Archive for the ‘Block ACKs’ Tag

How Stuff Works – 802.11n Frame Aggregation and Block ACKs

Here’s another guest post that I wrote for a friend of mine.  Check out his blog “Hip Technology“.  Again, the “How Stuff Works” posts have been so popular on that I am re-posting what I wrote here.


I have posted several times on technical improvements related to 802.11n and thought that I would continue that format here.  For this post, I would like to discuss frame aggregation and block acknowledgment.

In normal 802.11 operation, each directed data and management must be acknowledged.  This ACK takes the form of a 14 byte packet.  This is shown in the top row of the graphic.

802.11n MAC Improvements

802.11n MAC Improvements

With frame aggregation, up to 64 MSDUs (MAC Service Data Units – essentially layer 2 frames) can be sent at one time.  This “super” frame has one physical layer header, then data frames (each with their own MAC header).  Once all the data has been sent, a block acknowledgment is sent.   This is shown in the bottom row of the graphic.

This is more efficient for several reasons:

  • A physical layer header does not have to be transmitted for each data frame.  
  • The block acknowledgment is much shorter than 64 separate ACKs.   
  • There are far fewer interframe spaces as all the data is aggregated into one burst and all the acknowledgments are bundled together.  

Block Acknowledgments are also used under 802.11e Quality of Service.