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

Oct 21, 2008 IMC 2008

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 (Setup)

Oct 21, 2008 IMC 2008

TransmitterTransmitter

ReceiverReceiver

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

Thank you.

Questions ?

Oct 21, 2008 IMC 2008

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

Oct 21, 2008 IMC 2008

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

Co-existence with 802.11g

Oct 21, 2008 IMC 2008

Co-existence with 802.11g

Oct 21, 2008 IMC 2008

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)

Experimental Setup

Oct 21, 2008 IMC 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, 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)

Channel Width : To double or not to double !

Oct 21, 2008 IMC 2008

Link separation distance : 120 ft Link separation distance : 120 ft