Power Saving Techniques
Why it is so Important?
As you are aware that most of the Wifi Devices are Battery Operated such as Mobile Phones, Laptops etc. It is very important to consider the Power consumed of every component of such devices. If the device drains its battery too often, then it limits it mobility capability. It is very essential to conserve the battery when the device is not using any of such components. For Ex: Screen times out if there is no activity by User. Similarly when there is no active data transfer over Wifi, it is best to put it in a low power consumption mode to avoid Battery Wastage.
1. STA sends out a packet to AP indicating that it is going to sleep mode.
2. AP Start buffering data for the client and indicates that there is data buffered in Beacon.
3. STA wakes up, listens to Beacon and sees if there is any data buffered for it. If no, go back to sleep. If yes, send a packet to AP to say that it is awake and send the buffered data.
4. AP after sending the buffered data, indicates the STA that there is no more data. STA goes back to sleep mode.
Let us see how a STA indicates that it is going to sleep and how AP indicates that there is some data Buffered for STA.
Every 802.11 Frame has a 802.11 MAC Header:
There are many important fields in the MAC header, but for now let us just stick to Power Management bit.
STA sends out a Null Data Packet ( NDP ) to the AP with Power Management Bit Set to indicate the AP that it is going to sleep and AP from now starts to Buffer the Data for that client.
AP indicates the client that there is data buffered for it by indicating in the Traffic Indication Map Field (TIM). AP needs to buffer both Broadcast/Multicast as well as Unicast traffic for the client. This field is present in every Beacon transmitted by the AP.
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. The beacon having DTIM count as 0 is a DTIM. If broadcast/multicast traffic buffered at the AP the first bit of Bitmap Control set to 1, otherwise 0
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 STA has unicast frames buffered at the AP. Each bit in this filed corresponds to a AID of a STA.
We have seen how the STA uses Power Management bit to indicate that it is going to sleep and how AP is buffering the data for the STA and indicates it if has any data buffered.
In this mechanism, when the STA sees that there is buffered data, it wakes up and sends a PS Poll Frame to the AP, indicating him to send the first buffered Frame. The AP on recieving the PS Poll frame, sends the first buffered frame to the STA. The STA keeps sending PS Poll frame to AP until all buffered frames are retrieved and there is no more frames buffered. This is know the STA by the "No More Data" Flag in the MAC header.
Disadvantages:
This is not efficient Power Saving Mechanism as the STA needs to send PS Poll control frame for every buffered frame.
As you are aware that most of the Wifi Devices are Battery Operated such as Mobile Phones, Laptops etc. It is very important to consider the Power consumed of every component of such devices. If the device drains its battery too often, then it limits it mobility capability. It is very essential to conserve the battery when the device is not using any of such components. For Ex: Screen times out if there is no activity by User. Similarly when there is no active data transfer over Wifi, it is best to put it in a low power consumption mode to avoid Battery Wastage.
Basic Mechanism:
1. STA sends out a packet to AP indicating that it is going to sleep mode.
2. AP Start buffering data for the client and indicates that there is data buffered in Beacon.
3. STA wakes up, listens to Beacon and sees if there is any data buffered for it. If no, go back to sleep. If yes, send a packet to AP to say that it is awake and send the buffered data.
4. AP after sending the buffered data, indicates the STA that there is no more data. STA goes back to sleep mode.
Let us see how a STA indicates that it is going to sleep and how AP indicates that there is some data Buffered for STA.
Every 802.11 Frame has a 802.11 MAC Header:
There are many important fields in the MAC header, but for now let us just stick to Power Management bit.
STA sends out a Null Data Packet ( NDP ) to the AP with Power Management Bit Set to indicate the AP that it is going to sleep and AP from now starts to Buffer the Data for that client.
AP indicates the client that there is data buffered for it by indicating in the Traffic Indication Map Field (TIM). AP needs to buffer both Broadcast/Multicast as well as Unicast traffic for the client. This field is present in every Beacon transmitted by the AP.
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. The beacon having DTIM count as 0 is a DTIM. If broadcast/multicast traffic buffered at the AP the first bit of Bitmap Control set to 1, otherwise 0
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 STA has unicast frames buffered at the AP. Each bit in this filed corresponds to a AID of a STA.
Legacy Power Saving Mechanism - PS Poll
We have seen how the STA uses Power Management bit to indicate that it is going to sleep and how AP is buffering the data for the STA and indicates it if has any data buffered.
In this mechanism, when the STA sees that there is buffered data, it wakes up and sends a PS Poll Frame to the AP, indicating him to send the first buffered Frame. The AP on recieving the PS Poll frame, sends the first buffered frame to the STA. The STA keeps sending PS Poll frame to AP until all buffered frames are retrieved and there is no more frames buffered. This is know the STA by the "No More Data" Flag in the MAC header.
Disadvantages:
This is not efficient Power Saving Mechanism as the STA needs to send PS Poll control frame for every buffered frame.
Null Data Power Save
In this technique the STA does not send a PS-Poll for every buffered frame, instead it just informs the AP that it is awake by sending a NDP frame with Power Management Bit Unset. Once AP receives this frame it starts sending all the buffered frames to the STA one by one.
There is one more very crucial Importance of NDP packet as this packet is used to indicate the AP if STA is going to Sleep or Awake using the Power Management Bit. This is also used when the Client is going to do a Scan (Active, Passive or Roam Scan ) When a STA will start a scan, it will not be on the connected channel and will have to hop on multiple channels. While the STA is Scanning, it cannot receive any data from the AP and requires buffering of frames. Thus STA send a NDP with P bit set before a Scan and sends a NDP with P bit Unset after the Scan. This way STA informs the AP to buffer all its data while it is scanning.
You can read this article for bettery understanding: http://www.my80211.com/home/2009/12/5/80211-null-data-frames.html
WMM-Power Save
To understand this it is essential to understand the Queueing concept of WMM.
If you still want to know about it, then please read this http://wirelesslearnings.blogspot.com/2015/09/uapsd-aka-wmm-power-save.html
Hi,
ReplyDeleteThanks for this blog. Could help me to do SMPS mechanism in any windows client?. By default it is disabled . So please help me here