802.11n Under the Microscope
Vivek Shrivastava Shravan Rayanchu Jongwon Yoon
Suman Banerjee
Department Of Computer Sciences
University of Wisconsin-Madison
Oct 21, 2008
IMC 2008
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)
Overall, claims to make the wireless
connection much more faster and robust
Oct 21, 2008
IMC 2008
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)
Rx
Tx
Multiple antennas at the
transmitter/receiver allows multiple
data streams to be sent/received
simultaneously.
Oct 21, 2008
IMC 2008
Frame Aggregation
A-MPDU: Combining
all packet payloads
with single MAC
header
Oct 21, 2008
A-MSDU: Sending
back to back packets
IMC 2008
Wider Channel Widths
Spectrum Mask for 40, 20 MHz channels
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
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
• 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.
Oct 21, 2008
IMC 2008
802.11n in Isolation (Setup)
Receiver
Transmitte
r
Oct 21, 2008
IMC 2008
802.11n In Isolation
• Packet aggregation provides up to 75% throughput gains.
• Wider channel widths provides up to 2X throughput gains.
Oct 21, 2008
IMC 2008
802.11n in Isolation
• Throughput improves with packet size.
• Aggregation is more effective for 600 byte packets
Oct 21, 2008
IMC 2008
Coexistence with 802.11g (Setup)
802.11n
Link
Data Rate: 300M
Link separation distance = 10 ft
Data Rate:
6M – 54M
Oct 21, 2008
IMC 2008
802.11g
Link
Co-existence with 802.11g
80Mbps
62Mbps
42Mbps
60Mbps
• 802.11n sees throughput reduction of ~ 84%
when an interfering 802.11g operates at
6Mbps.
• Frame aggregation is very helpful, channel
width is not.
Oct 21, 2008
IMC 2008
Co-existence with 802.11g
• Performance improves with increase
in data rate of interferer
• Throughput improvement is minimal
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
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
Spectrum Mask for 40, 20 MHz channels
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
40 MHz vs. 20 MHz
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
Link separation distance
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
Link separation distance : 15 ft
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
Link separation distance : 60 ft
Oct 21, 2008
IMC 2008
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
Oct 21, 2008
IMC 2008
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)
Transmitter
Oct 21, 2008
Multiple
receivers
IMC 2008
MAC diversity is still relevant !!
P(R1ΠR2) = P(R1) * P(R2) indicates that the losses are largely
independent across receiver R1 and R2.
Oct 21, 2008
IMC 2008
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
Oct 21, 2008
IMC 2008
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 !
Throughput achieved when both links
operate on 40MHz channels
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
Link separation distance : 15 ft
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
Link separation distance : 60 ft
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
Link separation distance : 120 ft
Oct 21, 2008
IMC 2008
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
Oct 21, 2008
IMC 2008
Co-existence with 802.11g
Oct 21, 2008
IMC 2008
Co-existence with 802.11g
Oct 21, 2008
IMC 2008
802.11n with interference
• 802.11n sees throughput reduction of ~ 84%
when an interfering 802.11g operates at
6Mbps.
Oct 21, 2008
• Frame aggregation is very helpful, channel
width is not.
IMC 2008
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.
Oct 21, 2008
IMC 2008
Co-existence with 802.11g
• Performance improves with increase
in data rate of interferer
• Throughput improvement is minimal
Oct 21, 2008
IMC 2008
802.11n In Isolation
• Packet aggregation provides up to 75%
throughput gains, more effective for smaller packet
size.
• Wider channel widths provides up to 2X
throughput gains.
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.
Spectrum Mask for 40, 20 MHz channels
Oct 21, 2008
IMC 2008
Gains from MRC
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.
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
• 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 !
Theoretical I-factor for different combinations
of transmitter-receiver widths
Oct 21, 2008
IMC 2008
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 Edimax (EW-7728In) 802.11n (Draft
2.0) PCI wireless cards.
•Based on Ralink chipset, support 3X3 MIMO operation.
Oct 21, 2008
IMC 2008
Physical layer diversity (MIMO)
•Intelligent mechanisms exploit such physical level diversity
•One such mechanism is Maximum Ratio Combining (MRC)
Maximum ratio combining selects the best
signal from all antennas at all time instants
Oct 21, 2008
IMC 2008
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.
Center
Center
Frequency Frequency
1
2
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.
Center
Center
Frequency Frequency
1
2
Oct 21, 2008
IMC 2008
Physical layer diversity (MIMO)
Rx
Tx
• 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)
Oct 21, 2008
IMC 2008
Physical layer diversity (MIMO)
Rx
Tx
• 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)
Oct 21, 2008
IMC 2008
Physical layer diversity (MIMO)
Rx
Tx
• Intelligent mechanisms exploit such physical level diversity
• One such mechanism is Maximum Ratio Combining (MRC)
Oct 21, 2008
IMC 2008
Channel Width : To double or not to double !
Link separation distance : 120 ft
Oct 21, 2008
IMC 2008