Power Management in IEEE 802.11

1. Power Management in IEEE 802.11 Yu-Chee Tseng @CS.NCTU Possible Access Sequencesfor a STA in PS Mode PS in Infrastructure Network PS in Ad Hoc Network

2. Motivation • Since mobile hosts are supported by battery power, saving battery as much as possible is very important. • Power management in 802.11 • in infrastructure network vs. ad hoc network • PCF vs. DCF

3. Introduction • Power management modes • Active mode (AM) • Power Save mode (PS) • Power consumption of ORiNOCO WLAN Card

4. 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.

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

6. Basic Idea:An Illustration Example

7. Possible Access Sequencesfor a STA in PS Mode immediate response immediate response with fragmentation deferred response

8. 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.

9. Immediate Response • AP can respond immediately to the PS-Poll • PS-Poll frame contains an Association ID in the Duration/ID field so AP can determine which frames were buffered for the MS. • Since Duration is not used, it assumes • NAV = SIFS + ACK • Although the NAV is too short, the medium is seized by data frame.

10. Example: Immediate Response

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

12. 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

13. fig. 3-21 • The PS station must stay awake until the next Beacon frame in which its bit in TIM is clear. • Fragmentation is possible too.

14. PS in Infrastructure Network

15. Power Management in Infrastructure Networks • All traffic for MSs must go through APs, so they are an ideal location to buffer traffic. • APs are aware of MSs’ power management state. • APs have two power management-related tasks. • Determine whether a frame should be delivered • Announce periodically which stations have frames waiting for them.

16. Assumptions and Models • Assumptions: • TIM interval (beacon interval) and DTIM interval are known by all hosts • requires time synchronization • Stations in PS mode are known or can be predicted. • Two Operational Models: • under DCF (contention-based) • under PCF (contention-free): omitted

17. Under DCF (Infrastructure Mode) • Basic assumption: • use CSMA/CA to access the channel • RTS, CTS, ACK, PS-Poll are used to overcome the hidden-terminal problem

18. Operations of TIM (in DCF) • AP periodically broadcasts beacon with TIM. • Hosts in PS must wake up to check TIM. • Check for their IDs. • If found having packets buffered in AP, send PS-Poll to AP (by contention?). • Otherwise, go back to PS mode. • AP replies PS-poll with ACK. • The receiver must remain in active mode until it receives the packet. • AP uses CSMA/CA to transmit to stations.

19. Buffered Frame Retrieval Process for Two Stations • Station 1 has a listen interval of 2 while Station 2 has a listen interval of 3.

20. Delivering Multicast and Broadcast Frames: the 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.

21. Buffer Transmission after DTIM • DTIM interval = 3

22. (omitted)Under PCF (Infrastructure Mode) • Basic Assumption: • Point coordinator uses CF-Polling to access the channel. • AP only maintains the CF-Pollable stations.

23. Operations of TIM (PCF) • AP broadcasts beacon with TIM. • Hosts in PS mode checks TIM for their IDs. • If there are buffered packets in AP, the host must remain in Active Mode until being polled. • O/w, the station goes back to PS mode. • Then AP polls those PS stations. • When being polled, the station (in PS mode) sends PS-Poll to AP. • Then AP sends buffered packets to the station. • (See next page.) • AP must poll stations in PS mode first.

24. Beacon_ Interval TIM TIM Poll Poll Data TIM Data AP STA 1 in PS mode PS-poll ACK STA 2 in PS mode PS-poll ACK

25. Operations of DTIM (PCF) • All CF-pollable stations need be in Active Mode when AP broadcasts DTIM. • Immediately after DTIM, AP sends out the buffered broadcast/multicast packets.

26. Beacon_ Interval Broadcast Data TIM TIM DTIM AP STA 1 in PS mode STA 2 in PS mode

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

28. Announcement TIM (ATIM) • The ATIM frame is a message to keep the transceiver on because there is a pending data frame. • 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.

29. ATIM Usage

30. 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.

31. ATIM Window • If the beacon is delayed due to a traffic overrun, the useable portion of the ATIM window shrinks.

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

33. 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 the beacon interval. • If the host is not in the name list, • it can go back to the PS mode. • After ATIM window, • all stations use CSMA/CA to send the buffered packets • basic idea: data packet >> ATIM control frames • only those hosts who have ACKed the ATIM have such opportunity.

34. ATIM Example

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

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