Power Conservation in 802.11

Powering down the transceiver can lead to great power savings in wireless network.

• when the transceiver is off
It is called sleeping, dozing or in power saving.
• when the transceiver is on
It is called awake, active or on.

Power conservation is achieved by minimizing the time spent in on state.

AP plays a key role in power management on infrastructure networks:

• buffer frames
• announcement of buffer status

Stations only need to power up the transmitter to transmit polling frames after being informed by the AP's announcement.

Association request contains a "Listen Interval" field.
The Listen Interval field is used to indicate to the AP how often a STA in power save mode wakes to listen to Beacon management frames.
The value 0 might be used by a STA that never enters power save mode.
An AP may use the Listen Interval information in determining the lifetime of frames that it buffers for a STA.

The Traffic Indication Map information element (IE)

element ID	length	DTIM count	DTIM period	bitmap control		partial virtual bitmap
				offset	broadcast

where:

• element ID(1 octet)
5, identifies a TIM element.
• length(1 octet)
the size of the whole element (5 to 255)
• DTIM_count(1 octet)
the number of beacons remaining before a DTIM (including this frame, so 0 means that this frame is a DTIM)
• DTIM_period(1 octet)
This indicates the number of Beacon intervals between DTIM frames. Zero is reserved and is not used. The DTIM count cycles through from this value down to 0.
• bitmap_control
The Bitmap Control field is divided into two subfields:
• broadcast(1 bit)
Bit-0 is reserved for multicast traffic. 1 when one or more broadcast or multicast frames are queued. This means that all stations should wake up.
• offset(7 bits)
The remaining 7 bits of the Bitmap Control field are used for the Bitmap Offset field. The Bitmap Offset is related to the start of the virtual bitmap. By using the Bitmap Offset and the Length, 802.11 stations can infer which part of the virtual bitmap is included.
• partial_virtual_bitmap(8 to 2008 bits)
The meat of the TIM is the virtual bitmap, a logical structure composed of 2,008 bits. Each bit is tied to the Association ID. When traffic is buffered for that Association ID, the bit is 1. If no traffic is buffered, the bit tied to the Association ID is 0.

Unicast frame buffering and delivery using the Traffic Indication Map(TIM)

To inform stations that frames are buffered, AP periodically assemble a TIM and transmit it in Beacon frames. Each bit in the TIM corresponds to a particular AID. If the STA is in power saving mode, it must wake up and enter the active mode to listen for Beacon frames to receive the TIM. To retrieve buffered frames, the STA uses PS-Poll Control frames.

• Each PS-Poll frame is used to retrieve one buffered frame.
• The delivered Data frame must be positively ACKed before it is removed from the buffer.
• The More Data bit in the Frame Control field is set to 1 if multiple frames are buffered for a STA. The STA can then issue additional PS-Poll until the More Data bit is 0.

A station cannot return to a low-power mode until it receives a Beacon frame in which its bit in the traffic indication map (TIM) is clear.

Delivering multicast and broadcast frames: the Delivery TIM(DTIM)

Whenever any station associated with the AP is sleeping, broadcast/multicast frames are buffered. Each BSS has a parameter called the DTIM period. At a fixed number of Beacon interval, a special type of TIM (DTIM) is sent. The TIM element in Beacon frames contain a counter that counts down to the next DTIM, this count is 0 in a DTIM frame. Buffered broadcast/multicast frames are transmitted after a DTIM Beacon. STA must wake up every DTIM interval to receive buffered broadcast/multicast frames..
The STA can be configured to sleep for its listen period without regard to DTIM transmissions if the battery life is more important.