802.11n Under the Microscope

1
802.11n Under the Microscope

• Vivek Shrivastava Shravan Rayanchu
  Jongwon Yoon
• Suman Banerjee
• Department Of Computer Sciences
• University of Wisconsin-Madison

2
What is 802.11n ?

• A proposed amendment to 802.11 standard

3
What is 802.11n ?

• A proposed amendment to 802.11 standard
• Significantly improved wireless speeds

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

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)

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
7
So what is the secret of 802.11n ?

• Smarter, faster PHY and MAC layers
• Physical layer diversity (MIMO)
• Frame Aggregation
• Wider Channel Width

8
Physical layer diversity (MIMO)
Rx
Multiple antennas at the transmitter/receiver
allows multiple data streams to be sent/received
simultaneously.
9
Frame Aggregation
A-MPDU Combining all packet payloads with single
MAC header
A-MSDU Sending back to back packets
10
Wider Channel Widths
Spectrum Mask for 40, 20 MHz channels
11
Outline

• Introducing 802.11n
• Our goals and takeaways
• Experimental evaluation of 802.11n mechanisms
• Insight into the use of wider channel widths

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.

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.

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

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

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 ?

17
Outline

• Introducing 802.11n
• Our goals and takeaways
• Experimental evaluation of 802.11n mechanisms
• Insight into the use of wider channel widths

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.

19
802.11n in Isolation (Setup)
Receiver
Transmitter
20
802.11n In Isolation

• Packet aggregation provides up to 75 throughput
  gains.
• Wider channel widths provides up to 2X
  throughput gains.

21
802.11n in Isolation

• Throughput improves with packet size.
• Aggregation is more effective for 600 byte
  packets

22
Coexistence with 802.11g (Setup)
802.11n Link
Data Rate 300M
Link separation distance 10 ft
802.11g Link
Data Rate 6M 54M
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.

24
Co-existence with 802.11g

• Performance improves with increase in data rate
  of interferer
• Throughput improvement is minimal

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

26
Channel Width To double or not to double !
Spectrum Mask for 40, 20 MHz channels
27
Channel Width To double or not to double !
40 MHz vs. 20 MHz
28
Channel Width To double or not to double !
Link separation distance
29
Channel Width To double or not to double !
Link separation distance 15 ft
30
Channel Width To double or not to double !
Link separation distance 60 ft
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
32
Thank you.

• Questions ?

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

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

35
Setup (MAC diversity)
Multiple receivers
Transmitter
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MAC diversity is still relevant !!
P(R1?R2) P(R1) P(R2) indicates that the
losses are largely independent across receiver R1
and R2.
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
38
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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

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

41
Channel Width To double or not to double !
Throughput achieved when both links operate on
40MHz channels
42
Channel Width To double or not to double !
Link separation distance 15 ft
43
Channel Width To double or not to double !
Link separation distance 60 ft
44
Channel Width To double or not to double !
Link separation distance 120 ft
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
46
Co-existence with 802.11g
47
Co-existence with 802.11g
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.

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.

50
Co-existence with 802.11g

• Performance improves with increase in data rate
  of interferer
• Throughput improvement is minimal

51
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.

52
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
53
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.
54
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

55
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

56
Channel Width To double or not to double !
Theoretical I-factor for different combinations
of transmitter-receiver widths
57
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.

58
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
59
Experimental Setup
60
Channel Width To double or not to double !

• We extend I-factor proposed earlier for partially
  overlapped channels to incorporate channel
  widths.

Center Frequency 1
Center Frequency 2
61
Channel Width To double or not to double !

• We extend I-factor proposed earlier for partially
  overlapped channels to incorporate channel
  widths.

Center Frequency 1
Center Frequency 2
62
Physical layer diversity (MIMO)
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)

63
Physical layer diversity (MIMO)
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)

64
Physical layer diversity (MIMO)
Rx

• Intelligent mechanisms exploit such physical
  level diversity
• One such mechanism is Maximum Ratio Combining
  (MRC)

65
Channel Width To double or not to double !
Link separation distance 120 ft