oct 21, 2008imc 2008 802.11n under the microscope vivek shrivastava shravan rayanchu jongwon yoon...
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Oct 21, 2008 IMC 2008
802.11n Under the Microscope
Vivek Shrivastava Shravan Rayanchu Jongwon Yoon
Suman Banerjee
Department Of Computer SciencesDepartment Of Computer Sciences
University of Wisconsin-MadisonUniversity of Wisconsin-Madison
What is 802.11n ?
A proposed amendment to 802.11 standard Significantly improved wireless speeds
Oct 21, 2008 IMC 2008
What is 802.11n ?
A proposed amendment to 802.11 standard Significantly improved wireless speeds
Raw physical layer data rate up to 600 Mbps
Oct 21, 2008 IMC 2008
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)
Oct 21, 2008 IMC 2008
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)
Oct 21, 2008 IMC 2008
Overall, claims to make the wireless connection much more faster and robust
Overall, claims to make the wireless connection much more faster and robust
So what is the secret of 802.11n ?
Smarter, faster PHY and MAC layers
Physical layer diversity (MIMO)
Frame Aggregation
Wider Channel Width
Oct 21, 2008 IMC 2008
Physical layer diversity (MIMO)
Oct 21, 2008 IMC 2008
Multiple antennas at the transmitter/receiver allows multiple data streams to be
sent/received simultaneously.
Multiple antennas at the transmitter/receiver allows multiple data streams to be
sent/received simultaneously.
Tx Rx
Frame Aggregation
Oct 21, 2008 IMC 2008
A-MSDU: Sending back to back
packets
A-MSDU: Sending back to back
packets
A-MPDU: Combining all
packet payloads with single MAC
header
A-MPDU: Combining all
packet payloads with single MAC
header
Wider Channel Widths
Oct 21, 2008 IMC 2008
Spectrum Mask for 40, 20 MHz channelsSpectrum Mask for 40, 20 MHz channels
Outline Introducing 802.11n
Our goals and takeaways
Experimental evaluation of 802.11n mechanisms
Insight into the use of wider channel widths
Oct 21, 2008 IMC 2008
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.
Oct 21, 2008 IMC 2008
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.
Oct 21, 2008 IMC 2008
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
Oct 21, 2008 IMC 2008
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
Oct 21, 2008 IMC 2008
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 ?
Oct 21, 2008 IMC 2008
Outline Introducing 802.11n
Our goals and takeaways
Experimental evaluation of 802.11n mechanisms
Insight into the use of wider channel widths
Oct 21, 2008 IMC 2008
Experimental Setup
Oct 21, 2008 IMC 2008
• 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.
• 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.
802.11n In Isolation
Oct 21, 2008 IMC 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, 2008 IMC 2008
• Throughput improves with packet size.• Aggregation is more effective for 600 byte
packets
• Throughput improves with packet size.• Aggregation is more effective for 600 byte
packets
Coexistence with 802.11g (Setup)
Oct 21, 2008 IMC 2008
Link separation distance = 10 ftLink separation distance = 10 ft
Data Rate: 6M – 54M
Data Rate: 6M – 54M
Data Rate: 300MData Rate: 300M
802.11n Link802.11n Link
802.11g Link802.11g Link
Co-existence with 802.11g
Oct 21, 2008 IMC 2008
• 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps.• Frame aggregation is very helpful, channel width is not.
• 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps.• Frame aggregation is very helpful, channel width is not.
