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DeepSleep: Power Saving Mode to Support a Large Number of Devices

This paper proposes a power-saving mechanism for low-power STAs in 802.11ah that reduces contention and improves energy efficiency by allowing STAs with longer sleep periods to have higher priority access to the channel.

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DeepSleep: Power Saving Mode to Support a Large Number of Devices

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  1. DeepSleep: Power Saving Mode to Support a Large Number of Devices Date: 2012-03-12 Authors: Hung-Yu Wei, National Taiwan University

  2. Abstract • In this contribution, we propose a power saving mechanism that supports low-power STAs in 802.11ah. Hung-Yu Wei, National Taiwan University

  3. Motivation • To save power, STAs could reduce the time in active state. • To save power, a STA could spend a long time in power saving mode. Upon waking up, the STA wakes up to communication shortly and then goes back to power save mode as soon as possible. Hung-Yu Wei, National Taiwan University

  4. Spend Less Time in Active State for Energy Efficiency • Reducing backoff time and listening time could improve energy efficiency. • A STA with reduced CWmin has shorter contention time and could go back to power saving mode sooner • There are concerns about reducing CWmin • Fairness to contention • Collision probability • To be fair, only STAs that have been stay in sleep mode for a while should be qualified for setting small CWmin value. Hung-Yu Wei, National Taiwan University

  5. Reduce Contention Nodes for Energy Efficiency • With large number of devices, reducing channel contention improves energy efficiency • A large percentage of STAs are configured to enter sleep mode to reduce contention and to save energy • Similar solutions to resolve issues caused by large number of STAs have been proposed for 11ah [1][2] Hung-Yu Wei, National Taiwan University

  6. 802.11 Contention and Waste of Energy Energy waste during this time period Hung-Yu Wei, National Taiwan University

  7. Proposal Overview • We propose an enhanced mechanism to save power for low-power 802.11ah STAs • The low-power STAs (which have been stayed in sleep mode for a long time) has higher priority when it wakes up. These STAs could have shorter idle listening period and going back to power-saving mode sooner • The channel access fairness is also considered. • Only STAs that do not access channel for a while are qualified for high priority access because of fairness Hung-Yu Wei, National Taiwan University

  8. Design Overview • STAs go to DeepSleep • Enter DeepSleep mode randomly • If (P < Prob_Threshold) • Optional extension: a STA considers its energy status • Stay in DeepSleep for a while (DeepSleep_Duration ) • STAs wake up • If the STA has slept for a long enough time • The wake-up STA applies smaller contention window to have high priority to transmit. • Otherwise, • The wake-up STA operates in the original 802.11 mode Hung-Yu Wei, National Taiwan University

  9. Our proposal • Algorithm: • Randomly Enter DeepSleep Mode if(P < Threshold) then{ Enter DeepSleepMode (Stay in sleep for DeepSleep_Duration) Bool_Sleep=TRUE; } • Wakeup if (Bool_Sleep) then{ CWmin = CW_Sleep; Bool_Sleep=FALSE; } else{ CWmin = CW_Original; } Hung-Yu Wei, National Taiwan University

  10. Simulation Settings • Use NS-2 simulator • Number of devices : 100 • traffic interval: 1s • beacon period: 1s • TX power: 550 mW • RX power: 250 mW • Idle power: 200 mW • Sleep power: 40 mW • CWmin_Orignal: 31 • CWmin_DeepSleep: 15 • DeepSleep_Duration : 10 beacon periods • Case 1: 100 devices use the proposed scheme • Case 2: 100 devices use default 802.11 scheme • Case 3: 50 devices with the proposed scheme, 50 default scheme Hung-Yu Wei, National Taiwan University

  11. Better Energy Efficiency Hung-Yu Wei, National Taiwan University

  12. Lower Collision Probability Hung-Yu Wei, National Taiwan University

  13. Stay in Sleep Mode Longer Sleep time rate is defined as the time ratio that a STA is stayed in PSM mode. For example, sleep time rate = 0.95 implies a STA operates in active mode 5% of time and stays in power save mode 95% of time. Hung-Yu Wei, National Taiwan University

  14. Lower Retransmission Count To successfully deliver a packet, how many time does a STA needs to transmit (e.g. transmission per packet delivered = 1, if there is no packet loss; transmission per packet delivered = 2, if the average re-transmission count is 1). Hung-Yu Wei, National Taiwan University

  15. The Same Packet Delivery Ratio Hung-Yu Wei, National Taiwan University

  16. Conclusions • We proposed a power saving mechanism for the STAs. • Reducing the contention level caused by large number of devices. • Reducing the backoff time and channel listening time upon a STA wakes up. • Simulation results show the proposed mechanism effectively improve energy efficiency. • Better energy efficiency • Lower collision probability • Stay in doze time more often • Lower collision probability • Suitable for scenarios with a large number of lower-power STAs Hung-Yu Wei, National Taiwan University

  17. References • [1] 11-11-1255-00-00ah-dcf-enhancements-for-large-number-of-stas • [2] 11-12-0028-01-00ah-power-saving-possibilities-for-networks-supporting-a-large-number-of-stas Hung-Yu Wei, National Taiwan University

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