No Time to Countdown: Backing Off in Frequency Domain Souvik Sen , - PowerPoint PPT Presentation

No Time to Countdown: Backing Off in Frequency Domain Souvik Sen , Romit Roy Choudhury, Srihari Nelakuditi

Current WiFi Channel Contention Random Random Backoff = 15 Backoff = 25 AP1 AP2 R1 R2 B1=15 AP1 DIFS AP2 Time B2=25 2

Current WiFi Channel Contention Random Random Backoff = 0 Backoff = 10 AP1 AP2 R1 R2 B1=15 B1=0 AP1 DIFS AP2 Time B2=25 B2=10 3

Current WiFi Channel Contention Carrier Transmit Busy AP1 AP2 R1 R2 B1=15 B1=0 AP1 Data and ACK DIFS Wait AP2 Time B2=25 B2=10 4

Current WiFi Channel Contention Random Random Backoff = 18 Backoff = 10 AP1 AP2 R1 R2 B1=15 B1=0 B1=18 AP1 Data and ACK DIFS B2=10 Wait AP2 Time B2=25 B2=10 5

Current WiFi Channel Contention Random Random Backoff = 8 Backoff = 0 AP1 AP2 R1 R2 B1=15 B1=0 B1=18 B1=8 AP1 Data and ACK DIFS B2=10 B2=0 Wait AP2 Time B2=25 B2=10 6

Current WiFi Channel Contention Carrier Transmit Busy AP1 AP2 R1 R2 B1=15 B1=0 B1=18 B1=8 AP1 Data and ACK Wait DIFS B2=10 B2=0 Wait Data and ACK AP2 Time B2=25 B2=10 7

Current WiFi Channel Contention Carrier Transmit Busy AP1 AP2 High channel wastage due to backoff R1 R2 B1=15 B1=0 B1=18 B1=8 35% overhead at 54Mbps AP1 Data and ACK Wait DIFS B2=10 B2=0 Wait Data and ACK AP2 Time 8

Current WiFi Channel Contention ! Backoff is not fundamentally a time domain operation " Its implementation is in time domain Time Frequency Domain Domain Can we implement backoff in frequency domain? Are there any benefits in doing so? 9

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 Subcarriers: 1 2 3 4 … 48 Frequency We propose Back2F ! MAC Opportunity: Pretend OFDM subcarriers as integers ! Emulate randomized backoff 10

Back2F: Main Idea ! Replace temporal with subcarrier transmission 6 18 0 47 0 47 AP1 AP2 Backoff = 6 Backoff = 18 R1 R2 11

Back2F: Main Idea ! Replace temporal with subcarrier transmission Other’s Other’s Backoff = 18 Backoff = 6 6 18 0 47 0 47 Listen Antenna Listen Antenna AP1 AP2 Backoff = 6 Backoff = 18 R1 R2 Both APs learn AP1 is the winner 12

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 Self Other’s Backoff Backoff 0 12 47 Rank in TDMA: 3 AP1 13

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 0 1 2 3 4 5 Subcarrier First Round 15

Will APs Collide During Contention? ! Introduce a second round of contention " Winners of first go to second 0 1 2 3 4 5 Subcarrier First Round 0 1 2 3 4 5 Subcarrier Second Round 16

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 0 1 2 3 4 5 Subcarrier First Round 0 1 2 3 4 5 Subcarrier Second Round 18

Optimize for TDMA ! Instead of only winners, a few more APs to second round 0 1 2 3 4 5 First Round 0 2 4 0 2 4 0 2 4 Enabling Rank 1 Rank 2 Rank 3 TDMA 0 1 2 3 4 5 Subcarrier Second Round 19

Improved Channel Utilization 0 2 4 0 2 4 0 2 4 Enabling Rank 1 Rank 2 Rank 3 TDMA 0 1 2 3 4 5 Subcarrier 20

Improved Channel Utilization 0 2 4 0 2 4 0 2 4 Enabling Rank 1 Rank 2 Rank 3 TDMA 0 1 2 3 4 5 Subcarrier Data/ACK Data/ACK Data/ACK .......... Time 802.11: Contention per packet Frequency Backoff TDMA Data/ACK Data/ACK Data/ACK .......... Time Back2F: Contention per TDMA Schedule 21 Time

