Automatic Power Save Delivery (APSD)

In IEEE 802.11 wireless LANs, Automatic Power Save Delivery (APSD) is a mechanism that aims to reduce power consumption by the devices connected in the network. It was originally introduced to IEEE 802.11e in 2005 to address the growing need for energy-efficient wireless communications.

How APSD Works

APSD is generally deployed in infrastructure BSS (basic service set). In infrastructure BSS, there are access points (APs) which are basically wireless routers forming the base stations for access. All the wireless devices (clients) communicate with each other through the APs. When a client has frames to send, it sends the frames to the corresponding AP along with the destination address. The AP then forwards the frames to the corresponding devices.

In APSD, the AP buffers frames and sends them to a client device immediately after the client device has delivered its frames to the AP. The client device then goes to a hibernation state until it has more frames waiting to be delivered, thus reducing power usage. This method eliminates the need to poll for each frame, reducing both polling time and energy consumption.

Types of APSD

APSD defines two delivery mechanisms:

• U-APSD (Unscheduled Automatic Power Save Delivery) − The client device triggers frame delivery by sending any frame to the AP. This is more flexible but requires the client to initiate communication.

• S-APSD (Scheduled Automatic Power Save Delivery) − Frame delivery occurs at predetermined scheduled intervals. This provides more predictable power savings but is less flexible than U-APSD.

Advantages and Use Cases

Conclusion

Automatic Power Save Delivery (APSD) is a crucial power management mechanism in modern WiFi networks that significantly reduces energy consumption by enabling intelligent frame buffering and delivery. Both U-APSD and S-APSD variants provide flexible solutions for different network scenarios and device requirements.