1. 1 Chapter 8 Power Management in IEEE 802.11 Yu-Chee Tseng @CS.NCTU 1. Possible Access Sequences for a STA in PS Mode 2. PS in Infrastructure Network 3. PS in Ad Hoc Network

2. 2 Introduction Power management modes Active mode (AM) Power Save mode (PS) Power consumption of ORiNOCO WLAN Card Transmit mode Receive mode Idle modeDoze mode 1400mW900mW700mW60mW

3. 3 Basic Idea AP or source hosts buffer packets for hosts in PS mode. AP or sources send TIM periodically. TIM = traffic indication map (a partial virtual bitmap associated with station id) TIM is associated with beacon. Hosts in PS mode only turn on antenna when necessary. Hosts in PS mode only “wake up” to monitor TIM.

4. 4 TIM Types Infrastructure mode: TIM : transmitted with every beacon (for Unicast) Delivery TIM (DTIM): transmitted less frequently (every DTIM_interval) for sending buffered broadcast packets Ad hoc mode: ATIM: transmitted in ATIM-Window by stations who want to send buffered packets structured the same as TIM

5. 5 Possible Access Sequences for a STA in PS Mode immediate response immediate response with fragmentation deferred response

6. 6 Power Saving Sequences 802.11 stations shut down the radio transceiver and sleeping periodically to increase battery life. During sleeping periods, access points buffer any unicast frames for sleeping stations. These frames are announced by subsequent Beacon frames. To retrieve buffered frames, newly awakened stations use PS-Poll frames.

7. 7 Immediate Response AP can respond immediately to the PS-Poll Since Duration is not used, it assumes NAV = SIFS + ACK Although the NAV is too short, the medium is seized by data frame.

8. 8 Immediate Response with Fragmentation If the buffered frame is large, it may require fragmentation. ** note: the change of NAVs

9. 9 Deferred Response After being polled, the AP may decide to respond with a simple ACK. although promised, AP does not act immediately AP may do regular DCF activities the PS station must remain awake until it is delivered

10. 10 PS in Infrastructure Network

11. 11 Assumptions and Models Assumptions: TIM interval (beacon interval) and DTIM interval are known by all hosts requires time synchronization Two Operational Models: under DCF (contention-based) under PCF (contention-free): omitted

12. 12 Operations of TIM (in DCF) AP periodically broadcasts beacon with TIM. but STAs may have different wake-up intervals. Hosts in PS must wake up to check TIM. If found having packets buffered in AP, send PS-Poll to AP (by contention). AP replies PS-poll with ACK or DATA. The receiver must remain in active mode until it receives the packet.

13. 13 Buffered Frame Retrieval Process for Two Stations Station 1 has a listen interval = 2. Station 2 has a listen interval = 3.

14. 14 Multicast and Broadcasting: Delivery TIM (DTIM) Frames are buffered whenever any station associated with the AP is sleeping. Buffered broadcast and multicast frames are saved using AID = 0. AP sets the first bit in the TIM to 0. At a fixed number of Beacon intervals, a DTIM is sent. Buffered broadcast and multicast traffic is transmitted after a DTIM Beacon.

15. 15 Buffer Transmission after DTIM DTIM interval = 3

16. 16 PS in Ad Hoc Mode (without base station)

17. 17 Announcement TIM (ATIM) ATIM: a kind of “awake” announcement All stations in an IBSS listen for ATIM frames during specified periods after Beacon transmissions. Stations that do not receive ATIM frames are free to conserve power.

18. 18 ATIM Window a fixed period after beacon If the beacon is delayed due to a traffic overrun, the useable portion of the ATIM window shrinks.

19. 19 PS in Ad Hoc Mode Assumptions: beacon interval & ATIM window are known by all hosts Each station predicts which stations are in PS mode. The network is fully connected. Basic Method: CSMA/CA is used to access the channel. RTS, CTS, ACK, PS-Poll are used to overcome hidden terminal.

20. 20 Operations of ATIM 1. All stations should be in active mode during ATIM window. 2. The station which completes its backoff procedure broadcasts a beacon. Sending beacon is based on contention. Once a beacon is heard, the rest beacons are inhibited.

21. 21 (cont.) 3. In ATIM window, each source station having buffered packets to be sent contends to send out its ATIM. If a host finds it is in the ATIM name list, send an ACK to the sender. remain in the ACTIVE mode throughout If the host is not in the name list, go back to the PS mode. 4. After ATIM window, stations use CSMA/CA to send buffered packets only those hosts who were ACKed can contend.

22. 22 ATIM Example

23. 23 ATIM Example (STA 1 Waking Up STAs 2, 3, and 4)

24. 24 Summary of PS infrastructure network PCF DCF (omitted) ad hoc network DCF