How Stuff Works – 802.11n and Short Guard Interval

Posted February 11, 2009
Filed under: 802.11n, Short Guard Interval | Tags: 802.11n, Geekster, how stuff works - 802.11n, intersymbol interference, 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!

16 comments so far

chethana on June 3, 2009

good one!!!

Reply
Douglas Haider on June 4, 2009

Thanks for reading and for your comment.

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

Respectfully,
Douglas

Reply
mag on March 12, 2010

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.

Reply
Skip on October 5, 2010

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

Reply
Anonymous on March 21, 2011

how can i change my wireless adapter transmission levels?

Reply
Not All 802.11n Networks Are Alike – Stephen Foskett, Pack Rat on July 2, 2011

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

Reply
Anonymous on June 21, 2012

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

Reply
Angel on July 19, 2012

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.

Reply
Aaron on October 12, 2012

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

Reply
ehab on December 15, 2012

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

Reply
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yash on July 19, 2013

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

Reply
link on November 16, 2013

I don’t comment, however after looking at a few of the remarks
on How Stuff Works – 802.11n and Short Guard Interval | WiFi Jedi.com.

I actually do have 2 questions for you if you do not mind.
Could it be only me or does it seem like a few of
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😛 And, if you are posting on other sites, I’d like
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Reply
Anonymous on March 17, 2014

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.

Reply
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