Multiple Collision Domains ! Does Back2F work with real-world scattered APs? 22

Multiple Collision Domains ! Does Back2F work with real-world scattered APs? BO = 5 BO = 2 BO = 3 BO = 1 ! Blue waits for Purple , but Purple waits for Green ! 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 23

Multiple Collision Domains ! Does Back2F work with real-world scattered APs? BO = 5 BO = 2 BO = 3 BO = 1 ! Blue waits for Purple , but Purple waits for Green ! But Blue and Green should transmit simultaneously Back2F Solution: Emulate 802.11 " Lost transmission opportunity " However 802.11 does not suffer from this problem " Blue will wait for DIFS, continue counting down and eventually transmit 24

Multiple Collision Domains BO = 5 BO = 2 BO = 3 BO = 1 Frequency Backoff DIFS R -> 5 4 G -> 1 P -> 2 Time B -> 3 2 DIFS Reduce BO Channel idle > DIFS by winner’s BO Frequency Frequency Backoff, Backoff Wait for turn Transmit My turn 25

Back2F: Performance Evaluation ! Three important questions: " Can Back2F detect subcarriers reliably? " What is Back2F’s collision probability? " How much throughput gain over 802.11? 26

Back2F: Performance Evaluation ! 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 27

Back2F: Performance Evaluation ! Three important questions: " Can Back2F detect subcarriers reliably? " Evaluated on USRP/Gnuradio Practical Challenge: High Self Signal Transmit Antenna Listen Antenna 28

Self Signal Overflows into Adjacent Subcarrier 60 60 64 pt. FFT 64 pt. FFT 50 50 Self Signal 40 40 SNR in dB SNR in dB 30 30 20 20 10 10 0 0 10 20 30 40 50 60 10 20 30 40 50 60 Subcarrier Number Subcarrier Number 29

Solution: Use a Higher Point FFT 60 60 128 pt. FFT 128 pt. FFT 50 50 Self Signal 40 40 SNR in dB SNR in dB 30 30 20 20 10 10 0 0 10 20 30 40 50 60 10 20 30 40 50 60 Subcarrier Number Subcarrier Number 30

Solution: Use a Higher Point FFT 60 60 256 pt. FFT 256 pt. FFT 50 Self Signal 50 40 SNR in dB 40 SNR in dB 30 30 20 20 10 10 0 0 10 20 30 40 50 60 10 20 30 40 50 60 Subcarrier Number Subcarrier Number 31

Subcarrier Detection Performance 1 Detection Accuracy 0.8 8dB SNR 0.6 10dB SNR 12dB SNR 0.4 14dB SNR 0.2 0 1 2 3 4 5 6 7 8 Distance in Subcarriers Reliable subcarrier detection at 14dB Robust subcarrier detection at 14dB 32

Back2F: Performance Evaluation ! Collect traces to answer: " What is Back2F’s collision probability? " How much throughput gain over 802.11 AP Client 20 AP locations 45 client locations 33

Back2F: Performance Evaluation ! Collect traces to answer: " What is Back2F’s collision probability? " How much throughput gain over 802.11 1. RSSI 2. Per Subcarrier SNR Collision Probability 3. Optimal Bitrate 4. Traffic pattern Emulate 802.11, Back2F for various topologies 34

Back2F: Collision Probability 0.7 Back2F: 2 rounds Collision Probability(%) Back2F: single round 0.6 802.11 0.5 Benefit of 0.4 second round 0.3 0.2 0.1 0 5 10 15 20 25 30 35 40 45 50 Number of APs Small collision probability in dense networks 35

Throughput Evaluation 1 HD streaming 0.8 Skype traffic Web browsing 0.6 CDF 0.4 0.2 0 0.1 0.2 0.3 0.4 0.5 0.6 Throughput gain over 802.11 Higher throughput gain for real time traffic 36

Throughput Evaluation Throughput gain (%) over 802.11 0.6 6 Mbps 18 Mbps 0.5 36 Mbps 54 Mbps 54 Mbps w/o batch 0.4 0.3 0.2 0.1 0 5 10 15 20 25 30 35 40 45 50 Number of clients Throughput Gain of Upto 50% 37

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 Group http://synrg.ee.duke.edu