Power Management and Conservation in Wireless Networks

Unicast frame buffer and DTIM

Mobile Devices run on battery.  It is essential to conserve the battery power , to avoid frequent recharging of batteries and to have continued network connectivity.  The best  way to conserve power in wireless network is to turn off the transceiver. When the transceiver is turned off , it is said to be in sleep state or Doze state. When the transceiver is on, the mobile station is said to be in active state. By keeping a mobile station in sleeping state for a longer period of time , power can be conserved.

To conserve power , stations go to sleeping mode also called power save mode. Station indicates to the AP that it is going to power save mode by sending a NULL data frame to the AP. This is a special kind of frame that has no frame body.  This  frame has only a MAC Header and FCS field.This frame can only be sent by a station to the AP. The power Management bit in the frame control field can be set to 1 or 0. Whenever the station goes to sleep mode, it sets this bit to 1 and sends the frame to AP.  When AP receives this frame and finds the bit to be '1', it knows that the station is in sleep  or power save mode.

When  a station goes to sleep mode, AP has to perform two tasks. AP needs to buffer any data meant for station in power save mode. How long the AP will buffer the data is determined by the 'Listen Interval' parameter. This parameter specifies how  many beacon periods  the station will choose to sleep. The AP has to buffer the data meant for the station at least for the listen interval period.

Secondly, the AP has to inform the station that it has buffered data for that station. Traffic Indication Map Information Element (TIM IE)  is present in every beacon frame to indicate the presence of unicast buffered data. The TIM IE in the beacon, lists the stations for which AP has unicast buffered data.The station's Association ID is indicated in the TIM IE.  A station in Power Save mode has to wake up periodically and listen to the beacon. If the station finds its AID in the TIM IE, then the station knows that the AP has buffered data for it. The station then sends a PS-Poll request frame to the AP.  AP sends the buffered data to the station. The station sends a ACK  frame.

Multicast frame buffering and DTIM

Access points also buffer multicast and broadcast frames whenever a station associated with the AP is in sleep mode. Unlike the case of unicast buffering, stations cannot send PS-Poll frames.   TIM IE is sent in every beacon. To indicate the presence of multicast or broad cast frames buffered at AP to the clients, AP uses a special kind of TIM IE, called the DTIM or the Delivery traffic indication Map. 

TIM IE is present in every beacon. DTIM is sent as part of beacon at a fixed number of beacon intervals called DTIM period. Following the beacon that includes the  DTIM IE, the AP releases any multicast or broadcast buffered frames. The stations in power save mode need to wake up and receive the broadcast/multicast frames

The following is the structure of TIM IE.

DTIM period is the number of beacon interval after which a DTIM is sent as part of the beacon. If DTIM period is 1, it indicates that every beacon has a DTIM IE or every beacon is a DTIM beacon.  

DTIM count  indicates how many beacon frames will be transmitted until the next DTIM beacon. A DTIM count of 0 indicates that the current beacon is a DTIM beacon


 

 

 

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