no time to countdown: backing off in frequency domain · no time to countdown: backing off in...

No Time to Countdown:Backing Off in Frequency Domain

Souvik Sen, Romit Roy Choudhury, Srihari Nelakuditi

2

Current WiFi Channel Contention

Time

DIFS

AP1

AP2

AP1 AP2 Random Backoff = 15

Random Backoff = 25

B1=15

R1 R2

B2=25

3

Current WiFi Channel Contention

Time

DIFS

AP1

AP2

AP1 AP2 Random Backoff = 0

Random Backoff = 10

B1=15

R1 R2

B1=0

B2=25 B2=10

4

Current WiFi Channel Contention

Time

DIFS

AP1

AP2

AP1 AP2 Transmit Carrier

Busy

B1=15 B1=0

R1 R2

Data and ACK

Wait

B2=25 B2=10

5

Current WiFi Channel Contention

Time

DIFS

AP1

AP2

AP1 AP2 Random Backoff = 18

Random Backoff = 10

B1=15 B1=0

R1 R2

Data and ACK

Wait

B1=18

B2=10

B2=25 B2=10

6

Current WiFi Channel Contention

Time

DIFS

AP1

AP2

AP1 AP2 Random Backoff = 8

Random Backoff = 0

B1=15 B1=0

R1 R2

Data and ACK

Wait

B1=18

B2=10

B2=0

B1=8

B2=25 B2=10

7

Current WiFi Channel Contention

Time

DIFS

AP1

AP2

AP1 AP2 Carrier

Busy Transmit

B1=15 B1=0

R1 R2

Data and ACK

Wait

B1=18

B2=10

B2=0

B1=8

Wait

Data and ACK

B2=25 B2=10

8

Current WiFi Channel Contention

Time

DIFS

AP1

AP2

AP1 AP2 Carrier

Busy Transmit

B1=15 B1=0

R1 R2

Data and ACK

Wait

B1=18

B2=10

B2=0

B1=8

Wait

Data and ACK

35% overhead at 54Mbps

High channel wastage due to backoff

9

Current WiFi Channel Contention

! Backoff is not fundamentally a time domain operation" Its implementation is in time domain

TimeDomain

Frequency Domain

Can we implement backoff in frequency domain?Are there any benefits in doing so?

Frequency domain contention resolution

! 802.11 a/g/n PHY adopts OFDM" Wideband channel divided into 48 narrowband subcarriers" Copes better with fast, frequency selective fading" Purely a PHY motivation

! MAC Opportunity: Pretend OFDM subcarriers as integers! Emulate randomized backoff

10

We propose Back2F

Frequency

Subcarriers: 1 2 3 4 … 48

Back2F: Main Idea

! Replace temporal with subcarrier transmission

11

AP1

Backoff = 18Backoff = 6

R1 R2

0 47

6

0 47

18

AP2

Back2F: Main Idea

! Replace temporal with subcarrier transmission

12

R1 R2

AP1

Backoff = 18Backoff = 6

AP2

0 47

6

0 47

18

Listen Antenna Listen Antenna

Other’sBackoff = 18

Other’sBackoff = 6

Both APs learn AP1 is the winner

Back2F: Scheduled Transmission

! Active subcarriers imply backoff chosen by other APs" Each AP knows its rank in the sequence" Enables back to back TDMA like transmission

13

Other’sBackoff

SelfBackoff

470 12

Rank in TDMA: 3

AP1

Is there a benefit with frequency domain backoff?

- 1500 bytes at 54Mbps ~ 250 micro sec.

- Avg. temporal backoff ~ 100 micro sec.

- Frequency backoff = 1 OFDM symbol = 4 micro sec

Will APs Collide During Contention?

! Introduce a second round of contention" Winners of first go to second

15

Subcarrier0 1 2 3 4 5

First Round

Subcarrier0 1 2 3 4 5

Second Round

Will APs Collide During Contention?

! Introduce a second round of contention" Winners of first go to second

16

Subcarrier0 1 2 3 4 5

First Round

Only a Few APs in Second Round?

TDMA will not be effective

Optimize for TDMA

! Instead of only winners, a few more APs to second round

18

Subcarrier0 1 2 3 4 5

Subcarrier0 1 2 3 4 5

First Round

Second Round

0 2 4 0 2 4 0 2 4 Rank 1 Rank 2 Rank 3

Enabling TDMA

Optimize for TDMA

! Instead of only winners, a few more APs to second round

19

0 1 2 3 4 5

Subcarrier0 1 2 3 4 5

First Round

Second Round

Improved Channel Utilization

20

0 2 4 0 2 4 0 2 4 Rank 1 Rank 2 Rank 3

Enabling TDMA

Subcarrier0 1 2 3 4 5

Improved Channel Utilization

21

0 2 4 0 2 4 0 2 4 Rank 1 Rank 2 Rank 3

Enabling TDMA

Subcarrier0 1 2 3 4 5

..........

802.11: Contention per packet

TDMA

Time

Time

..........

Back2F: Contention per TDMA Schedule Time

FrequencyBackoff

Data/ACK Data/ACK Data/ACK

Data/ACK Data/ACK Data/ACK

Multiple Collision Domains

22

! Does Back2F work with real-world scattered APs?