42Mbps
62Mbps
60Mbps
80Mbps
Co-existence with 802.11g
Oct 21, 2008 IMC 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 802.11n
Working of 802.11n
Our goals and takeaways
Experimental evaluation of 802.11n mechanisms
Insight into the use of wider channel widths
Oct 21, 2008 IMC 2008
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Spectrum Mask for 40, 20 MHz channelsSpectrum Mask for 40, 20 MHz channels
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
40 MHz vs. 20 MHz40 MHz vs. 20 MHz
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Link separation distanceLink separation distance
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Link separation distance : 15 ft Link separation distance : 15 ft
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Link separation distance : 60 ft Link separation distance : 60 ft
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Using 20/40 MHz channels has to take into account the distance between two
links
Using 20/40 MHz channels has to take into account the distance between two
links
Link separation: 15ftLink separation: 15ft
Link separation: 120ftLink separation: 120ft
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
Oct 21, 2008 IMC 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 802.11n
Does diversity gains at PHY layer preclude any MAC layer gains
Oct 21, 2008 IMC 2008
Setup (MAC diversity)
Oct 21, 2008 IMC 2008
Multiple receivers
Multiple receivers
TransmitterTransmitter
MAC diversity is still relevant !!
Oct 21, 2008 IMC 2008
P(R1ΠR2) = P(R1) * P(R2) indicates that the losses are largely independent across receiver R1 and R2.
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, 2008 IMC 2008
Gains from MAC level diversity in 802.11g/n. MAC diversity provides better gains in 802.11g then
802.11n
Gains from MAC level diversity in 802.11g/n. MAC diversity provides better gains in 802.11g then
802.11n
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
Oct 21, 2008 IMC 2008
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
Oct 21, 2008 IMC 2008
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Throughput achieved when both links operate on 40MHz channels
Throughput achieved when both links operate on 40MHz channels
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Link separation distance : 15 ft Link separation distance : 15 ft
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Link separation distance : 60 ft Link separation distance : 60 ft
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Link separation distance : 120 ft Link separation distance : 120 ft
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Using 20/40 MHz channels has to take into account the distance between two
links
Using 20/40 MHz channels has to take into account the distance between two
links
Link separation: 15ftLink separation: 15ft
Link separation: 120ftLink separation: 120ft
802.11n with interference
Oct 21, 2008 IMC 2008
• 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps.
• Frame aggregation is very helpful, channel width is not.
• 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps.
• Frame aggregation is very helpful, channel width is not.
Co-existence with 802.11g
Oct 21, 2008 IMC 2008
• 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps.• Frame aggregation is very helpful, channel width is not.
• 802.11n sees throughput reduction of ~ 84% when an interfering 802.11g operates at 6Mbps.• Frame aggregation is very helpful, channel width is not.
Co-existence with 802.11g
Oct 21, 2008 IMC 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, 2008 IMC 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, 2008 IMC 2008
Spectrum Mask for 40, 20 MHz channelsSpectrum Mask for 40, 20 MHz channels
Gains from MRC
Oct 21, 2008 IMC 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.
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 802.11n ?
A new 802.11 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, 2008 IMC 2008
What is 802.11n ?
A new 802.11 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, 2008 IMC 2008
Channel Width : To double or not to double !
Oct 21, 2008 IMC 2008
Theoretical I-factor for different combinations of transmitter-receiver
widths
Theoretical I-factor for different combinations of transmitter-receiver
widths
Experimental Setup
Oct 21, 2008 IMC 2008
•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 Edimax (EW-7728In) 802.11n (Draft 2.0) PCI wireless cards.
•Based on Ralink chipset, support 3X3 MIMO operation.
•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 Edimax (EW-7728In) 802.11n (Draft 2.0) PCI wireless cards.
•Based on Ralink chipset, support 3X3 MIMO operation.
Physical layer diversity (MIMO)
Oct 21, 2008 IMC 2008
Maximum ratio combining selects the best signal from all antennas at all time
instants
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)
Channel Width : To double or not to double !
We extend I-factor proposed earlier for partially overlapped channels to incorporate channel widths.
Oct 21, 2008 IMC 2008
Center Frequenc
y 1
Center Frequenc
y 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, 2008 IMC 2008
Center Frequenc
y 1
Center Frequenc
y 2
Physical layer diversity (MIMO)
Oct 21, 2008 IMC 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, 2008 IMC 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, 2008 IMC 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)
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