A wireless radio can perform one of the 4 activities. Power consumed by each activity increases in the given order ( 1-4).
2. Idle & awake
There are 3 methods of power management used in 802.11
1. 802.11 Power Management
2. Unscheduled Automatic Power Save Delivery (U-APSD) from 802.11e amendment
3. Power Save Multi-Poll (PSMP) from 802.11n amendment.
In any of the above method basic power management structure summarized into 4 steps
1. Before a STA goes into the doze state, it sends a frame, usually null data frame, to the AP indicating that power management is enabled.
2. Once STA indicate, that it is in Power Save mode, the AP begins to buffer all frames destined to that station.
3. When the station goes into awake state, it sends a frame to the AP in order to begin the data retrieval process.
4. When AP has finished sending all buffered data to the station, the station goes back into the doze state.
Every 802.11 power management method begin when the STA associates to the BSS. When AP send “Association Response” frame to the STA, an Association Identifier (AID) value present in the AID fixed parameter field (16 bit) as shown below. AID is presence in Association Response or Reassoication Response frames.
STA is allowed to go to “doze” state after the AP has been notified that station is about to enter power save mode. STA will use null data frame with Power Management bit set to 1 as shown below.
Stations will wake up from doze state for one of the 3 reason
1. if station has a frame to send
2. based on STA internal timing mechanism.
Traffic Indication Map (TIM)
TIM is an information element & it has below structure.
Element ID (1 byte) : Value 5 indicate it is a TIM
Length (1 byte) : Length of the info carrying fields (DTIM count, DTIM Period, Bitmap Control, Partial Virtual Bitmap)
DTIM Count (1 byte) : Incremental beacon frames until the next DTIM.
DTIM Period (1 byte) : number of beacon frames between DTIM beacon.
Bitmap Control (1 byte) : to indicate multicast/broadcast are buffered at the AP & use as space save (bitmap offset)
Partial Virtual Bitmap(1-251 byte) : Series of flags indicating whether each associated STAT has unicast frames buffered at the AP. Each bit in this filed corresponds to a AID of a STA.
Here is a TIM information element in a beacon frame. Note that DTIM period is 3 in the below, which mean every 3 beacon one DTIM will be advertised. DTIM count is 2 indicating in 2 beacon time there will be a DTIM
In bitmap control field, first bit set to 1 that indicate buffered traffic at AP is broadcast or multicast. Remaining 7 bits used as Bitmap Offset, which may have any value between 0 to 127 used as a space saver. For an example, if there is no buffered traffic to AID1-70 then all those values are 0 in Partial Virtual Bitmap section. To save some space, you can use Bitmap offset value to indicate how many bytes are Zero in Partial Virtual Bitmap (PVB). Let’s say Bitmap Offset is N, then 2xN bytes are zero in PVB. In hour case N=4 where 8 bytes (or 64 bits) can be zero & those 64 bits can be skipped by setting Bitmap Offset value to 4.
Delivery Traffic Indication Message (DTIM)
DTIM beacon is identical in structure to the ordinary beacon. The only difference is tha the content of the TIM IE will give information about broadcast/multicast traffic that is buffered at the AP, in addition to the typical information about buffered unicast frames that is always present in the TIM.
Below shows a DTIM beacon where DTIM count set to 0. If broadcast/multicast traffic buffered at the AP the first bit of Bitmap Control set to 1, otherwise 0 (which is the case in this DTIM)
802.11 Power Management:
In legacy (802.11 standard) power management is uese, the STA never sends a frame with power management flag set to 0. It is always set to 1 (figure 8.6) & then AP send a buffered frame. In this method, when station return to “doze”mode STA does not notify the AP, & AP has to always buffered frame intend to that STA.
When Power Save Poll ( PS-Poll) frames are used in 802.11 power management, then STA has to send a PS-Poll control frame to request AP to send a buffered unicast frame. In that frame if AP set “more data” bit to 1 the STA understand AP has more data to send & therefor remain in awake state & send another PS-Poll frame to get next frame.802.11 power management has two major limitations
1. Additional overhad added to wireless channel (decrease throughput)
2. STA must spend too much time in Transmitting state.
802.11e U-APSD :
This is introduced in 802.11e amendment & part of WMM-Power Save certification as well.
In this method, a STA typically sends a null data frame in order to retrieve buffered unicast frame from AP. Power Mgmt bit set to 0 in this frame (indicated STA in Active mode). Note that in this method, AP will send ALL buffered unicast frames to that STA.
When STA goes into Power Save mode, it has to send another null data frame with power management bit set to 1.
802.11n Power Management :
1. PSMP – Power Save Multi-Poll
This is a power management method that builds on schedulded automatic power save delivery (S-APSD) for network that use HCF Controlled Channel Access (HCCA)
2. SMSP- Spatial Multiplexing Power Save
SMSP involves STA reducing the number of data streams used during spatial multiplexing. SMSP will temporarily disabling spatial multiplexing to conserve battery life.
IBSS Power Management
In IBSS (ad hoc network) there is no AP to send TIM or DTIM. So if a STA goes into power save mode multiple other STAs has to buffer its data for specific STA. So in IBSS, Announcement Traffic Indication Message (ATIM) is use for power management. It is a management frame with no frame body. When a STA receives ATIM, that formally dozing station must begin the process of retrieving buffered frame from the stations that transmitted the ATIM.
1. CWAP Official Study Guide – Chapter 8
Great blog! I found it very useful while reviewing 802.11 concepts.
One minor issue I noticed. For the “802.11e U-APSD” section above, I believe the method you have mentioned (i.e. NULL data frame with PM bit=0 to trigger downlink frames) is actually an alternate implementation for the (legacy) power save mechanism to avoid PS-POLL signalling overhead.
The 802.11e defined U-APSD mechanism involves using QoS-NULL or QoS-DATA frames to trigger a downlink burst from the AP (well SIFS separation between frames) until the end of the service period (ESOP bit = 1 in QoS control field).
Just thought I’d let you know, incase it turns out that a correction is required. Thanks again for the excellent resource!
Thank you very much for the correction & detailed information.
I know its bit old post. Have a small doubt on this :
“In legacy (802.11 standard) power management is use, the STA never sends a frame with power management flag set to 0. It is always set to 1 (figure 8.6) & then AP send a buffered frame”
Does it mean every packet sent by legacy STA will have PM bit set and AP will be continuously buffering
It keeps me wondering why STA has to set PM bit in every frame? How does STA retrieve the data from AP?
I have to study it again as can’t remember 🙂
cc feng said:
“Stations will wake up from doze state for one of the 3 reason
1. if station has a frame to send
2. based on STA internal timing mechanism.”
Is there a third reason? I can not find it in your blog…
while performing 11r roaming also STA will go to Power save mode and perform off channeling and tries to find best AP in his surroundings under same MD(Mobility Domain)
Vinay kumar said:
Can anyone please tell “How can my wifi device is enter into the suspend mode by using Linux commands”
Thank you for this informative blog.
I still have some doubts, would be great if you could please help in solving them.
* If the client is in sleep state, then how will it listen to the Beacon with TIM and if it listens to all the beacons, then is the client in really the sleep state or awake ? how does it sleep then ?
* Also, like if AP has buffered frame for multiple clients, does it send multiple different beacons ??