Legacy Power Save Mode

In our previous post Understanding WLAN Power Save Mode, we have seen an overview of 802.11 power save standards, mechanisms, and tools to simulate power save clients. In this blog, we will delve deeper into legacy power save mode – mechanism, process, drawbacks and simulation of WiFi clients with legacy power save enabled.

Power Management Mechanism

Every 802.11 Power Management mechanism begins with an STA (WiFi Client Device) associating with a BSS and getting an Identifier. Every STA associated with an AP will have an Association Identifier (AID), which an AP assigns to an STA. The AID for an STA will be present in the Association response or Reassociation response.

The above association response frame to the device with Mac 9c:f4:8e:9d:ac:1c has AID set to 3

Step-By-Step Process

1. An STA will indicate the AP that it is going into the power save mode, by sending a Null data frame, by setting Power Management bit set / enabled (1) as below

2. While an STA is in power save mode, the AP will keep buffering the packets destined for the STA in a queue (one per STA). AP shall not drop any packets during this time. AP will then set the Traffic Indication Map (TIM) bit in beacon frame to indicate that the packets for that particular STA are being queued at the AP.

Above Beacon packet has the TIM bit set.

• DTIM (Delivery Traffic Indication Map), informs STA about the buffered multicast/broadcast traffic on the AP
• Bit 0 of Bitmap control field indicates that multicast data is present at the AP
• Partial Virtual Bitmap is a 2008 bit field organized into 251 octets. Each bit in this field indicates the presence of buffered data at AP, to the particular AID of STA

     Here, Partial Virtual Bitmap = 8

The AID mentioned above is “ 3 “, Which indicates that AP has buffered data corresponding to AID = 3.

3. An STA can wake up from sleep mode for any of the reasons below:

• • The STA has a frame to send    OR
  • STA’s internal timing mechanism has been triggered

The STA will send a Null frame to AP by setting Power management bit set to “ 0 “, indicating to the AP that the STA has woken up and is ready to receive and send data. AP can then send all the buffered data to the STA.

4. When AP has finished sending all the buffered data the STA will go to sleep mode again, by sending Null frame to AP by setting PWR MGT bit to 1.

Simulating Clients With Legacy Power Save Enabled

In this WiFi era, most of the mobile devices that support wireless connectivity come with legacy power save mode. It’s important to understand that the power management mechanism is not only WiFi client device dependent but also requires configuration and support on the network side (Access Points). Testing an Access Point for its power save feature at scale in the lab requires 100’s of clients with power save feature.

Emulation of 100’s to 1000’s of WLAN clients with power save enabled can be done in a few seconds using SWAT WiCheck. This ensures that even under a heavy load of 100s of clients, the Access Point is able to buffer data to all these clients and provide a superior experience with extended hours of use over mobile devices to every user in the network.

Drawbacks of Legacy Power Save

Legacy power save mode is time driven and not event driven. Hence it’s not an effective way of power saving for:

• Latency sensitive applications like VoIP & video streaming

To overcome these drawbacks and solve battery drain problems, Unscheduled Automatic Power Save Delivery (U-APSD) mechanism has been implemented as part of 802.11e spec. We will discuss how U-APSD works in our next post