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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