General question about throughput on different wireless access points.

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We have 121 extreme APs and our biggest complaint is poor wireless speeds.  We use Extreme AP3825i aps for the most part.  I have noticed that one of our APs is producing far less throughput than an AP with the same settings much further away. 80 mbps vs 5mbps at 20/40mhz channel width. This is just an example we normally run our APs at 20 mhz on the 5ghz. I wonder if this is a sign that the AP has gone bad? If so is there anyway to tell if other APs have gone bad without connecting to each one and running an iperf? I have shutdown the AP and rebooted it. It has the latest firmware. Another note we have set channels based off a site survey and I am using the Ekahau site survey software.

With a high number of wireless APs in the building should I turn on auto power so that I cant see more than a few APs at a time?   Will turning the power down reduce throughput speeds?  Would you run your 5 ghz at 20/40/80 mhz?  The max number of clients on one ap is in the 20's we total right around 550 guests on our network. How would you start to tackle this problem?
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Eric Ashton

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Posted 6 days ago

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Joshua Puusep

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Hi Eric,

How did you determine the recommended channels within Ekahau?  Did you use the auto-planner or did you survey the area and examine the channel usage at each location?  Channel width recommendations are very location specific.  You want to find a balance between width and utilization.  Being in an urban area, we almost exclusively use 20Mhz in all of our buildings because we cannot control our neighbors and there are over 600 radios being seen in some of our buildings.

The first thing I would do is take Ekahau and look at the spectrum analysis near the AP in question.  I would be more concerned with air space utilization and the noise floor, rather than client count.  Perhaps there is some localized interference or high utilization on that specific channel.

Also, When you connect to the the AP in question, are you seeing high latency or just slow throughput?  If the latency looks good but there is just slow throughput, I would look over my configuration again.  If you're using policy, make sure you are getting the same policy on the two AP's in your comparison.  If there is high latency, then it is more likely an issue with interference or perhaps the radio, although I've very rarely seen physical issues with radio's.

Another thing i would check is the switchport status that the AP is connected to.  Does the speed and duplex look okay, is it getting the correct PoE class and are there any in/out errors?

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Joshua Puusep

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also on a side note, you can see some utilization stats for the last hour within the wireless controller, although i rather use ekahau as you can see very short spikes.

For historical data, i've setup librenms and it has been a blessing for looking at the overall picture and trends for any given ap.  You can find more info on that here https://community.extremenetworks.com/extreme/topics/extremewireless-support-in-librenms 

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Eric Ashton

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Thank you for your input I will use this.
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Hawkins, Bruce, Employee

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Hello Eric ..

Part of what I think you are possibly fighting against is an issue of focusing on trying to get the highest data rate reported for clients as opposed to working to create the most smoothly and best and most consistently performing WLAN.

In general, there are very few use cases where 40/80Mhz channel widths are really needed, and when you double the channel width you also double the noise floor and decrease both range and SNR at the same time.  20Mhz channels offer the most aggregate capacity, help to minimize co-channel interference and play a very important role in high density deployments.

Here's a set of recommendations that many/most customers with high density deployments, get very good results with:

-- upgrade to the most current version of software available for the track of code you can run (for WLANs with AP 36xx's ... you must stay in the v9 track, for AP 37xx, 38xx and 39xx WLANs, you can upgrade to the most current version of v10)
-- set the AP Polling Timeout to 30 seconds for all APs
-- if you have a High Availability pair of controllers ... set the Fast Failover Timeout value to 20 seconds
-- set the DTIM to 2 for all radios on all APs
-- set the channel width for all radios on all APs to 20Mhz
-- set the 5Ghz radios to either A/N or A/N/AC (depending on the model of AP)
-- set the 2.4Ghz radios to G/N
-- set the Minimum Basic Rate for all radios to 12Mbps
-- set the Guard Interval for all radios on all APs to "Short"
-- enable ADDBA Support for all radios on all APs
-- enable Aggregate MPDUs for all radios on all APs
-- disable Aggregate MSDUs for all radios on all APs
-- enable STBC for all radios on all APs
-- enable LDPC for all radios on all APs
-- disable TXBF for all radios on all APs
-- for all WLAN services that use Encryption/Privacy, use ONLY WPAv2/AES
-- disable Fast Transition and Management Frame Protection for all WLAN services that use WPAv2/AES Encryption/Privacy
-- enable WMM for all WLAN services
-- disable Flexible Client Access for all WLAN services
-- disable Auto Tune Power Control (ATPC) and Dynamic Channel Selection (DCS)
-- if you are in a high density deployment (one AP per every classroom/room, etc ... ) set the Max TX power for all 5Ghz radios to between 10-12dBm and the Max TX power for all 2.4Ghz radios to between 5-7dBm

