1/49 Power Management in IEEE Yu-Chee Tseng 2/49 Motivation Since mobile hosts are supported by battery power, saving battery as much as possible is very important. Power management in in infrastructure network vs. ad hoc network PCF vs. DCF 3/49 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 4/49 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. 5/49 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 6/49 Basic Idea: An Illustration Example 7/49 Access Sequences immediate response immediate response with fragmentation deferred response 8/49 Power Saving Sequences 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. 9/49 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. NAV = SIFS + ACK Although the NAV is too short, the medium is seized by data frame. 10/49 Example: Immediate Response 11/49 Immediate Response with Fragmentation If the buffered frame is large, it may require fragmentation. ** note: the change of NAVs 12/49 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 13/49 fig The PS station must stay awake until the next Beacon frame in which its bit in TIM is clear. Fragmentation is possible too. 14/49 PS in Infrastructure Network 15/49 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. Stations only need to power up receiver to listen to the buffer status and transmitter to transmit polling frames after being informed. 16/49 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) 17/49 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 18/49 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. 19/49 Listen Interval The number of Beacon periods for which the mobile station may choose to sleep. One of the key parameter used in estimating the resources required to support an association. Longer listen intervals require more buffer space on the AP. If a MS fails to check for waiting frames after each listen interval, they may be discarded without notification. 20/49 PS-Poll Frame Retrieval 21/49 Buffered Frame Retrieval Process for Two Stations Station 1 has a listen interval of 2 while Station 2 has a listen interval of 3. 22/49 Traffic Indication Map (TIM) To inform stations that frames are buffered, APs periodically assemble a TIM and transmit it in Beacon frames. The TIM is a virtual bitmap compose of 2,008 bits. Offsets are used to transmit only a small portion of the virtual bit-map. Each bit in the TIM corresponds to a particular AID. Setting the bit indicates that the AP has buffered unicast frames for the corresponding station. 23/49 TIM (cont.) MSs must wake up to listen for Beacon frames to receive the TIM. By examining the TIM, a station can determine if the AP has buffered traffic on its behalf. To retrieve buffered frames, MSs use PS-Poll control frames. When multiple stations have buffered frames, stations use the random backoff algorithm before transmitting the PS-Poll. The buffered frames must be positively acknowledged before it is removed from buffer. 24/49 TIM (cont.) If multiple frames are buffered for a MS, then the More Data bit in the Frame Control field is set to 1. MSs can then issue additional PS-Poll to AP until More Data bit is set to 0. After transmitting the PS-Poll, a mobile station must remain awake until either The polling transaction has concluded or The bit corresponding to its AID is no longer set in the TIM 25/49 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 used AID = 0. APs set 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. 26/49 Buffer Transmission after DTIM DTIM interval = 3 27/49 Under PCF (Infrastructure Mode) Basic Assumption: Point coordinator uses CF-Polling to access the channel. AP only maintains the CF-Pollable stations. 28/49 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. 29/49 TIM Poll TIM Beacon_ Interval AP STA 2 in PS mode PS-poll STA 1 in PS mode Data ACK PS-poll Data ACK Poll 30/49 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. 31/49 DTIM Broadcast Data TIM Beacon_ Interval AP STA 2 in PS mode STA 1 in PS mode 32/49 PS in Ad Hoc Mode (without base station) 33/49 IBSS Power Management Power management is less efficient. Far more of the burden is placed on the sender to ensure that the receiver is active. Receiver must also be more available and cannot sleep for the same lengths of time. 34/49 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. 35/49 ATIM Usage 36/49 PS in Ad Hoc Mode Assumptions: ATIM interval (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. 37/49 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. 38/49 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. 39/49 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. The ATIM window is the only IBSS-specific parameter. setting it to 0 avoids using any power management. 40/49 An Example of ATIM Window If the beacon is delayed due to a traffic overrun, the useable portion of the ATIM window shrinks. 41/49 ATIM Effects A station is permitted to sleep only if it neither transmits nor receives an ATIM. When a station stays up due to ATIM traffic, it remains active until the conclusion of the next ATIM window. 42/49 ATIM Example 43/49 ATIM Window (cont.) Only certain control and management frames can be transmitted during the ATIM window. Beacons, RTS, CTS, ACK and ATIM frames. ATIM frames may be transmitted only during the ATIM window. ACKs are required for unicast ATIM frames. ACKs are not required for multicast ATIM frames. 44/49 ATIM Window (cont.) Buffered broadcast and multicast frames are transmitted after the conclusion of the ATIM window. After that, a station may attempt to transmit unicast frames announced with an ATIM and for which an ACK was received. Then, stations may transmit unbuffered frames to others that are known to be active. 45/49 Effects of ATIM 46/49 PS Summary infrastructure network PCF DCF ad hoc network DCF 47/49 Open Problem 1: PS for multi-hop MANET ATIM design in IEEE Problems: clock drift (asynchronous) network partitioning Ref: Tseng et al. INFOCOM 2002 48/49 Open Problem 2: ATIM window length ATIM window length has performance impact on an ad hoc network. How to choose a proper size of ATIM window? Ref: E. S. Jung and N. Vaidya, An Energy Efficient MAC Protocol for Wireless LANs, INFOCOM 2002. 49/49 Open Problem 3: for soft handover: only allows hard handover searching for new AP only after a STA loses its previous AP thus, causing long delay (e.g., VoIP services are delay-sensitive) to support soft handover, a STA must frequently scan new APs approach: using PS mode as an excuse to search for neighboring APs (through such as an full frame with PS bit = 1) the old AP will buffer frames during the PS period.