1. 15-446 Networked Systems Practicum Lecture 7 – Power Management 1

2. Outline 802.11 details BSD Catnap Secondary Radio Systems Localization 2

3. Power Management Approach(Infrastructure) Allow idle station to go to sleep stations power save mode stored in AP AP buffers packets for sleeping nodes AP announces which station have frames buffered Power Saving stations wake up periodically listen for Beacons TSF assures AP and Power save stations are synchronized TSF timer keeps running when stations are sleeping

4. WiFi 4 Time Zzz… Between packet bursts, WiFi switches to low-power sleep mode : Saving Energy through Sleep

5. 5 Simultaneous measurements at 5K hertz

6. Wireless Interface Power-Saving AWAKE: high power consumption, even if idle SLEEP: low power consumption, but can’t communicate Basic PSM strategy: Sleep to save energy, periodically wake to check for pending data PSM protocol: when to sleep and when to wake? A PSM-static protocol has a regular, unchanging, sleep/wake cycle while the network is inactive (e.g. 802.11) power time PSM offPSM on 750mW 50mW 100ms Measurements of Enterasys Networks RoamAbout 802.11 NIC

7. Power Management Approach(Infrastructure) Broadcast/multicast frames are also buffered at AP these frames are sent only at DTIM DTIM = time when multicast frames are to be delivered by AP, determined by AP this time is indicated in the Beacon frames as delivery traffic indication map(DTIM) Power Saving stations wake up prior to expected DTIM If TIM indicates frame buffered station sends PS-Poll and stays awake to receive data else station sleeps again

8. Infrastructure Power Management Operation Beacon-Interval DTIM Interval Time axis TIM (in Beacon): AP activity: Busy medium: DTIM: Broadcast: AP activity Poll PS station

9. 9/49 Buffered Frame Retrieval Process for Two Stations Station 1 has a listen interval of 2 while Station 2 has a listen interval of 3.

10. Outline 802.11 details BSD Catnap Secondary Radio Systems Localization 10

11. PSM-Static Impact on TCP (initial RTTs) SYN ACK DATA SLEEP PSM on Mobile Device Access Point Server 100ms 200ms 0ms AWAKE time Mobile Device Access Point Server PSM off

12. PSM-Static Impact on TCP (steady state) PSM on Mobile Device Access Point Server Time to send buffered window window < BWRTT Network interface sleeps window > BWRTT Network interface stays awake Server RTT

13. PSM-static Overall Impact on TCP The transmission of each TCP window takes 100ms until the window size grows to the product of the wireless link bandwidth and the server RTT Measured TCP Performance

14. Web Browsing is Slow with PSM-static Web browsing typically consists of small TCP data transfers RTTs are a critical determinant of performance PSM-static slows the initial RTTs to 100ms Slowdown is worse for fast server connections Many popular Internet sites have RTTs less than 30ms (due to increasing deployment of Web CDNs, proxies, caches, etc.) For a server RTT of 20ms, the average Web page retrieval slowdown is 2.4x

15. PSM-static Does Not Save Enough Energy Client workloads are bursty 99% of the total inactive time is spent in intervals lasting longer than 1 second (see paper) During long idle periods, waking up to receive a beacon every 100ms is inefficient Percentage of idle energy spent listening to beacons: Longer sleep times enable deeper sleep modes Basic tradeoff between reducing power and wakeup cost Current cards are optimized for 100ms sleep intervals Enterasys RoamAbout23%Used in our paper ORiNOCO PC Gold35%Based on data in: Cisco AIR-PCM35084%[Shih, MOBICOM 2002]

16. The PSM-static Dilemma Compromise between performance and energy If PSM-static is too coarse-grained, it harms performance by delaying network data If PSM-static is too fine-grained, it wastes energy by waking unnecessarily Solution: dynamically adapt to network activity to maintain performance while minimizing energy Stay awake to avoid delaying very fast RTTs Back off (listen to fewer beacons) while idle

17. request T wait T waitp Bounding Slowdown with Minimum Energy (Idealized) Bounded Slowdown Property: If T wait has elapsed since a request was sent, the network interface can sleep for a duration up to T waitp while bounding the RTT slowdown to (1+p) Idealized protocol: To minimize energy: sleep as much as possible To bound slowdown: wakeup to check for response data as governed by above property

18. Synchronization Mobile device and AP should be synchronized with a fixed beacon period (T bp ) May delay response by one beacon period during first sleep interval To bound slowdown, initially stay awake for 1/p beacon periods Round sleep intervals down to a multiple of T bp Requires minimal changes to 802.11 (1/p)T bp T bp

19. Bounded-Slowdown (BSD) Protocol BSD-10%: BSD-20%: BSD-50%: BSD-100%: PSM-static: beacon period : Parameterized BSD protocol exposes trade-off between performance and energy Compared to PSM-static: awake energy increases, listen energy decreases

20. Web Browsing Energy BSD would have large energy savings for other cards: 25% for ORiNOCO PC Gold, and 70% for Cisco AIR-PCM350 Sleep energy could be reduced by going into deeper sleep during long sleep intervals Shorter beacon-period can reduce awake energy (see paper)

21. Outline 802.11 details BSD Catnap Secondary Radio Systems Localization 21

22. Using Sleep Modes 802.11 PSM Yes, but only when no application is using the wireless interface Can hurt application performance, e.g. VoIP “Enter sleep mode if no network activity for X amount of time” S3 Mode Cannot use it while applications are running Sleep modes not useful during data transfers 22

23. Typical Home Scenario 23

24. Catnap Design 24

25. Catnap Scheduler 25

26. 26

27. Outline 802.11 details BSD Catnap Secondary Radio Systems Localization 27

28. WiFi Sleep Under Contention 28 Time Zzz… Time Zzz…

29. Beacon Wakeups 29 Bad wakeups = burst contention Bad wakeups = burst contention Key intuition: move beacons, spread apart traffic, let clients sleep faster Traffic Download

30. Reducing Idle Power The Problem To receive a phone call the device and the wireless NIC has to be in a “listening” state i.e. they have to be on. Our Proposal When not in use, turn the wireless NIC and the device off. Create a separate control channel. Operate the control channel using very low power, possibly in a different freq. band. Use this channel to “wake-up” device when necessary. Proof of Concept & Implementation Short Term: Add a low power RF transceiver to the 802.11 enabled handheld device Long term: Integrate lower power functionality into 802.11 or integrate lower power radio into mother board and/or 802.11 Access Points.

31. Front View The MiniBrick PCB Back View Audio Plug 915 MHz Radio Vibrator Tilt SensorIR Range Speaker Accelerometer Temperature Sensor Crystal PIC Modular design allows removal of components

32. Radio Power Consumption Radio: RFM TR 1000 ASH Modulation: ASK Voltage: 3V Range: 30 feet (approx) ChipsetReceive (mW) Transmit (mW) Standby (mW) Rate (Mbps) Intersil PRISM 2 (802.11b) 40010002011 Silicon Wave SiW1502 (BT) 160140201 RFM TR100014360.0150.115 Comparing against 802.11 and BT Radios

33. MiniBrick Power Consumption ModePower Consumption Transmit39 mW Receive16 mW Standby7.8 mW Theses numbers include the power consumption by the PIC Microcontroller and the RFM TR1000

34. Outline 802.11 details BSD Catnap Secondary Radio Systems Localization 34