Wireless Access

Hello all,

Whilst I don't have it in front of me right now, I'm pretty sure the Aruba high density VRD suggests disabling low data rates. From memory, it's either everything below 11Mbps or everything below 5.5Mbps. Regardless, I'm really very 50/50 about this (happy with the rest of this VRD).

So let's assume I've Walked the site floor and we get >11Mbps everywhere. On that basis, the proposition sounds solid. However, the gap we potentially create, is isolation of let's call them "weak" client devices. I.e. those with terrible drivers/nics or antennas. I.e. we're elevating the playing field for the greater good, but potentially discriminating against the weak. Be mindful this is a BYOD scenario, so we're talking about all manner of "uncontrolled" devices (from an RF point of view).

My question is to those with direct experience with trying low rates off or on in v. High density environments. Which way did you go and how successful was it?

The goal of reducing lower rates is to reduce channel utilization, that is IF you have high channel utilization.  A side effect of doing this properly is that roaming can improve, as well.

 

You first should start considering removing lower rates if your RF utilization on the Aruba Controller Dashboard is at 20% at Midnight (low to no users on access points) in a dense deployment.  During the day, you do not want your utilization at 50% if you have few or little users on, either.  It is quite possible that you could go to 70% utilization during the day, but that could be because of congestion or interference, so you would probably want to measure it at night so that you can measure the next night after you remove lower rates to see if that solves your issue.  If you have Airwave, you should also be able to tell historically if you have RF utilization at 20% at night by looking at the b/g radio of an access point and going back to midnight and looking at the utilization graph.  In addition in Airwave, if more airtime is spent "Receiving" under channel utilization is another indicator that management traffic, instead of data is consuming airtime and you should consider trimming rates.  You should enable Drop Broadcast and Multicast as a prerequisite to removing rates to ensure that is not the issue.

 

Once again, you would only consider trimming rates if you have (1) High Utilization and (2) a dense deployment.  Once you trim rates, you effectively reduce the radius at which clients can connect.  Management frames at the speeds of 1, 5, 6, 9, 11 transmit further than the rates of 12 and above, but they consume more airtime.  I would not start with removing rates if you have a sparse deployment

 

With that being said, 90% of the time you would consider trimming rates on the b/g (2.4ghz band) side, because it definitely has more clients, interference and fewer channels to service them with.  If you want to consider trimming rates, everything below 12 (removing "b" clients effectively) is not an unreasonable place to start if you have a dense deployment.  To remove everything under 12, these are the 3 parameters in the SSID profile> Advanced that you would change:

 

 

If you have 3 SSIDs being broadcast in an Ap-Group, you would have to change this on all 3.  A packet capture in the Air should show that all management frames are now being sent at 12 to confirm...

 

"So let's assume I've Walked the site floor and we get >11Mbps everywhere. On that basis, the proposition sounds solid. However, the gap we potentially create, is isolation of let's call them "weak" client devices. I.e. those with terrible drivers/nics or antennas. I.e. we're elevating the playing field for the greater good, but potentially discriminating against the weak. Be mindful this is a BYOD scenario, so we're talking about all manner of "uncontrolled" devices (from an RF point of view)." -  The Racking.Monkey

 

Do not make decisions based on if you get 11Mbps everywhere, because the negotiated rate of a client is determined by a number of factors that you do not have direct control over.  Use the utilization number at midnight, because that is something that can be measured after the change...

 

I hope this helps.

Great article Joseph, thanks!

 

I came across a customer where the low speed channles have been disabled but the 802.11g beacon rate is on "Default". As I still have some problems in this customer, what is the side efects of not setting the beacon rate? Can you give me some hints?

 

Thanks again!

Mo

Beacons are sent out all the time, so they potentially have the biggest impact on utilization.

 

Colin,

I was going through this older thread and read this comment that you made. ".....if more airtime is spent "Receiving" under channel utilization is another indicator that management traffic, instead of data is consuming airtime..."

 

I was a little confused at this. Can you explain how you extrapolate that an AP with more Rx traffic is an indicator that management traffic instead of data is consuming air time?

 

thanks for your time,

Sky.

EP,

 

That statement should be read with the entire paragraph that it was part of.  Please read the entire paragraph and let me know if you still have questions.

Colin,

Thanks for your response.

I read the paragraph prior and was unclear as it seemd as though the referenced line  was a 'free standing statement. Were you therefore saying that if RF utilization is at 20% at night (during low usage/no clients) And the AP is spending most of its Airtime on Rx traffic  that this is an indicator that channel utilization was primarily mamagement trafic as there are no users/user traffic?

thanks,

sky

EP,

 

Correct.  When there should be  little data traffic, utilization is high and most of the traffic on an access point is being received, it could mean there is too much management traffic.

cjoseph wrote:If you have 3 SSIDs being broadcast in an Ap-Group, you would have to change this on all 3.  A packet capture in the Air should show that all management frames are now being sent at 12 to confirm...

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we have one or two locations when we have to keep supporting 11b for 2-3 devices.

However, all users will connect with their devices on another SSID.

Can we set speed/beacon on 12Mbps for the users, but keep the slow ones for the 11b SSID?

Apart from not having a great improvement due to the 'slow' beacons, will we have any other side effects? Maybe something like 11b devices not receiving the 'fast' beacons and transmitting at random times, interfering with everyone in that area?

 

You can make the minimum tx rate 11 and minimum basic rate 11 and still support 802.11b clients.

Hi Colin,

 

I have read your article and am thankful for sharing this useful article with us. 

But in this point, I am confused with your comment about not killing 802.11b clients with enabling 11 Mbps too.

I know that basic rates is just about associating with an AP. So if a client ,that supports only 802.11b, wants to associate with an AP, it is neccessary to be enabled the basic rates of 1 and 2. How can it connect to an AP when 1 and 2 basic rates are disabled but 11 is enabled? 

 

Kind Regards,

Emre