Archive for the ‘intersymbol interference’ Tag

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 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.  🙂


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%.

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