Archive for the ‘how stuff works – 802.11n’ 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 https://wifijedi.wordpress.com that I am re-posting what I wrote here.

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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.

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How Stuff Works – 802.11n and Short Guard Interval

This is a post that I wrote the other day as a “guest post” for a co-worker’s blog.   It is a Xirrus sponsored blog, titled “Geekster”.  The URL for the blog is http://geekster1.blogspot.com/ The guest post was part of a series, “How Stuff Works”, which has been one of the most successful portions of my own blog, so I am going to re-post for my faithful readers.  🙂

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In the first several “How Stuff Works” posts, I have been talking about technical improvements to 802.11n such as MIMO antennas, Spatial Multiplexing, and Channel Bonding.  In this post, I want to talk about another such technical improvement, Short Guard Interval.

The guard interval is the space between symbols (characters) being transmitted.  This is often confused with the space between packets, which is the interframe space (IFS).  The guard interval is there to eliminate intersymbol interference, which is referred to as ISI.  ISI happens when echos or reflections from one symbol interfere with another.  Adding time between symbol transmission allows these echos and reflections to settle in before the next symbol is transmitted.  In normal 802.11 operation, the guard interval is 800 ns.

With 802.11n, short guard intervals are possible.   The short guard interval time is 400ns, or half of what it used to be.  Shorter wait time (guard interval) between symbols increases throughput.  However, if it’s too short, the amount of ISI will increase, and throughput will decrease.  On the other hand, if the guard interval is too long, there is increased overhead due to the additional idle time.    If you look at an 802.11 Modulation and Coding Scheme (MCS) chart, you will see that Short Guard Interval increases the data rate by roughly 10-11%.

Check out my blog at http://wifijedi.wordpress.org for other “How Stuff Works” postings as well as other information and opinions on wireless networking and security!

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