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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16 comments so far

  1. chethana on

    good one!!!

  2. Douglas Haider on

    Thanks for reading and for your comment.

    Let me know if you ever have any wireless related questions.

    Respectfully,
    Douglas

  3. mag on

    Thanks for sharing. I’m new to 802.11n 2.0 Draft/Approved Wi-Fi certification. I found your articles are very helpful for someone like me to build some foundation to the wireless world.

  4. Skip on

    Hi Douglas,

    This is a very informative article. Thank you! So, if I understand you correctly, the chance for incorrectly transmitted characters (packet collisions?) increases. I’m asking because I have a client who daily has issues with staying connected to a SonicPoint Wireless N wap. If I reboot the wap, they can connect successfully again. When they lose their connection, they are prompted again for the password. They enter it correctly but continue to get the prompt, as if the wifi device doesn’t get the correct password. I’m turning off the Short Guard Interval just to see if things improve. Any further advice would be greatly appreciated.
    Thanks again!
    Skip

  5. Anonymous on

    how can i change my wireless adapter transmission levels?

  6. […] MAC Service Data Units (MSDUs) or MAC Protocol Data Units (MPDUs). Another optional enhancement is a shorter guard interval between […]

  7. Anonymous on

    This is very helpful for my understanding.. Thanks a ton

  8. Angel on

    good article. thanks.
    I am curious about how the real short GI throughput is?
    what will be the tradeoff of 11% data rate improvement and bad ISI?
    thanks.

  9. Aaron on

    Hello,

    I am having an issue getting a kindle to roam between APs. It holds on to the connected AP until there is absolutely no signal left and then will connect to the secondary AP. To your knowledge, is there a setting on my SonicPoint AP that I can change so that the Kindle will release and re-connect to the strongest AP?

    Stumped,
    IT Support Guy

  10. ehab on

    hi
    Douglas please i need your help
    i have huwaie router b368 and i cant open my play store at Samsung s2(connection timed out) either on my Nokia c7 i cant open 90% of internet sites but when i go to another city or region every thing becomes okay note that the speed is the same every where its good

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  12. yash on

    Sir, I am a fresher can you please suggest a book which covers all the aspects of wlan,as I am a fresher so difficult to understand the 802.11 spec

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  14. Anonymous on

    thx. I got how 802.11n SGI works. However, it still refuse me how to set the SGI on the AP or router. It seems that some routers do not provide the item to select the SGI mode.

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