How valid is a 802.11n site survey for 802.11ac?

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Ville Franck

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Posted 5 years ago

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Daniel Zeller

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Shouldn't be too different, except you need room for wider channels.
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Ronald Esveld

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The big question would be if you want to use the 80Mhz channels or not.

802.11ac is as you know only on the 5GHz so cell size will be the same for both 802.11n and 802.11ac on 5GHz.

Surveying for ac would be different because there's not so mut non-overlapping channels which will be a pain in the but.
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Crowdie, Champ

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We have found at sites where we deployed AP370s with 40 MHz channels that the clients reported slightly higher RSSI values than with the AP330s and the coverage per access point is slightly larger.  This was most obvious at one site where the AP330s couldn't provide coverage to a public cafe in the same shared tenant building but the AP370s did.  The coverage in the public cafe wasn't stunning but it was good enough for web surfing and E-mail.
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steven

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The main difference for site surveying 11ac would be channel planning and extra SNR to support for the 256 QAM modulation.

Andrew von Nagy did a great article on 11ac channel planning: http://revolutionwifi.blogspot.nl/201...
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Matthew Gast

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There's actually not much you need to do with channels. What you want to do is choose the widest unoccupied channel you can with your automatic radio tuning algorithm. If there's an 80 MHz channel unoccupied, take it. If not, take a 40 MHz channel if you can. Only as a last resort should you take a 20 MHz channel. This figure from my book illustrates the basic process:


There is a matter of how much spectrum there is -- and bad vendors have cost us the opportunity to use several wide channels in 802.11ac by not following the rules. One day after work, I read all the FCC enforcement reports on DFS violations, and wrote up the results for the Aerohive blog: http://blogs.aerohive.com/blog/the-wi...

256-QAM requires some planning adjustments because it requires a higher SNR. It's about 5-7 dB. A good rule of thumb is to say that if you want 256-QAM rates, put the AP in the same room as the receiver. Most walls cause a 3-4 dB loss.
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EvaldasOu

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Matthew, but how this will work when you use two, three, or four 802.11ac capable APs? How theu will share the channels?
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steven

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The channel width is determined per transmission, so my guess would be plan for 40Mhz or even 80Mhz and let the 160Mhz magic come when it wants.
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Matthew Gast

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Exactly right - I posted a different figure that shows how channel sharing works between two APs in this other thread: http://community.aerohive.com/aerohiv...
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Craig Mathias

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Excellent points, Matthew - thanks!
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Craig Mathias

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I should mention here that I don't usually recommend a detailed site survey in most open-office settings - it's much easier to look at the usage logs to get an idea of demand on a geographic basis, and then deploy, filling in the difficult spots with additional APs as required (this needs to be done on an ongoing basis regardless). This is way cheaper than a site survey, and, a real bonus, results in greater capacity.
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Prakash Parasuram

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Craig,
I have read another article of yours on the value of site survey in office spaces, the delays caused while contracting a surveyor and the fact that almost every different site surveyor gives a different resulting ap placement for the same site. Additionally with auto radio and power calculations, spatial streaming, and 5 Ghz channels, it is impossible for a single human being to take a laptop and ap and make appropriate decisions on ap placement. I endorse your method, but this topic is taboo in enterprises because of the site survey "lobby". Especially in office spaces, what you state is a resonable way to handle ap placement. Have you read any other experts endorsing your point of view with regards to site surveys?
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Craig Mathias

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You know, I've not checked that, but I hope so. I've always wanted to do a bake-off of my method (estimate where the demand is, deploy with appropriate density in each location, and fix errors by installing more APs) vs. a site survey on a cost-benefit basis for the completed installation. I'd win every time! Site surveys (a/k/a the WLAN Installer Full Employment Act) are a serious waste of money in most venues, especially open offices. They're all about coverage, not capacity. BTW - I will do site surveys in spaces that I consider to be pathological (building construction, mostly), and I always do an RF sweep with a spectrum analyzer to check for any serious interference in advance. Overall, though, I let the system set channels and power levels, and then check everything regularly to look for problems.

Thank you for the note!

Craig.
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Crowdie, Champ

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In some sites a site survey is a necessary evil - or at least a partial one is.  I do a lot of large hospitals and trying to get data, voice and Vocera (or equivalent) medical badges going without getting your arse out on site is almost impossible.
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Terry Bates

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To get the maximum throughput benefit will we need more APs when using 802.11ac than currently required for a high throughput deployment using 802.11n?
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Matthew Gast

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Not necessarily. If you have plenty of spectrum available and get the benefits of 802.11ac from wider channels, you'll get better throughput with the same number of APs. But for maximum benefit, you'll probably want a few more to start using 256-QAM at short range.
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Craig Mathias

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And you'll probably need more APs as your user base grows regardless...

Thx. Craig.