Multiple Collision Domains

23

! Does Back2F work with real-world scattered APs?

BO = 1

! Blue waits for Purple, but Purple waits for Green

BO = 5

BO = 3 BO = 2

! But Blue and Green should transmit simultaneously" Lost transmission opportunity" However 802.11 does not suffer from this problem" Blue will wait for DIFS, continue counting down and eventually transmit

Multiple Collision Domains

24

! Does Back2F work with real-world scattered APs?

! Blue waits for Purple, but Purple waits for Green

BO = 1

BO = 5

BO = 3 BO = 2

! But Blue and Green should transmit simultaneously" Lost transmission opportunity" However 802.11 does not suffer from this problem" Blue will wait for DIFS, continue counting down and eventually transmit

Back2F Solution: Emulate 802.11

DIFS

Multiple Collision Domains

25

BO = 1

BO = 5

BO = 3 BO = 2

G -> 1 P -> 2

Time

Frequency Backoff,Wait for turn

Transmit

Reduce BOby winner’s BO

Channel idle > DIFS

My turn

Frequency Backoff

B -> 3 2

DIFS R -> 5 4

FrequencyBackoff

Back2F: Performance Evaluation

26

! Three important questions:" Can Back2F detect subcarriers reliably?" What is Back2F’s collision probability?" How much throughput gain over 802.11?

Back2F: Performance Evaluation

27

! Three important questions:" Can Back2F detect subcarriers reliably?

" Evaluated on USRP/Gnuradio

" What is Back2F’s collision probability?" How much throughput gain over 802.11?

" Evaluated using traces at 65 locations

Back2F: Performance Evaluation

28

! Three important questions:" Can Back2F detect subcarriers reliably?

" Evaluated on USRP/Gnuradio

Practical Challenge: High Self Signal

Listen AntennaTransmit Antenna

10 20 30 40 50 600

10

20

30

40

50

60

Subcarrier Number

SN

R in

dB

64 pt. FFT

Self Signal Overflows into Adjacent Subcarrier

29

Self Signal

10 20 30 40 50 60 Subcarrier Number

60

50

40

30

20

10

0

SN

R in

dB

64 pt. FFT

Solution: Use a Higher Point FFT

30

10 20 30 40 50 600

10

20

30

40

50

60

SN

R in

dB

Subcarrier Number

128 pt. FFT

10 20 30 40 50 60 Subcarrier Number

60

50

40

30

20

10

0

SN

R in

dB

128 pt. FFT

Self Signal

Solution: Use a Higher Point FFT

31

10 20 30 40 50 600

10

20

30

40

50

60

SN

R in

dB

Subcarrier Number

256 pt. FFT

10 20 30 40 50 60 Subcarrier Number

60

50

40

30

20

10

0

SN

R in

dB

256 pt. FFTSelf Signal

Subcarrier Detection Performance

32

Reliable subcarrier detection at 14dB

1 2 3 4 5 6 7 80

0.2

0.4

0.6

0.8

1

Distance in Subcarriers

De

tectio

n A

ccu

racy

8dB SNR

10dB SNR

12dB SNR

14dB SNR

Robust subcarrier detection at 14dB

Back2F: Performance Evaluation

33

! Collect traces to answer:" What is Back2F’s collision probability?" How much throughput gain over 802.11

AP

Client

20 AP locations45 client locations

Back2F: Performance Evaluation

34

! Collect traces to answer:" What is Back2F’s collision probability?" How much throughput gain over 802.11

2. Per Subcarrier SNR3. Optimal Bitrate

1. RSSI

Collision Probability

Emulate 802.11, Back2F for various topologies

4. Traffic pattern

Back2F: Collision Probability

35

Small collision probability in dense networks

Benefit of second round

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

5 10 15 20 25 30 35 40 45 50

Coll

isio

n P

robab

ilit

y(%

)

Number of APs

Back2F: 2 roundsBack2F: single round

802.11

Throughput Evaluation

36

0.1 0.2 0.3 0.4 0.5 0.60

0.2

0.4

0.6

0.8

1

Throughput gain over 802.11

CD

F

HD streaming

Skype traffic

Web browsing

Higher throughput gain for real time traffic

Throughput Evaluation

37

Throughput Gain of Upto 50%

0

0.1

0.2

0.3

0.4

0.5

0.6

5 10 15 20 25 30 35 40 45 50Thro

ughput

gai

n (

%)

over

802.1

1

Number of clients

6 Mbps18 Mbps36 Mbps54 Mbps

54 Mbps w/o batch

Limitation and Discussion! Robustness of subcarrier based backoff

" Back2F more sensitive to channel fluctuation

! Need for additional antenna" Back2F is complementary to MIMO

! Gain over packet aggregation" Aggregation may not be possible for real time traffic" Back2F provides gain with aggregation at higher rates

! Interoperability with 802.11" May interoperate but will cause unfairness

38

Summary

! Randomization is an effective method for contention resolution

! 802.11 time domain backoff requires channel to remain idle

! Observation: randomization possible in frequency domain" Using OFDM subcarriers

! Back2F: practical system realizing frequency domain contention

! Prevents collisions, provides upto 50% improvement in throughput

39

Questions, comments?

Thank you

Duke SyNRG Research Grouphttp://synrg.ee.duke.edu