It's also worth saying, that you would ideally be putting APs in the rooms you want service clients for, and not in hallways where you are hoping the signal will "bleed through" into the rooms.  Avoid hallway placement of APs if at all possible.

You may have to adjust the signal strength recommendations a bit in different areas of your deployment or in all of them ... as I can't know what the RF obstacles are in your environment ... walls ... ceilings ... floors .... doors ... windows etc ... and what they are made of ... and most deployments don't have a perfectly equal distribution of APs in terms of distance etc ... but those are good basic starting points to work with.

Additional steps could be to turn off "every other" 2.4Ghz radio (again, if your deployment is highly dense) as you only have three channels with which to work, and to enable Band Preferencing to try and move as many 5Ghz capable devices off the 2.4Ghz side of things.

What you should find is that with those settings ... clients connect smoothly and consistently and if adjusted well ... roam well also. If clients aren't roaming well enough, and "stick" to APs that are in other rooms than where they are ... you could try increasing the Minimum Basic Rate to 24Mbps.

Your data rates won't be 900Mbps etc ... but the truth is ... APs are half duplex hubs with fancy radios that split the aggregate available data across the number of clients connecting to a given AP and only one client can "talk" at a time and unless you have only a single client on a radio in perfectly optimum RF conditions with perfect signal ... you're never going to push that kind of data out the ethernet port of any of your APs anyway.  What you will find is very good performance ... and data rates in acceptable ranges, much as what was described in this earlier discussion:  https://community.extremenetworks.com/extreme/topics/general-question-about-throughput-and-ping-ms .

By minimizing co-channel interference, keeping the noise floor low, and keeping the cell sizes of your APs small and tight with just enough overlap to facilitate good roaming, you should get reliable results with very good performance.

Hope this helps.

(Edited)
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Eric Ashton

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Thank you for your response I will try this.
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Hawkins, Bruce, Employee

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Good luck!  I should also have mentioned, that if you are using WPAv2/AES with PSK, and you have the options to enable Opportunistic Key Caching (OKC) and Pre-Authentication ... you should enable both as additional measures to promote the smoothest roaming possible between neighboring APs.

If you have any issues using Multi-Edit to push out those changes to all your APs or challenges finding some of those settings, or just need assistance from us in general while working through this ... never hesitate to open a case with us. That's why we're here! 
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Laura

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What do these acronyms stand for?
enable STBC for all radios on all APs
-- enable LDPC for all radios on all APs
-- disable TXBF for all radios on all APs
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Gareth Mitchell, Extreme Escalation Support Engineer

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These are all complex technologies, in brief:

STBC = Space Time Block Encoding - Used to spread copies of a single data stream across multiple (2 or more) antennas to improve the reliability of data transfer.
LDPC = Low Density Parity Check - is a forward error correction encoding scheme, the idea being to correct single bit errors and improve SNR. 
TxBF = Transmit beamforming Allows the antenna to adjust the phase and amplitude  in such away that MIMO is utilised and allows a signal to arrive at a receiver in phase, therefore using multipath in a constructive way - the net result being higher SNR at the receiver and increased throughput.

-Gareth