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Occasional Contributor II
Posts: 36
Registered: ‎04-03-2007

Spectrum Load Balancing

We turned on SLB in a large classroom with 7 AP's in it. In the past we used the old method of load balancing by capping users at 20 per access point. The problem we ran into was we had 2 ssid per AP. (A potential for 40 users) Since I turned on the SLB in this room I am seeing 43 and 45 users on a particular AP and it is the highest bandwidth of any AP and some AP's in that same room with only 10 to 20 users. My questions are at what point of bandwidth utilization does
SLB kick in and what would it be in megabits. Also, why are the other AP's not loadbalancing more evenly. I am getting my stats from Airwave.
Brian
Guru Elite
Posts: 20,002
Registered: ‎03-29-2007

Statistics


We turned on SLB in a large classroom with 7 AP's in it. In the past we used the old method of load balancing by capping users at 20 per access point. The problem we ran into was we had 2 ssid per AP. (A potential for 40 users) Since I turned on the SLB in this room I am seeing 43 and 45 users on a particular AP and it is the highest bandwidth of any AP and some AP's in that same room with only 10 to 20 users. My questions are at what point of bandwidth utilization does
SLB kick in and what would it be in megabits. Also, why are the other AP's not loadbalancing more evenly. I am getting my stats from Airwave.




A few questions:

What types of APs are these?
If they are dual-radio APs, do the clients support dual bands, as well?
How well do those APs hear all the clients?
Is there anything blocking the APs so that they cannot hear each other?

Spectrum Load balancing is supposed to load balance based on spectrum, which means for example it will attempt to make the number of users on channels, not access points, even. If the APs are so far apart that clients hear a few of the APs weakly, the clients will not even attempt to connect to those APs, so they will not participate in spectrum load balancing on those APs.
Colin Joseph
Aruba Customer Engineering

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Frequent Contributor II
Posts: 110
Registered: ‎12-07-2007

Re: Spectrum Load Balancing

Since you say classroom, you are in an educational environment like me?

Couple of answers since I'm seeing some issues but SLB seems to have helped some:

1) The client has the ultimate say in where it wants to go. SLB 'encourages' it to do what we want. Many PC client drivers are very stubborn.

2) If the users are moving around the room, it's entirely possible to get too many people on 1 AP. If they start in one corner then spread out, it's possible the environment hasn't changed enough for the client to attempt to roam. I see this quite often in our dense deployment.

3) As stated, it depends on how dense your AP deployment is for the clients. A very dense deployment can definitely cause grouping. You have to know the precise measurements used by the client driver(s) to determine when/if a roaming event should occur. Unfortunately, many WLAN NIC suppliers are stingy with the exactness of their answer making roaming hard to predict for their device.


I do not believe SLB uses any traffic metrics for it's calculations. It is strictly based on number of clients on APs in the area.

I would suggest a 2-fold approach: cap the max clients around 25-30 and turn on SLB. That's what I'm doing in my testing with our dense deployments. It can get a bit tricky so you have to understand how your entire environment is laid out so you don't mess yourself up somewhere else. You might need to develop a couple of different profiles so you can set some schools/buildings to one maximum and other sites to a more liberal number.
Occasional Contributor II
Posts: 36
Registered: ‎04-03-2007

Re: Spectrum Load Balancing


A few questions:

What types of APs are these?
If they are dual-radio APs, do the clients support dual bands, as well?
How well do those APs hear all the clients?
Is there anything blocking the APs so that they cannot hear each other?

Spectrum Load balancing is supposed to load balance based on spectrum, which means for example it will attempt to make the number of users on channels, not access points, even. If the APs are so far apart that clients hear a few of the APs weakly, the clients will not even attempt to connect to those APs, so they will not participate in spectrum load balancing on those APs.



Colin,
Sorry for the late responce. We are using AP 65's and 70's in these classroom. We spaced them around 25 feet apart. They are in an open area just above the cealing tile. How much of the spectrum of a particular AP would be used, say on channel 1 before it starts to load balance to the other AP in the classroom which is also on channel 1?
Thanks Brian
Brian
Frequent Contributor I
Posts: 97
Registered: ‎08-19-2008

Re: Spectrum Load Balancing

I am seeing the same behavior in buildings with heavy usage. The old LB did a pretty good job of putting equal amount of users per AP, now with SLB, I see some APs much more loaded than others. I understand it is doing it based on spectrum usage.
Out of about 300 users in this building, I think about 20% are only .a capable, so most users concentrate on the 3 channels available. Number of APs is adequate, but I think the 2.4ghz spectrum is getting saturated. I have recently turned on "Co-Channel Interference Mitigation" (turns APs RF almost off if not being used) and went from a 3 channel architecture (1, 6, 11) to a 4 channel one (1, 4, 8, 11). This seem to have helped some...
Marcelo Lew
Wireless Network Architect-Engineer
University of Denver
Guru Elite
Posts: 20,002
Registered: ‎03-29-2007

4 Channel Plan


I am seeing the same behavior in buildings with heavy usage. The old LB did a pretty good job of putting equal amount of users per AP, now with SLB, I see some APs much more loaded than others. I understand it is doing it based on spectrum usage.
Out of about 300 users in this building, I think about 20% are only .a capable, so most users concentrate on the 3 channels available. Number of APs is adequate, but I think the 2.4ghz spectrum is getting saturated. I have recently turned on "Co-Channel Interference Mitigation" (turns APs RF almost off if not being used) and went from a 3 channel architecture (1, 6, 11) to a 4 channel one (1, 4, 8, 11). This seem to have helped some...




Be careful with a 4-Channel Plan. A great deal of time it is not optimal, especially if your APs can see foreign APs. Elizabeth and David discuss it here: http://airheads.arubanetworks.com/vBulletin/showthread.php?t=1381

You should do a "show ap active" to make sure that the "L" flag is on the AP. That would mean that client load balancing (SLB) is really on.
Colin Joseph
Aruba Customer Engineering

Looking for an Answer? Search the Community Knowledge Base Here: Community Knowledge Base

Validated Reference Design Guides : http://community.arubanetworks.com/t5/Validated-Reference-Design/tkb-p/Aruba-VRDs
Contributor I
Posts: 50
Registered: ‎04-29-2008

Re: Spectrum Load Balancing

I show SLB on in the GUI with no domain. The CLI does not show the "L" flag. 3.4.06.
Guru Elite
Posts: 20,002
Registered: ‎03-29-2007

in the 802.11g Radio Profile




There is a "Spectrum Load Balancing" checkbox in the 802.11a or 802.11g profile. (Configuration> Wireless> AP Configuration> Edit your AP Group. Under RF management 802.11g profile). That checkbox being ON is the only thing you need to enable it. Do not enter anything into the Spectrum Load Balancing domain. The "L" is next to an AP in "show ap active" when it recognizes that it's spectrum neighbors have less clients than he does, so he will deny the next association. It does not have to be on all APs, but if you have enough density, it should be on *some* of them.

Colin Joseph
Aruba Customer Engineering

Looking for an Answer? Search the Community Knowledge Base Here: Community Knowledge Base

Validated Reference Design Guides : http://community.arubanetworks.com/t5/Validated-Reference-Design/tkb-p/Aruba-VRDs
Contributor I
Posts: 50
Registered: ‎04-29-2008

Re: Spectrum Load Balancing

They are on. We may not have the density needed in many locations. Your first post made it sound like the L is always present. I'll keep an eye on it.

Thanks Colin.
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