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Power Savings on IEEE-802.11

IEEE-802.11 was designed with power saving in mind for stations. This section tries to summarize what the standard defines for all aspects of power saving. It's a work in progress.

Power saving basics

To assist stations with power saving, Access Points (APs) are designed to buffer frames for a station when that station is in power save mode and to transmit them later to the station when the AP knows the station will listen. When a station is in power save mode, it turns off its transmitter and receiver to preserve energy. It takes less power for a station to turn its receiver on to listen to frames than to turn it its transmitter on to transmit frames. For this reason, it's more power-efficient for an AP to inform a station if it has buffered frames present on the AP than to have the station poll the AP querying if frames are present.

Target Beacon Transmission Time Time (TBTT) and beacon interval

Target Beacon Transmission Time Time (TBTT) is the time at which a node (AP or station when in Ad-hoc) must send a beacon. The time difference between two TBTTs is known as the beacon interval. The beacon interval is given in Time Units (TU), each TU represents 1024 microseconds. The beacon interval is typically set to 100 TUs (102400 microseconds, or 102.4 ms) and its length is two bytes.

Listen interval

During association, a "Listen Interval" field is provided by the station. The listen interval is given in beacon interval units, so essentially it tells the AP how many beacons it wants to ignore before turning the receiver on. Two bytes are used to represent he listen interval. Depending on how the AP is tuned, usually based on the amount of space desired for buffered frames, the association may or may not be rejected. After the listen interval the AP does not guarantee it will buffer frames for the station anymore and may discard them. As might be expected then, the listen interval can be used by the AP as a guaranteed maximum time before stations listen to one of their beacons.

The TIM information element

The TIM information element is covered under section of 802.11-1999.pdf. The IEEE-802.11 standards chose to use a bitmap to indicate to any sleeping listening stations if the AP has any buffered frames present for it. Because stations should listen to at least one beacon before the listen interval, the AP periodically sends this bitmap on its beacons as an information element. The bitmask is called the Traffic Indication Map and consists of 2008 bits, each bit representing the Association Id (AID) of a station. For example, the the TIM information element allows you to transfer 1 byte up to the entire 251 bytes (2008 bits) of the TIM. You are allowed to transmit a smaller TIM bitmap as it is expected that only a few number of stations will be asleep. Because of this the bitmap values passed in the TIM information element is called a partial virtual bitmap. To allow you to transmit only a partial bitmap you must make use of the bitmap control and length fields of the TIM information element.

This is what the TIM information element looks like:

|Element ID|Length|DTIM count|DTIM period|Bitmap control|PVM|

PVM: Partial Virtual Bitmap
Note: all fields are 1-byte long except the PVM which can be 
      1-256 bytes long

The last 7 bits of the Bitmap control represent the bitmap offset. To best describe length and bitmap offset, let us just quote the relevant section:

The Partial Virtual Bitmap field consists of octets numbered
N1 through N2 of the traffic indication virtual bitmap, where
N1 is the largest even number such that bits numbered 1 through
(N1 × 8) – 1 in the bitmap are all 0 and N2 is the smallest
number such that bits numbered (N2 + 1) × 8 through 2007 in
the bitmap are all 0. In this case, the Bitmap Offset subfield
value contains the number N1 / 2, and the Length field will
be set to (N2 – N1) + 4.

To better illustrate this let we provide an image:


If no station has unicast frames buffered on the AP, the partial virtual bitmap is encoded as a single zero-filled byte and the bitmap offset is set to zero.

Retrieving buffered unicast frames

After a station receives a TIM and if its sees that the AP has buffered frames for it, it must send a Power Save Poll (PS-Poll) control frame to retrieve each buffered frame on the AP. A station may go back to sleep after the PS-Poll <-> frame exchange or once the TIM no longer has its AID present.

Retrieving buffered multicast and broadcast frames

When dealing with multicast or broadcast frames the AP will buffer in similar way all broadcast and multicast frames should any associated station be sleeping. To indicate to stations that multicast or broadcast frames are buffered the AID of 0 is enabled in the TIM, which corresponds to the first bit on the TIM bitmap control, and not the first bit of the TIM bitmap (XXX needs verification). This will not be set on all TIMs on every beacon but on a special kind of TIM.

Delivery Traffic Indication Map (DTIM), DTIM period

We use a special type of TIM to to announce that the AP is about to transmit all buffered broadcast and multicast frames called the Delivery Traffic Indication Map (DTIM). After this DTIM the AP will send all buffered broadcast and multicast frames. The DTIM will be sent every DTIM period. The DTIM period is set on the TIM information element on the DTIM period field. This field is one byte and represents the number of beacon intervals that must go by before a new DTIM is sent. The DTIM count on the TIM information element tells stations how many beacons must be transmitted before receiving the next DTIM. The DTIM count will be 0 when we've reached a DTIM.

Some devices can be configured to ignore the DTIMs, and this is sometimes considered as additional power saving mechanisms with different marketing names.

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