio Combining (MRC) Other examples are Space Time Block Coding (STBC), Cyclic Shift Diversity (CSD) Intelligent mechanisms exploit such physical level diversity One such mechanism is Maximum Ratio Combining (MRC) Other examples are Space Time Block Coding (STBC), Cyclic Shift Diversity (CSD)

Physical layer diversity (MIMO) Oct 21, 2008IMC 2008 Tx Rx Intelligent mechanisms exploit such physical level diversity One such mechanism is Maximum Ratio Combining (MRC) Intelligent mechanisms exploit such physical level diversity One such mechanism is Maximum Ratio Combining (MRC)

Channel Width : To double or not to double ! Oct 21, 2008IMC 2008 Link separation distance : 120 ft